< draft-ietf-avt-srtp-08.txt   draft-ietf-avt-srtp-09.txt >
Internet Engineering Task Force Baugher, McGrew, Internet Engineering Task Force Baugher, McGrew (Cisco)
AVT Working Group Oran (Cisco) AVT Working Group Carrara, Naslund,
INTERNET-DRAFT Blom, Carrara, Naslund, INTERNET-DRAFT Norrman (Ericsson)
EXPIRES: November 2003 Norrman (Ericsson) EXPIRES: December 2003 July 2003
May 2003
The Secure Real-time Transport Protocol The Secure Real-time Transport Protocol
<draft-ietf-avt-srtp-08.txt> <draft-ietf-avt-srtp-09.txt>
Status of this memo Status of this memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
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Abstract Abstract
This document describes the Secure Real-time Transport Protocol This document describes the Secure Real-time Transport Protocol
(SRTP), a profile of the Real-time Transport Protocol (RTP), which (SRTP), a profile of the Real-time Transport Protocol (RTP), which
can provide confidentiality, message authentication, and replay can provide confidentiality, message authentication, and replay
protection to the RTP/RTCP traffic. protection to the RTP traffic and to the control traffic for RTP,
the Real-time Transport Control Protocol (RTCP).
TABLE OF CONTENTS TABLE OF CONTENTS
1. Introduction......................................................3 1. Introduction.......................................................3
1.1. Notational Conventions..........................................3 1.1. Notational Conventions.........................................4
2. Goals and Features................................................4 2. Goals and Features.................................................4
3. SRTP Framework....................................................5 2.1 Features........................................................5
3.1 Secure RTP......................................................6 3. SRTP Framework.....................................................5
3.2 SRTP Cryptographic Contexts.....................................7 3.1 Secure RTP......................................................6
3.2.1 Transform-independent parameters............................7 3.2 SRTP Cryptographic Contexts.....................................8
3.2.2 Transform-dependent parameters.............................10 3.2.1 Transform-independent parameters............................8
3.2.3 Mapping SRTP Packets to Cryptographic Contexts.............10 3.2.2 Transform-dependent parameters.............................10
3.3 SRTP Packet Processing.........................................11 3.2.3 Mapping SRTP Packets to Cryptographic Contexts.............10
3.3.1 Packet Index Determination, and ROC, s_l Update............12 3.3 SRTP Packet Processing.........................................11
3.3.2 Replay Protection..........................................14 3.3.1 Packet Index Determination, and ROC, s_l Update............13
3.4 Secure RTCP....................................................15 3.3.2 Replay Protection..........................................15
4. Pre-Defined Cryptographic Transforms.............................18 3.4 Secure RTCP....................................................16
4.1 Encryption.....................................................19 4. Pre-Defined Cryptographic Transforms..............................19
4.1.1 AES in Counter Mode........................................20 4.1 Encryption.....................................................20
4.1.2 AES in f8-mode.............................................21 4.1.1 AES in Counter Mode........................................21
4.1.3 NULL Cipher................................................23 4.1.2 AES in f8-mode.............................................23
4.2 Message Authentication and Integrity...........................24 4.1.3 NULL Cipher................................................25
4.2.1 HMAC-SHA1..................................................24 4.2 Message Authentication and Integrity...........................26
4.3 Key Derivation.................................................25 4.2.1 HMAC-SHA1..................................................26
4.3.1 Key Derivation Algorithm...................................25 4.3 Key Derivation.................................................27
4.3.2 SRTCP Key Derivation.......................................27 4.3.1 Key Derivation Algorithm...................................27
4.3.3 AES-CM PRF.................................................27 4.3.2 SRTCP Key Derivation.......................................29
5. Default and mandatory-to-implement Transforms....................27 4.3.3 AES-CM PRF.................................................29
5.1 Encryption: AES-CM and NULL....................................28 5. Default and mandatory-to-implement Transforms.....................29
5.2 Message Authentication/Integrity: HMAC-SHA1....................28 5.1 Encryption: AES-CM and NULL....................................30
5.3 Key Derivation: AES-CM PRF.....................................28 5.2 Message Authentication/Integrity: HMAC-SHA1....................30
6. Adding SRTP Transforms...........................................28 5.3 Key Derivation: AES-CM PRF.....................................30
7. Rationale........................................................29 6. Adding SRTP Transforms............................................30
7.1 Key derivation.................................................29 7. Rationale.........................................................31
7.2 Salting key....................................................30 7.1 Key derivation.................................................31
7.3 Message Integrity from Universal Hashing.......................30 7.2 Salting key....................................................32
7.4 Data Origin Authentication Considerations......................30 7.3 Message Integrity from Universal Hashing.......................32
7.5 Short and Zero-length Message Authentication...................31 7.4 Data Origin Authentication Considerations......................32
8. Key Management Considerations....................................32 7.5 Short and Zero-length Message Authentication...................33
8.1. Re-keying.....................................................33 8. Key Management Considerations.....................................34
8.2. Key Management parameters.....................................34 8.1. Re-keying.....................................................35
9. Security Considerations..........................................35 8.1.1 Use of the <From, To> for re-keying........................35
9.1 SSRC collision and two-time pad................................35 8.2. Key Management parameters.....................................36
9.2 Key Usage......................................................36 9. Security Considerations...........................................37
9.3 Confidentiality of the RTP Payload.............................38 9.1 SSRC collision and two-time pad................................37
9.4 Confidentiality of the RTP Header..............................38 9.2 Key Usage......................................................38
9.5 Integrity of the RTP payload and header........................39 9.3 Confidentiality of the RTP Payload.............................40
9.5.1. Risks of Weak or Null Message Authentication..............40 9.4 Confidentiality of the RTP Header..............................41
9.5.2 Implicit Header Authentication.............................41 9.5 Integrity of the RTP payload and header........................41
9.5.1. Risks of Weak or Null Message Authentication..............42
10. Interaction with Forward Error Correction mechanisms............41 9.5.2 Implicit Header Authentication.............................44
11. Scenarios.......................................................42 10. Interaction with Forward Error Correction mechanisms.............44
11.1 Unicast.......................................................42 11. Scenarios........................................................44
11.2 Multicast (one sender)........................................43 11.1 Unicast.......................................................44
11.3 Re-keying and access control..................................44 11.2 Multicast (one sender)........................................45
11.4 Summary of basic scenarios....................................44 11.3 Re-keying and access control..................................46
12. IANA Considerations.............................................45 11.4 Summary of basic scenarios....................................47
13. Acknowledgements................................................45 12. IANA Considerations..............................................47
14. Author's Addresses..............................................45 13. Acknowledgements.................................................47
15. References......................................................46 14. Author's Addresses...............................................48
16. Intellectual Property Right Considerations......................49 15. References.......................................................48
17. Full Copyright Statement........................................50 16. Intellectual Property Right Considerations.......................51
Appendix A: Pseudocode for Index Determination......................50 17. Full Copyright Statement.........................................52
Appendix B: Test Vectors............................................51 Appendix A: Pseudocode for Index Determination.......................53
B.1 AES-f8 Test Vectors............................................51 Appendix B: Test Vectors.............................................53
B.2 AES-CM Test Vectors............................................52 B.1 AES-f8 Test Vectors............................................53
B.3 Key Derivation Test Vectors....................................52 B.2 AES-CM Test Vectors............................................54
B.3 Key Derivation Test Vectors....................................55
1. Introduction 1. Introduction
This document describes the Secure Real-time Transport Protocol This document describes the Secure Real-time Transport Protocol
(SRTP), a profile of the Real-time Transport Protocol (RTP), which (SRTP), a profile of the Real-time Transport Protocol (RTP), which
can provide confidentiality, message authentication, and replay can provide confidentiality, message authentication, and replay
protection to the RTP/RTCP traffic. protection to the RTP traffic and to the control traffic for RTP,
RTCP (the Real-time Transport Control Protocol) [RTPNEW].
SRTP provides a framework for encryption and message authentication SRTP provides a framework for encryption and message authentication
of RTP and RTCP streams (Section 3). SRTP defines a set of default of RTP and RTCP streams (Section 3). SRTP defines a set of default
cryptographic transforms (Sections 4 and 5), and it allows new cryptographic transforms (Sections 4 and 5), and it allows new
transforms to be introduced in the future (Section 6). With transforms to be introduced in the future (Section 6). With
appropriate key management (Sections 7 and 8), SRTP is secure appropriate key management (Sections 7 and 8), SRTP is secure
(Sections 9 and 10) for unicast and multicast RTP applications (Sections 9) for unicast and multicast RTP applications (Section
(Section 11). 11).
SRTP can achieve high throughput and low packet expansion. SRTP SRTP can achieve high throughput and low packet expansion. SRTP
proves to be a suitable protection for heterogeneous environments. proves to be a suitable protection for heterogeneous environments
To get such features, default transforms are described, based on an (mix of wired and wireless networks). To get such features, default
additive stream cipher for encryption, a keyed-hash based function transforms are described, based on an additive stream cipher for
for message authentication, and an "implicit" index for encryption, a keyed-hash based function for message authentication,
sequencing/synchronization based on the RTP sequence number for SRTP and an "implicit" index for sequencing/synchronization based on the
and an index number for Secure RTCP (SRTCP). RTP sequence number for SRTP and an index number for Secure RTCP
(SRTCP).
1.1. Notational Conventions 1.1. Notational Conventions
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
The terminology conforms to [RFC2828]. The terminology conforms to [RFC2828] with the following exception.
For simplicity we use the term "random" throughout the document to
denote randomly or pseudo-randomly generated values. Large amounts
of random bits may be difficult to obtain, and for the security of
SRTP, pseudo-randomness is sufficient.
By convention, the adopted representation is the network byte order, By convention, the adopted representation is the network byte order,
i.e. the left most bit (octet) is the most significant one. By XOR i.e. the left most bit (octet) is the most significant one. By XOR
we mean bitwise addition modulo 2 of binary strings, and || denotes we mean bitwise addition modulo 2 of binary strings, and || denotes
concatenation. In other words, if C = A || B, then the most concatenation. In other words, if C = A || B, then the most
significant bits of C are the bits of A, and the least significant significant bits of C are the bits of A, and the least significant
bits of C equal the bits of B. Hexadecimal numbers are prefixed by bits of C equal the bits of B. Hexadecimal numbers are prefixed by
0x. 0x.
The word "encryption" includes also use of the NULL algorithm (which The word "encryption" includes also use of the NULL algorithm (which
in practice does leave the data in the clear). in practice does leave the data in the clear).
With slight abuse of notation, we use the terms "message With slight abuse of notation, we use the terms "message
authentication" and "authentication tag" as is common practice even authentication" and "authentication tag" as is common practice, even
though in some circumstances, e.g. group communication, the service though in some circumstances, e.g. group communication, the service
provided is actually only integrity protection and not data origin provided is actually only integrity protection and not data origin
authentication. authentication.
2. Goals and Features 2. Goals and Features
The security goals for SRTP are to ensure: The security goals for SRTP are to ensure:
* the confidentiality of the RTP and RTCP payloads, and * the confidentiality of the RTP and RTCP payloads, and
* the integrity of the entire RTP and RTCP packets, together with * the integrity of the entire RTP and RTCP packets, together with
protection against replayed packets. protection against replayed packets.
These security services are optional and independent from each These security services are optional and independent from each
other, except that SRTCP integrity protection is mandatory other, except that SRTCP integrity protection is mandatory
(malicious or erroneous alteration of RTCP messages could disrupt (malicious or erroneous alteration of RTCP messages could otherwise
the processing of the RTP stream). disrupt the processing of the RTP stream).
Other, functional, goals for the protocol are: Other, functional, goals for the protocol are:
* a framework that permits upgrading with new cryptographic * a framework that permits upgrading with new cryptographic
transforms, transforms,
* low bandwidth cost, i.e., a framework preserving RTP header * low bandwidth cost, i.e., a framework preserving RTP header
compression efficiency, compression efficiency,
and, asserted by the pre-defined transforms: and, asserted by the pre-defined transforms:
* a low computational cost, * a low computational cost,
* a small footprint (i.e. small code size and data memory for keying * a small footprint (i.e. small code size and data memory for
information and replay lists), keying information and replay lists),
* limited packet expansion to support the bandwidth economy goal, * limited packet expansion to support the bandwidth economy goal,
* independence from the underlying transport, network, and physical
layers used by RTP, in particular high tolerance to packet loss * independence from the underlying transport, network, and
and re-ordering. physical layers used by RTP, in particular high tolerance to
packet loss and re-ordering.
These properties ensure that SRTP is a suitable protection scheme These properties ensure that SRTP is a suitable protection scheme
for RTP/RTCP in both wired and wireless scenarios. for RTP/RTCP in both wired and wireless scenarios.
2.1 Features 2.1 Features
Besides the above mentioned direct goals, SRTP provides for some Besides the above mentioned direct goals, SRTP provides for some
additional features. They have been introduced to lighten the burden additional features. They have been introduced to lighten the
on key management and to further increase security. They include: burden on key management and to further increase security. They
include:
* A single "master key" provides keying material for * A single "master key" can provide keying material for
confidentiality and integrity protection, both for the SRTP stream confidentiality and integrity protection, both for the SRTP stream
and the corresponding SRTCP stream. This is achieved with a key and the corresponding SRTCP stream. This is achieved with a key
derivation function (see Section 4.3), providing "session keys" derivation function (see Section 4.3), providing "session keys"
for the respective security primitive, securely derived from for the respective security primitive, securely derived from
the master key. the master key.
* In addition, the key derivation can be configured to periodically * In addition, the key derivation can be configured to periodically
"refresh" the session keys, which limits the amount of ciphertext refresh the session keys, which limits the amount of ciphertext
produced by a fixed key, available for an adversary to produced by a fixed key, available for an adversary to
cryptanalyze. cryptanalyze.
* "Salting keys" are used to protect against pre-computation and * "Salting keys" are used to protect against pre-computation and
time-memory tradeoff attacks [MF00,BS00]. time-memory tradeoff attacks [MF00,BS00].
Detailed rationale for these features can be found in Section 7. Detailed rationale for these features can be found in Section 7.
3. SRTP Framework 3. SRTP Framework
RTP is the Real-time Transport Protocol [RTPNEW]. We define SRTP as RTP is the Real-time Transport Protocol [RTPNEW]. We define SRTP as
a profile of RTP. This profile is an extension to the RTP a profile of RTP. This profile is an extension to the RTP
Audio/Video Profile [AVPNEW]. Except where explicitly noted, all Audio/Video Profile [AVPNEW]. Except where explicitly noted, all
aspects of that profile apply, with the addition of the SRTP aspects of that profile apply, with the addition of the SRTP
security features. Conceptually, we consider SRTP to be a "bump in security features. Conceptually, we consider SRTP to be a "bump in
the stack" implementation which resides between the RTP application the stack" implementation which resides between the RTP application
and the transport layer. SRTP intercepts RTP packets and then and the transport layer. SRTP intercepts RTP packets and then
forwards an equivalent SRTP packet on the sending side, and which forwards an equivalent SRTP packet on the sending side, and
intercepts SRTP packets and passes an equivalent RTP packet up the intercepts SRTP packets and passes an equivalent RTP packet up the
stack on the receiving side. stack on the receiving side.
Secure RTCP (SRTCP) provides the same security services to RTCP as Secure RTCP (SRTCP) provides the same security services to RTCP as
SRTP does to RTP. SRTCP message authentication is MANDATORY and SRTP does to RTP. SRTCP message authentication is MANDATORY and
thereby protects the RTCP fields to keep track of membership, thereby protects the RTCP fields to keep track of membership,
provide feedback to RTP senders, or maintain packet sequence provide feedback to RTP senders, or maintain packet sequence
counters. SRTCP is described in Section 3.4. counters. SRTCP is described in Section 3.4.
3.1 Secure RTP 3.1 Secure RTP
skipping to change at page 6, line 29 skipping to change at page 6, line 42
| .... | | | .... | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| RTP extension (OPTIONAL) | | | RTP extension (OPTIONAL) | |
+>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | +>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | payload ... | | | | payload ... | |
| | +-------------------------------+ | | | +-------------------------------+ |
| | | RTP padding | RTP pad count | | | | | RTP padding | RTP pad count | |
+>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+ +>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+
| ~ SRTP MKI (OPTIONAL) ~ | | ~ SRTP MKI (OPTIONAL) ~ |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| ~ authentication tag (RECOMMENDED) ~ | | : authentication tag (RECOMMENDED) : |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | | |
+- Encrypted Portion* Authenticated Portion ---+ +- Encrypted Portion* Authenticated Portion ---+
Figure 1. The format of an SRTP packet. *Encrypted Portion is the Figure 1. The format of an SRTP packet. *Encrypted Portion is the
same size as the plaintext for the Section 4 pre-defined transforms. same size as the plaintext for the Section 4 pre-defined transforms.
The Encrypted Portion of an SRTP packet consists of the encryption The "Encrypted Portion" of an SRTP packet consists of the encryption
of the RTP payload (including RTP padding when present) of the of the RTP payload (including RTP padding when present) of the
equivalent RTP packet. The "Encrypted Portion" MAY be the exact equivalent RTP packet. The Encrypted Portion MAY be the exact size
size of the plaintext or MAY be larger. It is exact for the pre- of the plaintext or MAY be larger. Figure 1 shows the RTP payload
defined transforms. Figure 1 shows the RTP payload including any including any possible padding for RTP [RTPNEW].
possible padding for RTP [RTPnew]. The presence of RTP padding is
transparent to SRTP, i.e. is treated as part of the plaintext. Note
that an encryption transform used in SRTP MAY choose to specify its
own padding, independently of the RTP pad, in which case the
encrypted portion may be larger than the original RTP packet. None
of the pre-defined transforms, however, uses any padding so for
these, the sizes match exactly. Each future addition to SRTP MUST
specify the amount and format of its padding, if any. While it
could seem more attractive to specify a fixed padding scheme for all
transforms, security and flexibility of transform specifications
REQUIRE that each transform specify a secure padding method.
The OPTIONAL MKI and RECOMMENDED authentication tag are the only None of the pre-defined encryption transforms uses any padding; for
fields defined by SRTP that are not in RTP. Only 8-bit alignment is these, the RTP and SRTP payload sizes match exactly. New transforms
added to SRTP (following Section 6) may require padding, and may
hence produce larger payloads. RTP provides its own padding format
(as seen in Fig. 1), which due to the padding indicator in the RTP
header has merits in terms of compactness relative to paddings using
prefix-free codes. This RTP padding SHALL be the default method for
transforms requiring padding. Transforms MAY specify other padding
methods, and MUST then specify the amount, format, and processing of
their padding. It is important to note that encryption transforms
that use padding are vulnerable to subtle attacks, especially when
message authentication is not used [V02]. Each specification for a
new encryption transform needs to carefully consider and describe
the security implications of the padding that it uses. Message
authentication codes define their own padding, so this default does
not apply to authentication transforms.
The OPTIONAL MKI and the RECOMMENDED authentication tag are the only
fields defined by SRTP that are not in RTP. Only 8-bit alignment is
assumed. assumed.
MKI (Master Key Identifier): configurable length, OPTIONAL MKI (Master Key Identifier): configurable length, OPTIONAL
The MKI is defined, signaled, and used by key management. The MKI is defined, signaled, and used by key management.
The MKI identifies the master key from which the session The MKI identifies the master key from which the session
key(s) were derived that authenticate and/or encrypt the key(s) were derived that authenticate and/or encrypt the
particular packet. Note that the MKI SHALL NOT identify the particular packet. Note that the MKI SHALL NOT identify the
SRTP cryptographic context, which is identified according to SRTP cryptographic context, which is identified according to
Section 3.2.3. The MKI MAY be used by key management for the Section 3.2.3. The MKI MAY be used by key management for the
purposes of re-keying, identifying a particular master key purposes of re-keying, identifying a particular master key
within the cryptographic context (Section 3.2.1). within the cryptographic context (Section 3.2.1).
Authentication tag: configurable length, RECOMMENDED Authentication tag: configurable length, RECOMMENDED
The authentication tag is used to carry message The authentication tag is used to carry message
authentication data. The Authenticated Portion of an SRTP authentication data. The Authenticated Portion of an SRTP
packet consists of the RTP header followed by the Encrypted packet consists of the RTP header followed by the Encrypted
Portion of the SRTP packet. Thus, if both encryption and Portion of the SRTP packet. Thus, if both encryption and
authentication are applied, encryption SHALL be applied authentication are applied, encryption SHALL be applied
before authentication on the sender side and conversely on before authentication on the sender side and conversely on
the receiver side. The authentication tag provides the receiver side. The authentication tag provides
authentication of the RTP header and payload, and it authentication of the RTP header and payload, and it
indirectly provides replay protection by authenticating the indirectly provides replay protection by authenticating the
sequence number. Note that the MKI is not integrity protected sequence number. Note that the MKI is not integrity
as this does not provide any extra protection. protected as this does not provide any extra protection.
3.2 SRTP Cryptographic Contexts 3.2 SRTP Cryptographic Contexts
Each SRTP stream requires the sender and receiver to maintain Each SRTP stream requires the sender and receiver to maintain
cryptographic state information. This information is called the cryptographic state information. This information is called the
"cryptographic context". "cryptographic context".
SRTP uses two types of keys: session keys and master keys. By a SRTP uses two types of keys: session keys and master keys. By a
"session key", we mean a key which is used directly in a "session key", we mean a key which is used directly in a
cryptographic transform (e.g. encryption or message authentication), cryptographic transform (e.g. encryption or message authentication),
and by a "master key", we mean a random bit string (given by the key and by a "master key", we mean a random bit string (given by the key
management protocol) from which session keys are derived in a management protocol) from which session keys are derived in a
cryptographically secure way. The master key(s) and other parameters cryptographically secure way. The master key(s) and other
in the cryptographic context are provided by key management parameters in the cryptographic context are provided by key
mechanisms external to SRTP, see Section 8. management mechanisms external to SRTP, see Section 8.
3.2.1 Transform-independent parameters 3.2.1 Transform-independent parameters
"Transform-independent parameters" are present in the cryptographic
Transform-independent parameters are present in the cryptographic
context independently of the particular encryption or authentication context independently of the particular encryption or authentication
transforms that are used. The transform-independent parameters of transforms that are used. The transform-independent parameters of
the cryptographic context for SRTP consist of: the cryptographic context for SRTP consist of:
* a 32-bit unsigned rollover counter (ROC), which records how many * a 32-bit unsigned rollover counter (ROC), which records how many
times the 16-bit RTP sequence number has been reset to zero after times the 16-bit RTP sequence number has been reset to zero after
passing through 65,535. Unlike the sequence number (SEQ), which passing through 65,535. Unlike the sequence number (SEQ), which
SRTP extracts from the RTP packet header, the ROC is maintained by SRTP extracts from the RTP packet header, the ROC is maintained by
SRTP as described in Section 3.3.1. SRTP as described in Section 3.3.1.
We define the index of the SRTP packet corresponding to a given We define the index of the SRTP packet corresponding to a given
ROC and RTP sequence number to be the 48-bit quantity ROC and RTP sequence number to be the 48-bit quantity
i = 2^16 * ROC + SEQ. i = 2^16 * ROC + SEQ.
* for the receiver only, a 16-bit sequence number s_l, which is the * for the receiver only, a 16-bit sequence number s_l, which can
highest received RTP sequence number, which SHOULD be be thought of as the highest received RTP sequence number (see
authenticated since message authentication is RECOMMENDED, Section 3.3.1 for its handling), which SHOULD be authenticated
since message authentication is RECOMMENDED,
* an identifier for the encryption algorithm, i.e., the cipher and * an identifier for the encryption algorithm, i.e., the cipher and
its mode of operation, its mode of operation,
* an identifier for the message authentication algorithm, * an identifier for the message authentication algorithm,
* a replay list, maintained by the receiver only (when * a replay list, maintained by the receiver only (when
authentication and replay protection are provided), containing authentication and replay protection are provided), containing
indices of recently received and authenticated SRTP packets, indices of recently received and authenticated SRTP packets,
* an MKI indicator (0/1) as to whether an MKI is present in SRTP and * an MKI indicator (0/1) as to whether an MKI is present in SRTP and
SRTCP packets, SRTCP packets,
* if the MKI indicator is set to one, the length (in octets) of the * if the MKI indicator is set to one, the length (in octets) of the
MKI field, and (for the sender) the actual value of the currently MKI field, and (for the sender) the actual value of the currently
active MKI (the value of the MKI indicator and length MUST be active MKI (the value of the MKI indicator and length MUST be
kept fixed for the lifetime of the context), kept fixed for the lifetime of the context),
* the master key(s), which MUST be random and kept secret, * the master key(s), which MUST be random and kept secret,
* for each master key, there is a counter of the number of SRTP * for each master key, there is a counter of the number of SRTP
packets that have been processed (sent) with that master key packets that have been processed (sent) with that master key
(essential for security, see Sections 3.3.1 and 9), (essential for security, see Sections 3.3.1 and 9),
* non-negative integers n_e, and n_a, determining the length of the * non-negative integers n_e, and n_a, determining the length of the
session keys for encryption, and message authentication. session keys for encryption, and message authentication.
In addition, for each master key, an SRTP stream MAY use the In addition, for each master key, an SRTP stream MAY use the
following associated values: following associated values:
* a master salt, to be used in the key derivation of session keys. * a master salt, to be used in the key derivation of session keys.
This value, when used, MUST be random, but MAY be public. Use of This value, when used, MUST be random, but MAY be public. Use of
master salt is strongly RECOMMENDED, see Section 9.2. A "NULL" master salt is strongly RECOMMENDED, see Section 9.2. A "NULL"
salt is treated as 00...0. salt is treated as 00...0.
* an integer in the set {1,2,4,...,2^24}, the "key_derivation_rate", * an integer in the set {1,2,4,...,2^24}, the "key_derivation_rate",
where an unspecified value is treated as zero. The constraint to where an unspecified value is treated as zero. The constraint to
be a power of 2 simplifies the session-key derivation be a power of 2 simplifies the session-key derivation
implementation, see Section 4.3. implementation, see Section 4.3.
* an MKI, * an MKI value,
* <"From", "To"> values, specifying the lifetime for a master key, * <"From", "To"> values, specifying the lifetime for a master key,
expressed in terms of the two 48-bit index values inside whose expressed in terms of the two 48-bit index values inside whose
range (including the range end-points) the master key is valid. For range (including the range end-points) the master key is valid.
the use of <From, To>, see Section 8.1.1. <"From", "To"> is an For the use of <From, To>, see Section 8.1.1. <"From", "To"> is an
alternative to the MKI and assumes that a master key is in one-to- alternative to the MKI and assumes that a master key is in one-to-
one correspondence with the SRTP session key on which the <"From", one correspondence with the SRTP session key on which the <"From",
"To"> range is defined. "To"> range is defined.
SRTCP SHALL by default share the crypto context with SRTP, except: SRTCP SHALL by default share the crypto context with SRTP, except:
* no rollover counter and s_l-value need to be maintained as the * no rollover counter and s_l-value need to be maintained as the
RTCP index is explicitly carried in each SRTCP packet, RTCP index is explicitly carried in each SRTCP packet,
* a separate replay list is maintained (when replay protection is * a separate replay list is maintained (when replay protection is
provided), provided),
* SRTCP maintains a separate counter for its master key (even if the * SRTCP maintains a separate counter for its master key (even if the
master key is the same as that for SRTP, see below), as a means to master key is the same as that for SRTP, see below), as a means to
maintain a count of the number of SRTCP packets that have been maintain a count of the number of SRTCP packets that have been
processed with that key. processed with that key.
Note in particular that the master key(s) MAY be shared between SRTP Note in particular that the master key(s) MAY be shared between SRTP
and the corresponding SRTCP, if the pre-defined transforms and the corresponding SRTCP, if the pre-defined transforms
(including the key derivation) are used but the session key(s) MUST (including the key derivation) are used but the session key(s) MUST
NOT be so shared. NOT be so shared.
In addition, there can be cases (see Sections 8 and 9.1) where In addition, there can be cases (see Sections 8 and 9.1) where
several SRTP streams within a given RTP session, identified by their several SRTP streams within a given RTP session, identified by their
SSRCs, share most of the crypto context parameters (including synchronization source (SSRCs, which is part of the RTP header),
possibly master and session keys). In such cases, just as in the share most of the crypto context parameters (including possibly
normal SRTP/SRTCP parameter sharing above, separate replay lists and master and session keys). In such cases, just as in the normal
packet counters for each stream (SSRC) MUST still be maintained. SRTP/SRTCP parameter sharing above, separate replay lists and packet
Also, separate SRTP indices MUST then be maintained. counters for each stream (SSRC) MUST still be maintained. Also,
separate SRTP indices MUST then be maintained.
A summary of parameters, pre-defined transforms, and default values A summary of parameters, pre-defined transforms, and default values
for the above parameters (and other SRTP parameters) can be found in for the above parameters (and other SRTP parameters) can be found in
Sections 5 and 8.2. Sections 5 and 8.2.
3.2.2 Transform-dependent parameters 3.2.2 Transform-dependent parameters
All encryption, authentication/integrity, and key derivation All encryption, authentication/integrity, and key derivation
parameters are defined in the transforms section (Section 4). parameters are defined in the transforms section (Section 4).
Typical examples of such parameters are block size of ciphers, Typical examples of such parameters are block size of ciphers,
session keys, data for IV formation, etc. Future SRTP transform session keys, data for the Initialization Vector (IV) formation,
specifications MUST include a section to list the additional etc. Future SRTP transform specifications MUST include a section to
cryptographic context's parameters for that transform, if any. list the additional cryptographic context's parameters for that
transform, if any.
3.2.3 Mapping SRTP Packets to Cryptographic Contexts 3.2.3 Mapping SRTP Packets to Cryptographic Contexts
Recall that an RTP session for each participant is defined [RTPNEW] Recall that an RTP session for each participant is defined [RTPNEW]
by a pair of destination transport addresses (one network address by a pair of destination transport addresses (one network address
plus a port pair for RTP and RTCP), and that a multimedia session is plus a port pair for RTP and RTCP), and that a multimedia session is
defined as a collection of RTP sessions. For example, a particular defined as a collection of RTP sessions. For example, a particular
multimedia session could include an audio RTP session, a video RTP multimedia session could include an audio RTP session, a video RTP
session, and a text RTP session. session, and a text RTP session.
A cryptographic context SHALL be uniquely identified by the triplet A cryptographic context SHALL be uniquely identified by the triplet
context identifier: context identifier:
context id = <SSRC, destination network address, destination context id = <SSRC, destination network address, destination
transport port number> transport port number>
where the destination network address and the destination transport where the destination network address and the destination transport
port are the ones in the SRTP packet. It is assumed that, when port are the ones in the SRTP packet. It is assumed that, when
presented with this information, the key management returns a presented with this information, the key management returns a
context with the information as described in Section 3.2. context with the information as described in Section 3.2.
As noted above, SRTP and SRTCP by default share the bulk of the As noted above, SRTP and SRTCP by default share the bulk of the
parameters in the cryptographic context. Thus, retrieving the crypto parameters in the cryptographic context. Thus, retrieving the
context parameters for an SRTCP stream in practice may imply a crypto context parameters for an SRTCP stream in practice may imply
binding to the correspondent SRTP crypto context. It is up to the a binding to the correspondent SRTP crypto context. It is up to the
implementation to assure such binding, since the RTCP port may not implementation to assure such binding, since the RTCP port may not
be directly deducible from the RTP port only. Alternatively, the key be directly deducible from the RTP port only. Alternatively, the
management may choose to provide separate SRTP- and SRTCP-contexts, key management may choose to provide separate SRTP- and SRTCP-
duplicating the common parameters (such as master key(s)). The contexts, duplicating the common parameters (such as master key(s)).
latter approach then also enables SRTP and SRTCP to use, e.g., The latter approach then also enables SRTP and SRTCP to use, e.g.,
distinct transforms, if so desired. Similar considerations arise distinct transforms, if so desired. Similar considerations arise
when multiple SRTP streams, forming part of one single RTP session, when multiple SRTP streams, forming part of one single RTP session,
share keys and other parameters. share keys and other parameters.
If no valid context can be found for a packet corresponding to a If no valid context can be found for a packet corresponding to a
certain context identifier, that packet MUST be discarded from certain context identifier, that packet MUST be discarded from
further SRTP processing. further processing.
3.3 SRTP Packet Processing 3.3 SRTP Packet Processing
The following applies to SRTP. SRTCP is described in Section 3.4. The following applies to SRTP. SRTCP is described in Section 3.4.
Assuming initialization of the cryptographic context(s) has taken Assuming initialization of the cryptographic context(s) has taken
place via key management, the sender SHALL do the following to place via key management, the sender SHALL do the following to
construct an SRTP packet: construct an SRTP packet:
1. Determine which cryptographic context to use as described in 1. Determine which cryptographic context to use as described in
Section 3.2.3. Section 3.2.3.
2. Determine the index of the SRTP packet using the rollover 2. Determine the index of the SRTP packet using the rollover
counter, the highest sequence number in the cryptographic context, counter, the highest sequence number in the cryptographic context,
and the sequence number in the RTP packet, as described in Section and the sequence number in the RTP packet, as described in Section
3.3.1. 3.3.1.
3. Determine the master key and master salt. This is done using the 3. Determine the master key and master salt. This is done using the
index determined in the previous step or the current MKI in the index determined in the previous step or the current MKI in the
cryptographic context, according to Section 8.1. cryptographic context, according to Section 8.1.
4. Determine the session keys and session salt (if they are used by 4. Determine the session keys and session salt (if they are used by
the transform) as described in Section 4.3, using master key, master the transform) as described in Section 4.3, using master key, master
salt, key_derivation_rate, and session key-lengths in the salt, key_derivation_rate, and session key-lengths in the
cryptographic context with the index, determined in Steps 2 and 3. cryptographic context with the index, determined in Steps 2 and 3.
5. Encrypt the RTP payload to produce the Encrypted Portion of the 5. Encrypt the RTP payload to produce the Encrypted Portion of the
packet (see Section 4.1, for the defined ciphers). This step uses packet (see Section 4.1, for the defined ciphers). This step uses
the encryption algorithm indicated in the cryptographic context, the the encryption algorithm indicated in the cryptographic context, the
session encryption key and the session salt (if used) found in Step session encryption key and the session salt (if used) found in Step
4 together with the index found in Step 2. 4 together with the index found in Step 2.
6. If the MKI indicator is set to one, append the MKI to the packet. 6. If the MKI indicator is set to one, append the MKI to the packet.
7. For message authentication, compute the authentication tag for 7. For message authentication, compute the authentication tag for
the Authenticated Portion of the packet, as described in Section the Authenticated Portion of the packet, as described in Section
4.2. This step uses the current rollover counter, the authentication 4.2. This step uses the current rollover counter, the
algorithm indicated in the cryptographic context, and the session authentication algorithm indicated in the cryptographic context, and
authentication key found in Step 4. Append the authentication tag to the session authentication key found in Step 4. Append the
the packet. authentication tag to the packet.
8. If necessary, update the ROC as in Section 3.3.1, using the 8. If necessary, update the ROC as in Section 3.3.1, using the
packet index determined in Step 2. packet index determined in Step 2.
To authenticate and decrypt an SRTP packet, the receiver SHALL do To authenticate and decrypt an SRTP packet, the receiver SHALL do
the following: the following:
1. Determine which cryptographic context to use as described in 1. Determine which cryptographic context to use as described in
Section 3.2.3. Section 3.2.3.
2. Run the algorithm in Section 3.3.1 to get the index of the SRTP 2. Run the algorithm in Section 3.3.1 to get the index of the SRTP
packet. The algorithm uses the rollover counter and highest packet. The algorithm uses the rollover counter and highest
sequence number in the cryptographic context with the sequence sequence number in the cryptographic context with the sequence
number in the SRTP packet, as described in Section 3.3.1. number in the SRTP packet, as described in Section 3.3.1.
3. Determine the master key and master salt. If the MKI indicator in 3. Determine the master key and master salt. If the MKI indicator
the context is set to one, use the MKI in the SRTP packet, otherwise in the context is set to one, use the MKI in the SRTP packet,
use the index from the previous step, according to Section 8.1. otherwise use the index from the previous step, according to Section
8.1.
4. Determine the session keys, and session salt (if used by the 4. Determine the session keys, and session salt (if used by the
transform) as described in Section 4.3, using master key, master transform) as described in Section 4.3, using master key, master
salt, key_derivation_rate and session key-lengths in the salt, key_derivation_rate and session key-lengths in the
cryptographic context with the index, determined in Steps 2 and 3. cryptographic context with the index, determined in Steps 2 and 3.
5. For message authentication and replay protection, first check if 5. For message authentication and replay protection, first check if
the packet has been replayed (Section 3.3.2), using the Replay List the packet has been replayed (Section 3.3.2), using the Replay List
and the index as determined in Step 2. If the packet is judged to be and the index as determined in Step 2. If the packet is judged to
replayed, then the packet MUST be discarded, and the event SHOULD be be replayed, then the packet MUST be discarded, and the event SHOULD
logged. be logged.
Next, perform verification of the authentication tag, using the Next, perform verification of the authentication tag, using the
rollover counter from Step 2, the authentication algorithm indicated rollover counter from Step 2, the authentication algorithm indicated
in the cryptographic context, and the session authentication key in the cryptographic context, and the session authentication key
from Step 4. If the result is "AUTHENTICATION FAILURE" (see Section from Step 4. If the result is "AUTHENTICATION FAILURE" (see Section
4.2), the packet MUST be discarded from further processing and the 4.2), the packet MUST be discarded from further processing and the
event SHOULD be logged. event SHOULD be logged.
6. Decrypt the Encrypted Portion of the packet (see Section 4.1, for 6. Decrypt the Encrypted Portion of the packet (see Section 4.1, for
the defined ciphers), using the decryption algorithm indicated in the defined ciphers), using the decryption algorithm indicated in
the cryptographic context, the session encryption key and salt (if the cryptographic context, the session encryption key and salt (if
used) found in Step 4 with the index from Step 2. used) found in Step 4 with the index from Step 2.
7. Update the rollover counter and highest sequence number, s_l, in 7. Update the rollover counter and highest sequence number, s_l, in
the cryptographic context as in Section 3.3.1, using the packet the cryptographic context as in Section 3.3.1, using the packet
index estimated in Step 2. If replay protection is provided, also index estimated in Step 2. If replay protection is provided, also
update the Replay List as described in Section 3.3.2. update the Replay List as described in Section 3.3.2.
8. When present, remove the MKI and authentication tag fields from 8. When present, remove the MKI and authentication tag fields from
the packet. the packet.
3.3.1 Packet Index Determination, and ROC, s_l Update 3.3.1 Packet Index Determination, and ROC, s_l Update
SRTP implementations use an "implicit" packet index for sequencing, SRTP implementations use an "implicit" packet index for sequencing,
i.e., not all of the index is explicitly carried in the SRTP packet. i.e., not all of the index is explicitly carried in the SRTP packet.
For the pre-defined transforms, the index i is used in replay For the pre-defined transforms, the index i is used in replay
protection (Section 3.3.2), encryption (Section 4.1), message protection (Section 3.3.2), encryption (Section 4.1), message
authentication (Section 4.2), and for the key derivation (Section authentication (Section 4.2), and for the key derivation (Section
4.3). 4.3).
When the session starts, the sender side MUST set the rollover When the session starts, the sender side MUST set the rollover
counter, ROC, to zero. Each time the RTP sequence number, SEQ, wraps counter, ROC, to zero. Each time the RTP sequence number, SEQ,
modulo 2^16, the sender side MUST increment ROC by one, modulo 2^32 wraps modulo 2^16, the sender side MUST increment ROC by one, modulo
(see security aspects below). The sender's packet index is then 2^32 (see security aspects below). The sender's packet index is
defined as then defined as
i = 2^16 * ROC + SEQ. i = 2^16 * ROC + SEQ.
Receiver-side implementations use the RTP sequence number to Receiver-side implementations use the RTP sequence number to
determine the correct index of a packet, which is the location of determine the correct index of a packet, which is the location of
the packet in the sequence of all SRTP packets. A robust approach the packet in the sequence of all SRTP packets. A robust approach
for the proper use of a rollover counter requires its handling and for the proper use of a rollover counter requires its handling and
use to be well defined. In particular, out-of-order RTP packets with use to be well defined. In particular, out-of-order RTP packets
sequence numbers close to 2^16 or zero must be properly handled. with sequence numbers close to 2^16 or zero must be properly
handled.
The index estimate is based on the receiver's locally maintained ROC The index estimate is based on the receiver's locally maintained ROC
and s_l values. At the setup of the session, the ROC MUST be set to and s_l values. At the setup of the session, the ROC MUST be set to
zero. Receivers joining an on-going session MUST be given the current zero. Receivers joining an on-going session MUST be given the
ROC value using out-of-band signaling such as key-management current ROC value using out-of-band signaling such as key-management
signaling. Furthermore, the receiver SHALL initialize s_l to the RTP signaling. Furthermore, the receiver SHALL initialize s_l to the RTP
sequence number (SEQ) of the first observed SRTP packet (unless the sequence number (SEQ) of the first observed SRTP packet (unless the
initial value is provided by out of band signaling such as key initial value is provided by out of band signaling such as key
management). management).
On consecutive SRTP packets, the receiver SHOULD estimate the index On consecutive SRTP packets, the receiver SHOULD estimate the index
as as
i = 2^16 * v + SEQ, i = 2^16 * v + SEQ,
where v is chosen from the set { ROC-1, ROC, ROC+1 } (modulo 2^32) where v is chosen from the set { ROC-1, ROC, ROC+1 } (modulo 2^32)
such that i is closest (in modulo 2^48 sense) to the value 2^16 * ROC such that i is closest (in modulo 2^48 sense) to the value 2^16 * ROC
+ s_l (see Appendix A for pseudocode). + s_l (see Appendix A for pseudocode).
After the packet has been processed and authenticated (when enabled After the packet has been processed and authenticated (when enabled
for SRTP packets for the session), the receiver MUST use v to for SRTP for SRTP packets for the session), the receiver MUST use v
conditionally update its s_l and ROC variables as follows. If to conditionally update its s_l and ROC variables as follows. If
v=(ROC-1) mod 2^32, then there is no update to s_l or ROC. If v=ROC, v=(ROC-1) mod 2^32, then there is no update to s_l or ROC. If v=ROC,
then s_l is set to SEQ if and only if SEQ is larger; there is no then s_l is set to SEQ if and only if SEQ is larger than the current
change to ROC. If v=(ROC+1) mod 2^32, then s_l is set to SEQ and ROC s_l; there is no change to ROC. If v=(ROC+1) mod 2^32, then s_l is
is set to v. set to SEQ and ROC is set to v.
After a re-keying occurs (changing to a new master key), the After a re-keying occurs (changing to a new master key), the
rollover counter always maintains its sequence of values, i.e., it rollover counter always maintains its sequence of values, i.e., it
MUST NOT be reset to zero. MUST NOT be reset to zero.
As the rollover counter is 32 bits long and the sequence number is As the rollover counter is 32 bits long and the sequence number is
16 bits long, the maximum number of packets belonging to a given 16 bits long, the maximum number of packets belonging to a given
SRTP stream that can be secured with the same key is 2^48 using the SRTP stream that can be secured with the same key is 2^48 using the
pre-defined transforms. After that number of SRTP packets have been pre-defined transforms. After that number of SRTP packets have been
sent with a given (master or session) key, the sender MUST NOT send sent with a given (master or session) key, the sender MUST NOT send
any more packets with that key. (There exists a similar limit for any more packets with that key. (There exists a similar limit for
SRTCP, which in practice may be more restrictive, see Section 9.2.) SRTCP, which in practice may be more restrictive, see Section 9.2.)
This limitation enforces a security benefit by providing an upper This limitation enforces a security benefit by providing an upper
bound on the amount of traffic that can pass before cryptographic bound on the amount of traffic that can pass before cryptographic
keys are changed. Re-keying (see Section 8.1) MUST be triggered, keys are changed. Re-keying (see Section 8.1) MUST be triggered,
before this amount of traffic, and MAY be triggered earlier, e.g., before this amount of traffic, and MAY be triggered earlier, e.g.,
for increased security and access control to media. Recurring key for increased security and access control to media. Recurring key
derivation by means of a non-zero key_derivation_rate (see Section derivation by means of a non-zero key_derivation_rate (see Section
4.3), also gives stronger security but does not change the above 4.3), also gives stronger security but does not change the above
absolute maximum value. absolute maximum value.
On the receiver side, there is a caveat to updating s_l and ROC: if On the receiver side, there is a caveat to updating s_l and ROC: if
message authentication is not present, neither the initialization of message authentication is not present, neither the initialization of
s_l, nor the ROC update can be made completely robust. The s_l, nor the ROC update can be made completely robust. The
receiver's "implicit index" approach works for the pre-defined receiver's "implicit index" approach works for the pre-defined
transforms as long as the reorder and loss of the packets are not transforms as long as the reorder and loss of the packets are not
too great and bit-errors do not occur in unfortunate ways. In too great and bit-errors do not occur in unfortunate ways. In
particular, 2^15 packets would need to be lost, or a packet would particular, 2^15 packets would need to be lost, or a packet would
need to be 2^15 packets out of sequence before synchronization is need to be 2^15 packets out of sequence before synchronization is
lost. Such drastic loss or reorder is likely to disrupt the RTP lost. Such drastic loss or reorder is likely to disrupt the RTP
application itself. application itself.
The algorithm for the index estimate and ROC update is a matter of The algorithm for the index estimate and ROC update is a matter of
implementation, and should take into consideration the environment implementation, and should take into consideration the environment
(e.g., packet loss rate) and the cases when synchronization is (e.g., packet loss rate) and the cases when synchronization is
likely to be lost, e.g. when the initial sequence number (randomly likely to be lost, e.g. when the initial sequence number (randomly
chosen by RTP) is not known in advance (not sent in the key chosen by RTP) is not known in advance (not sent in the key
management protocol) but may be near to wrap modulo 2^16. management protocol) but may be near to wrap modulo 2^16.
A more elaborate and more robust scheme than the one given above is A more elaborate and more robust scheme than the one given above is
the handling of RTP's own "rollover counter", see Appendix A.1 of the handling of RTP's own "rollover counter", see Appendix A.1 of
[RTPNEW]. [RTPNEW].
3.3.2 Replay Protection 3.3.2 Replay Protection
Secure replay protection is only possible when integrity protection Secure replay protection is only possible when integrity protection
is present. It is RECOMMENDED to use replay protection, both for RTP is present. It is RECOMMENDED to use replay protection, both for
and RTCP, as integrity protection alone cannot assure security RTP and RTCP, as integrity protection alone cannot assure security
against replay attacks. against replay attacks.
A packet is "replayed" when it is stored by an adversary, and then A packet is "replayed" when it is stored by an adversary, and then
re-injected into the network. When message authentication is re-injected into the network. When message authentication is
provided, SRTP protects against such attacks through a "Replay provided, SRTP protects against such attacks through a Replay List.
List". Each SRTP receiver maintains a Replay List, which Each SRTP receiver maintains a Replay List, which conceptually
conceptually contains the indices of all of the packets which have contains the indices of all of the packets which have been received
been received and authenticated. In practice, the list can use a and authenticated. In practice, the list can use a "sliding window"
"sliding window" approach, so that a fixed amount of storage approach, so that a fixed amount of storage suffices for replay
suffices for replay protection. Packet indices which lag behind the protection. Packet indices which lag behind the packet index in the
packet index in the context by more than SRTP-WINDOW-SIZE can be context by more than SRTP-WINDOW-SIZE can be assumed to have been
assumed to have been received, where SRTP-WINDOW-SIZE is a receiver- received, where SRTP-WINDOW-SIZE is a receiver-side, implementation-
side, implementation-dependent parameter and MUST be at least 64, dependent parameter and MUST be at least 64, but which MAY be set to
but which MAY be set to a higher value. a higher value.
The receiver checks the index of an incoming packet against the The receiver checks the index of an incoming packet against the
replay list and the window. Only packets with index ahead of the replay list and the window. Only packets with index ahead of the
window, or, inside the window but not already received, SHALL be window, or, inside the window but not already received, SHALL be
accepted. accepted.
After the packet has been authenticated (if necessary the window is After the packet has been authenticated (if necessary the window is
first moved ahead), the replay list SHALL be updated with the new first moved ahead), the replay list SHALL be updated with the new
index. index.
The Replay List can be efficiently implemented by using a bitmap to The Replay List can be efficiently implemented by using a bitmap to
represent which packets have been received, as described in the represent which packets have been received, as described in the
Security Architecture for IP [RFC2401]. Security Architecture for IP [RFC2401].
3.4 Secure RTCP 3.4 Secure RTCP
Secure RTCP follows the definition of Secure RTP. SRTCP adds three Secure RTCP follows the definition of Secure RTP. SRTCP adds three
mandatory new fields (the SRTCP index, an "encrypt-flag", and the mandatory new fields (the SRTCP index, an "encrypt-flag", and the
authentication tag) and one optional field (the MKI) to the RTCP authentication tag) and one optional field (the MKI) to the RTCP
packet definition. The three mandatory fields MUST be appended to an packet definition. The three mandatory fields MUST be appended to
RTCP packet in order to form an equivalent SRTCP packet. The added an RTCP packet in order to form an equivalent SRTCP packet. The
fields follow any other profile-specific extensions. added fields follow any other profile-specific extensions.
According to Section 6.1 of [RTPNEW], there is a REQUIRED packet According to Section 6.1 of [RTPNEW], there is a REQUIRED packet
format for compound packets. SRTCP MUST be given packets according format for compound packets. SRTCP MUST be given packets according
to that requirement in the sense that the first part MUST be a to that requirement in the sense that the first part MUST be a
sender report or a receiver report. However, the RTCP encryption sender report or a receiver report. However, the RTCP encryption
prefix (a random 32-bit quantity) specified in that Section MUST NOT prefix (a random 32-bit quantity) specified in that Section MUST NOT
be used since, as is stated there, it is only applicable to the be used since, as is stated there, it is only applicable to the
encryption method specified in [RTPNEW] and is not needed by the encryption method specified in [RTPNEW] and is not needed by the
cryptographic mechanisms used in SRTP. cryptographic mechanisms used in SRTP.
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+
|V=2|P| RC | PT=SR or RR | length | | |V=2|P| RC | PT=SR or RR | length | |
skipping to change at page 16, line 44 skipping to change at page 17, line 44
| | | |
+-- Encrypted Portion Authenticated Portion -----+ +-- Encrypted Portion Authenticated Portion -----+
Figure 2. An example of the format of a Secure RTCP packet, Figure 2. An example of the format of a Secure RTCP packet,
consisting of an underlying RTCP compound packet with a Sender consisting of an underlying RTCP compound packet with a Sender
Report and SDES packet. Report and SDES packet.
The Encrypted Portion of an SRTCP packet consists of the encryption The Encrypted Portion of an SRTCP packet consists of the encryption
(Section 4.1) of the RTCP payload of the equivalent compound RTCP (Section 4.1) of the RTCP payload of the equivalent compound RTCP
packet, from the first RTCP packet, i.e., from the ninth (9) octet packet, from the first RTCP packet, i.e., from the ninth (9) octet
to the end of the compound packet. The Authenticated Portion of an to the end of the compound packet. The Authenticated Portion of an
SRTCP packet consists of the entire equivalent (eventually compound) SRTCP packet consists of the entire equivalent (eventually compound)
RTCP packet, the E flag, and the SRTCP index (after any encryption RTCP packet, the E flag, and the SRTCP index (after any encryption
has been applied to the payload). has been applied to the payload).
The added fields are: The added fields are:
E-flag: 1 bit, REQUIRED E-flag: 1 bit, REQUIRED
The E-flag indicates if the current SRTCP packet is encrypted The E-flag indicates if the current SRTCP packet is encrypted
or unencrypted. Section 9.1 of [RTPNEW] allows the split of a or unencrypted. Section 9.1 of [RTPNEW] allows the split of
compound RTCP packet into two lower-layer packets, one to be a compound RTCP packet into two lower-layer packets, one to
encrypted and one to be sent in the clear. The E bit set to be encrypted and one to be sent in the clear. The E bit set
"1" indicates encrypted packet, and "0" indicates non- to "1" indicates encrypted packet, and "0" indicates non-
encrypted packet. encrypted packet.
SRTCP index: 31 bits, REQUIRED SRTCP index: 31 bits, REQUIRED
The SRTCP index is a 31-bit counter for the SRTCP packet. The The SRTCP index is a 31-bit counter for the SRTCP packet.
index is explicitly included in each packet, in contrast to The index is explicitly included in each packet, in contrast
the "implicit" index approach used for SRTP. The SRTCP index to the "implicit" index approach used for SRTP. The SRTCP
MUST be set to zero before the first SRTCP packet is sent, index MUST be set to zero before the first SRTCP packet is
and MUST be incremented by one, modulo 2^31, after each SRTCP sent, and MUST be incremented by one, modulo 2^31, after each
packet is sent. In particular, after a re-key, the SRTCP SRTCP packet is sent. In particular, after a re-key, the
index MUST NOT be reset to zero again (Section 3.3.1). SRTCP index MUST NOT be reset to zero again (Section 3.3.1).
Authentication Tag: configurable length, REQUIRED Authentication Tag: configurable length, REQUIRED
The authentication tag is used to carry message The authentication tag is used to carry message
authentication data. authentication data.
MKI: configurable length, OPTIONAL MKI: configurable length, OPTIONAL
The MKI is the Master Key Indicator, and functions according The MKI is the Master Key Indicator, and functions according
to the MKI definition in Section 3. to the MKI definition in Section 3.
SRTCP uses the cryptographic context parameters and packet SRTCP uses the cryptographic context parameters and packet
processing of SRTP by default, with the following changes: processing of SRTP by default, with the following changes:
* The receiver does not need to "estimate" the index, as it is * The receiver does not need to "estimate" the index, as it is
explicitly signaled in the packet. explicitly signaled in the packet.
* Pre-defined SRTCP encryption is as specified in Section 4.1, but * Pre-defined SRTCP encryption is as specified in Section 4.1, but
using the definition of the SRTCP Encrypted Portion given in this using the definition of the SRTCP Encrypted Portion given in this
section, and using the SRTCP index as the index i. The encryption section, and using the SRTCP index as the index i. The encryption
transform and related parameters SHALL by default be the same transform and related parameters SHALL by default be the same
selected for the protection of the associated SRTP stream(s), while selected for the protection of the associated SRTP stream(s), while
the NULL algorithm SHALL be applied to the RTCP packets not to be the NULL algorithm SHALL be applied to the RTCP packets not to be
encrypted. SRTCP may have a different encryption transform than the encrypted. SRTCP may have a different encryption transform than the
one used by the corresponding SRTP. The expected use for this one used by the corresponding SRTP. The expected use for this
feature is when the former has NULL-encryption and the latter has a feature is when the former has NULL-encryption and the latter has a
non NULL-encryption. non NULL-encryption.
The E-flag is assigned a value by the sender depending on whether The E-flag is assigned a value by the sender depending on whether
the packet was encrypted or not. the packet was encrypted or not.
* SRTCP decryption is performed as in Section 4, but only if the E * SRTCP decryption is performed as in Section 4, but only if the E
flag is equal to 1. If so, the Encrypted Portion is decrypted, using flag is equal to 1. If so, the Encrypted Portion is decrypted,
the SRTCP index as the index i. In case the E-flag is 0, the payload using the SRTCP index as the index i. In case the E-flag is 0, the
is simply left unmodified. payload is simply left unmodified.
* SRTCP replay protection is as defined in Section 3.3.2, but using * SRTCP replay protection is as defined in Section 3.3.2, but using
the SRTCP index as the index i and a separate replay list that is the SRTCP index as the index i and a separate Replay List that is
specific to SRTCP. specific to SRTCP.
* The pre-defined SRTCP authentication tag is specified as in * The pre-defined SRTCP authentication tag is specified as in
Section 4.2, but with the Authenticated Portion of the SRTCP packet Section 4.2, but with the Authenticated Portion of the SRTCP packet
given in this section (which includes the index). The authentication given in this section (which includes the index). The
transform and related parameters (e.g., key size) SHALL by default authentication transform and related parameters (e.g., key size)
be the same as selected for the protection of the associated SRTP SHALL by default be the same as selected for the protection of the
stream(s)). associated SRTP stream(s)).
* In the last step of the processing, only the sender needs to * In the last step of the processing, only the sender needs to
update the value of the SRTCP index by incrementing it modulo 2^31 update the value of the SRTCP index by incrementing it modulo 2^31
and for security reasons the sender MUST also check the number of and for security reasons the sender MUST also check the number of
SRTCP packets processed, see Section 9.2. SRTCP packets processed, see Section 9.2.
Message authentication for RTCP is REQUIRED, as it is the control Message authentication for RTCP is REQUIRED, as it is the control
protocol (e.g., it has a BYE packet) for RTP. protocol (e.g., it has a BYE packet) for RTP.
Precautions must be taken so that the packet expansion in SRTCP (due Precautions must be taken so that the packet expansion in SRTCP (due
to the added fields) does not cause SRTCP messages to use more than to the added fields) does not cause SRTCP messages to use more than
their share of RTCP bandwidth. To avoid this, the following two their share of RTCP bandwidth. To avoid this, the following two
measures MUST be taken: measures MUST be taken:
1. When initializing the RTCP variable "avg_rtcp_size" defined in 1. When initializing the RTCP variable "avg_rtcp_size" defined in
chapter 6.3 of [RTPNEW], it MUST include the size of the fields that chapter 6.3 of [RTPNEW], it MUST include the size of the fields that
will be added by SRTCP (index, E-bit, authentication tag, and when will be added by SRTCP (index, E-bit, authentication tag, and when
present, the MKI). present, the MKI).
2. When updating the "avg_rtcp_size" using the variable packet_size" 2. When updating the "avg_rtcp_size" using the variable packet_size"
(section 6.3.3 of [RTPNEW]), the value of "packet_size" MUST include (section 6.3.3 of [RTPNEW]), the value of "packet_size" MUST include
the size of the additional fields added by SRTCP. the size of the additional fields added by SRTCP.
With these measures in place the SRTCP messages will not use more With these measures in place the SRTCP messages will not use more
than the allotted bandwidth. The effect of the size of the added than the allotted bandwidth. The effect of the size of the added
fields on the SRTCP traffic will be that messages will be sent with fields on the SRTCP traffic will be that messages will be sent with
longer packet intervals. The increase in the intervals will be longer packet intervals. The increase in the intervals will be
directly proportional to size of the added fields. For the pre- directly proportional to size of the added fields. For the pre-
defined transforms, the size of the added fields will be at least 14 defined transforms, the size of the added fields will be at least 14
octets, and upper bounded depending on MKI and the authentication octets, and upper bounded depending on MKI and the authentication
tag sizes. tag sizes.
4. Pre-Defined Cryptographic Transforms 4. Pre-Defined Cryptographic Transforms
While there are numerous encryption and message authentication While there are numerous encryption and message authentication
algorithms that can be used in SRTP, we define below default algorithms that can be used in SRTP, we define below default
algorithms in order to avoid the complexity of specifying the algorithms in order to avoid the complexity of specifying the
encodings for the signaling of algorithm and parameter identifiers. encodings for the signaling of algorithm and parameter identifiers.
The defined algorithms have been chosen as they fulfill the goals The defined algorithms have been chosen as they fulfill the goals
listed in Section 2. Recommendations on how to extend SRTP with new listed in Section 2. Recommendations on how to extend SRTP with new
transforms are given in Section 6. transforms are given in Section 6.
4.1 Encryption 4.1 Encryption
The following parameters are common to both pre-defined, non-NULL, The following parameters are common to both pre-defined, non-NULL,
encryption transforms specified in this section. encryption transforms specified in this section.
* BLOCK_CIPHER-MODE indicates the block cipher used and its mode of * BLOCK_CIPHER-MODE indicates the block cipher used and its mode of
operation operation
* n_b is the bit-size of the block for the block cipher * n_b is the bit-size of the block for the block cipher
* k_e is the session encryption key * k_e is the session encryption key
* n_e is the bit-length of k_e * n_e is the bit-length of k_e
* k_s is the session salting key * k_s is the session salting key
* n_s is the bit-length of k_s * n_s is the bit-length of k_s
* SRTP_PREFIX_LENGTH is the octet length of the keystream prefix, an * SRTP_PREFIX_LENGTH is the octet length of the keystream prefix, an
non-negative integer, specified by the message authentication code non-negative integer, specified by the message authentication code
in use. in use.
The distinct session keys and salts for SRTP/SRTCP are by default The distinct session keys and salts for SRTP/SRTCP are by default
derived as specified in Section 4.3. derived as specified in Section 4.3.
The encryption transforms defined in SRTP map the SRTP packet index The encryption transforms defined in SRTP map the SRTP packet index
and secret key into a pseudorandom keystream segment. Each keystream and secret key into a pseudo-random keystream segment. Each
segment encrypts a single RTP packet. The process of encrypting a keystream segment encrypts a single RTP packet. The process of
packet consists of generating the keystream segment corresponding to encrypting a packet consists of generating the keystream segment
the packet, and then bitwise exclusive-oring that keystream segment corresponding to the packet, and then bitwise exclusive-oring that
onto the payload of the RTP packet to produce the Encrypted Portion keystream segment onto the payload of the RTP packet to produce the
of the SRTP packet. Decryption is done the same way, but swapping Encrypted Portion of the SRTP packet. In case the payload size is
the roles of the plaintext and ciphertext. not an integer multiple of n_b bits, the excess (least significant)
bits of the keystream are simply discarded. Decryption is done the
same way, but swapping the roles of the plaintext and ciphertext.
+----+ +------------------+---------------------------------+ +----+ +------------------+---------------------------------+
| KG |-->| Keystream Prefix | Keystream Suffix |---+ | KG |-->| Keystream Prefix | Keystream Suffix |---+
+----+ +------------------+---------------------------------+ | +----+ +------------------+---------------------------------+ |
| |
+---------------------------------+ v +---------------------------------+ v
| Payload of RTP Packet |->(*) | Payload of RTP Packet |->(*)
+---------------------------------+ | +---------------------------------+ |
| |
+---------------------------------+ | +---------------------------------+ |
| Encrypted Portion of SRTP Packet|<--+ | Encrypted Portion of SRTP Packet|<--+
+---------------------------------+ +---------------------------------+
Figure 3: Default SRTP Encryption Processing. Here KG denotes the Figure 3: Default SRTP Encryption Processing. Here KG denotes the
keystream generator, and (*) denotes bitwise exclusive-or. keystream generator, and (*) denotes bitwise exclusive-or.
The definition of how the keystream is generated, given the index, The definition of how the keystream is generated, given the index,
depends on the cipher and its mode of operation. Below, two such depends on the cipher and its mode of operation. Below, two such
keystream generators are defined. The NULL cipher is also defined, keystream generators are defined. The NULL cipher is also defined,
to be used when encryption of RTP is not required. to be used when encryption of RTP is not required.
The SRTP definition of the keystream is illustrated in Figure 3. The The SRTP definition of the keystream is illustrated in Figure 3.
initial octets of each keystream segment MAY be reserved for use in The initial octets of each keystream segment MAY be reserved for use
a message authentication code, in which case the keystream used for in a message authentication code, in which case the keystream used
encryption starts immediately after the last reserved octet. The for encryption starts immediately after the last reserved octet.
initial reserved octets are called the "keystream prefix" (not to be The initial reserved octets are called the "keystream prefix" (not
confused with the "encryption prefix" of [RTPNEW, Section 6.1]), and to be confused with the "encryption prefix" of [RTPNEW, Section
the remaining octets are called the "keystream suffix". The 6.1]), and the remaining octets are called the "keystream suffix".
keystream prefix MUST NOT be used for encryption. The process is The keystream prefix MUST NOT be used for encryption. The process
illustrated in Figure 3. is illustrated in Figure 3.
The number of octets in the keystream prefix is denoted as The number of octets in the keystream prefix is denoted as
SRTP_PREFIX_LENGTH. The keystream prefix is indicated by a positive, SRTP_PREFIX_LENGTH. The keystream prefix is indicated by a
non-zero value of SRTP_PREFIX_LENGTH. This means that, even if positive, non-zero value of SRTP_PREFIX_LENGTH. This means that,
confidentiality is not to be provided, the keystream generator even if confidentiality is not to be provided, the keystream
output may still need to be computed for packet authentication, in generator output may still need to be computed for packet
which case the default keystream generator (mode) SHALL be used. authentication, in which case the default keystream generator (mode)
SHALL be used.
The default cipher is the Advanced Encryption Standard (AES), and we The default cipher is the Advanced Encryption Standard (AES), and we
define two modes of running AES, (1) Segmented Integer Counter Mode define two modes of running AES, (1) Segmented Integer Counter Mode
AES and (2) AES in f8-mode. In the remainder of this section, let AES and (2) AES in f8-mode. In the remainder of this section, let
E(k,x) be AES applied to key k and input block x. E(k,x) be AES applied to key k and input block x.
4.1.1 AES in Counter Mode 4.1.1 AES in Counter Mode
Conceptually, counter mode [AES-CTR] consists of encrypting Conceptually, counter mode [AES-CTR] consists of encrypting
successive integers. The actual definition is somewhat more successive integers. The actual definition is somewhat more
complicated, in order to randomize the starting point of the integer complicated, in order to randomize the starting point of the integer
sequence. Each packet is encrypted with a distinct keystream sequence. Each packet is encrypted with a distinct keystream
segment, which SHALL be computed as follows. segment, which SHALL be computed as follows.
A keystream segment SHALL be the concatenation of the 128-bit output A keystream segment SHALL be the concatenation of the 128-bit output
blocks of the AES cipher in the encrypt direction, using key k = blocks of the AES cipher in the encrypt direction, using key k =
k_e, in which the block indices are in increasing order. k_e, in which the block indices are in increasing order.
Symbolically, each keystream segment looks like Symbolically, each keystream segment looks like
E(k, IV) || E(k, IV + 1 mod 2^128) || E(k, IV + 2 mod 2^128) ... E(k, IV) || E(k, IV + 1 mod 2^128) || E(k, IV + 2 mod 2^128) ...
where the 128-bit integer value IV SHALL be defined by the SSRC, the where the 128-bit integer value IV SHALL be defined by the SSRC, the
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The inclusion of the SSRC allows the use of the same key to protect The inclusion of the SSRC allows the use of the same key to protect
distinct SRTP streams within the same RTP session, see the security distinct SRTP streams within the same RTP session, see the security
caveats in Section 9.1. caveats in Section 9.1.
In the case of SRTCP, the SSRC of the first header of the compound In the case of SRTCP, the SSRC of the first header of the compound
packet MUST be used, i SHALL be the 31-bit SRTCP index and k_e, k_s packet MUST be used, i SHALL be the 31-bit SRTCP index and k_e, k_s
SHALL be replaced by the SRTCP session key and salt. SHALL be replaced by the SRTCP session key and salt.
Note that the initial value, IV, is fixed for each packet and is Note that the initial value, IV, is fixed for each packet and is
formed by "reserving" 16 zeros in the least significant bits for the formed by "reserving" 16 zeros in the least significant bits for the
purpose of the counter. The number of blocks of keystream generated purpose of the counter. The number of blocks of keystream generated
for any fixed value of IV MUST NOT exceed 2^16 to avoid key stream for any fixed value of IV MUST NOT exceed 2^16 to avoid key stream
re-use, see below. The AES has a block size of 128 bits, so 2^16 re-use, see below. The AES has a block size of 128 bits, so 2^16
output blocks are sufficient to generate the 2^23 bits of keystream output blocks are sufficient to generate the 2^23 bits of keystream
needed to encrypt the largest possible RTP packet (except for IPv6 needed to encrypt the largest possible RTP packet (except for IPv6
"jumbograms" [RFC2675], which are not likely to be used for RTP- "jumbograms" [RFC2675], which are not likely to be used for RTP-
based multimedia traffic). This restriction on the maximum bit-size based multimedia traffic). This restriction on the maximum bit-size
of the packet that can be encrypted ensures the security of the of the packet that can be encrypted ensures the security of the
encryption method by limiting the effectiveness of probabilistic encryption method by limiting the effectiveness of probabilistic
attacks [BDJR]. attacks [BDJR].
For a fixed Counter Mode key, each IV value used as an input MUST be For a particular Counter Mode key, each IV value used as an input
distinct, in order to avoid the security exposure of a two-time pad MUST be distinct, in order to avoid the security exposure of a two-
situation (Section 9.1). To satisfy this constraint, an time pad situation (Section 9.1). To satisfy this constraint, an
implementation MUST ensure that the values of the SRTP packet index implementation MUST ensure that the values of the SRTP packet index
of ROC || SEQ, and the SSRC used in the construction of the IV are of ROC || SEQ, and the SSRC used in the construction of the IV are
distinct for any fixed key. The failure to ensure this uniqueness distinct for any particular key. The failure to ensure this
could be catastrophic for Secure RTP. This is in contrast to the uniqueness could be catastrophic for Secure RTP. This is in
situation for RTP itself, which may be able to tolerate such contrast to the situation for RTP itself, which may be able to
failures. It is RECOMMENDED that, if a dedicated security module is tolerate such failures. It is RECOMMENDED that, if a dedicated
present, the RTP sequence numbers and SSRC either be generated or security module is present, the RTP sequence numbers and SSRC either
checked by that module (i.e., sequence-number and SSRC processing in be generated or checked by that module (i.e., sequence-number and
an SRTP system needs to be protected as well as the key). SSRC processing in an SRTP system needs to be protected as well as
the key).
4.1.2 AES in f8-mode 4.1.2 AES in f8-mode
To encrypt UMTS (Universal Mobile Telecommunications System, as 3G To encrypt UMTS (Universal Mobile Telecommunications System, as 3G
networks) data, a solution (see [f8-a], [f8-b]) known as the f8- networks) data, a solution (see [f8-a], [f8-b]) known as the f8-
algorithm has been developed. On a high level, the proposed scheme algorithm has been developed. On a high level, the proposed scheme
is a variant of Output Feedback Mode (OFB) [HAC], with a more is a variant of Output Feedback Mode (OFB) [HAC], with a more
elaborate initialization and feedback function. As in normal OFB, elaborate initialization and feedback function. As in normal OFB,
the core consists of a block cipher. We also define here the use of the core consists of a block cipher. We also define here the use of
AES as a block cipher to be used in what we shall call "f8-mode of AES as a block cipher to be used in what we shall call "f8-mode of
operation" RTP encryption. The AES f8-mode SHALL use the same operation" RTP encryption. The AES f8-mode SHALL use the same
default sizes for session key and salt as AES counter mode. default sizes for session key and salt as AES counter mode.
Figure 4 shows the structure of block cipher, E, running in f8-mode. Figure 4 shows the structure of block cipher, E, running in f8-mode.
IV IV
| |
v v
+------+ +------+
| | | |
+--->| E | +--->| E |
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| | | | | | | | | | | | | | | | | | | | | | | |
k_e ---+--->| E | | | E | | | E | | | E | k_e ---+--->| E | | | E | | | E | | | E |
| | | | | | | | | | | | | | | | | | | | | |
+------+ | +------+ | +------+ | +------+ +------+ | +------+ | +------+ | +------+
| | | | | | | | | | | | | |
+------+ +--------+ +-- ... ----+ | +------+ +--------+ +-- ... ----+ |
| | | | | | | |
v v v v v v v v
S(0) S(1) S(2) . . . S(L-1) S(0) S(1) S(2) . . . S(L-1)
Figure 4. f8-mode of operation (asterisk, (*), denotes bitwise XOR). Figure 4. f8-mode of operation (asterisk, (*), denotes bitwise
The figure represents the KG in Figure 3, when AES-f8 is used. XOR). The figure represents the KG in Figure 3, when AES-f8 is
used.
4.1.2.1 f8 Keystream Generation 4.1.2.1 f8 Keystream Generation
The Initialization Vector (IV) SHALL be determined as described in The Initialization Vector (IV) SHALL be determined as described in
Section 4.1.2.2 (and in Section 4.1.2.3 for SRTCP). Section 4.1.2.2 (and in Section 4.1.2.3 for SRTCP).
Let IV', S(j), and m denote n_b-bit blocks. The keystream, S(0) || Let IV', S(j), and m denote n_b-bit blocks. The keystream, S(0) ||
... || S(L-1), for an N-bit message SHALL be defined by setting IV' ... || S(L-1), for an N-bit message SHALL be defined by setting IV'
= E(k_e XOR m, IV), and S(-1) = 00..0. For j = 0,1,..,L-1 where L = = E(k_e XOR m, IV), and S(-1) = 00..0. For j = 0,1,..,L-1 where L =
N/n_b (rounded up to nearest integer) compute N/n_b (rounded up to nearest integer) compute
S(j) = E(k_e, IV' XOR j XOR S(j-1)) S(j) = E(k_e, IV' XOR j XOR S(j-1))
Notice that the IV is not used directly. Instead it is fed through E Notice that the IV is not used directly. Instead it is fed through
under another key to produce an internal, "masked" value (denoted E under another key to produce an internal, "masked" value (denoted
IV') to prevent an attacker from gaining known input/output pairs. IV') to prevent an attacker from gaining known input/output pairs.
The role of the internal counter, j, is to prevent short keystream The role of the internal counter, j, is to prevent short keystream
cycles. The value of the key mask m SHALL be cycles. The value of the key mask m SHALL be
m = k_s || 0x555..5, m = k_s || 0x555..5,
i.e. the session salting key, appended by the binary pattern 0101.. i.e. the session salting key, appended by the binary pattern 0101..
to fill out the entire desired key size, n_e. to fill out the entire desired key size, n_e.
The sender SHOULD NOT generate more than 2^32 blocks, which is The sender SHOULD NOT generate more than 2^32 blocks, which is
sufficient to generate 2^39 bits of keystream. Unlike counter mode, sufficient to generate 2^39 bits of keystream. Unlike counter mode,
there is no absolute threshold above (below) which f8 is guaranteed there is no absolute threshold above (below) which f8 is guaranteed
to be insecure (secure). The above bound has been chosen to limit, to be insecure (secure). The above bound has been chosen to limit,
with sufficient security margin, the probability of degenerative with sufficient security margin, the probability of degenerative
behavior in the f8 keystream generation. behavior in the f8 keystream generation.
4.1.2.2 f8 SRTP IV Formation 4.1.2.2 f8 SRTP IV Formation
The purpose of the following IV formation is to provide a feature The purpose of the following IV formation is to provide a feature
which we call implicit header authentication (IHA), see Section 9.5. which we call implicit header authentication (IHA), see Section 9.5.
The SRTP IV for 128-bit block AES-f8 SHALL be formed in the The SRTP IV for 128-bit block AES-f8 SHALL be formed in the
following way: following way:
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RTP session, see Section 9.1. RTP session, see Section 9.1.
4.1.2.3 f8 SRTCP IV Formation 4.1.2.3 f8 SRTCP IV Formation
The SRTCP IV for 128-bit block AES-f8 SHALL be formed in the The SRTCP IV for 128-bit block AES-f8 SHALL be formed in the
following way: following way:
IV= 0..0 || E || SRTCP index || V || P || RC || PT || length || SSRC IV= 0..0 || E || SRTCP index || V || P || RC || PT || length || SSRC
where V, P, RC, PT, length, SSRC SHALL be taken from the first where V, P, RC, PT, length, SSRC SHALL be taken from the first
header in the RTCP compound packet. E and SRTCP index are the 1-bit header in the RTCP compound packet. E and SRTCP index are the 1-bit
and 31-bit fields added to the packet. and 31-bit fields added to the packet.
4.1.3 NULL Cipher 4.1.3 NULL Cipher
The NULL cipher is used when no confidentiality for RTP/RTCP is
requested. The keystream can be thought of as "000..0", i.e. the The NULL cipher is used when no confidentiality for RTP/RTCP is
encryption SHALL simply copy the plaintext input into the ciphertext requested. The keystream can be thought of as "000..0", i.e. the
output. encryption SHALL simply copy the plaintext input into the ciphertext
output.
4.2 Message Authentication and Integrity 4.2 Message Authentication and Integrity
Throughout this section, M will denote data to be integrity Throughout this section, M will denote data to be integrity
protected. In the case of SRTP, M SHALL consist of the Authenticated protected. In the case of SRTP, M SHALL consist of the
Portion of the packet (as specified in Figure 1) concatenated with Authenticated Portion of the packet (as specified in Figure 1)
the ROC, M = Authenticated Portion || ROC; in the case of SRTCP, M concatenated with the ROC, M = Authenticated Portion || ROC; in the
SHALL consist of the Authenticated Portion (as specified in Figure case of SRTCP, M SHALL consist of the Authenticated Portion (as
2) only. specified in Figure 2) only.
Common parameters: Common parameters:
* AUTH_ALG is the authentication algorithm * AUTH_ALG is the authentication algorithm
* k_a is the session message authentication key * k_a is the session message authentication key
* n_a is the bit-length of the authentication key * n_a is the bit-length of the authentication key
* n_tag is the bit-length of the output authentication tag * n_tag is the bit-length of the output authentication tag
* SRTP_PREFIX_LENGTH is the octet length of the keystream prefix as * SRTP_PREFIX_LENGTH is the octet length of the keystream prefix as
defined above, a parameter of AUTH_ALG defined above, a parameter of AUTH_ALG
The distinct session authentication keys for SRTP/SRTCP are by The distinct session authentication keys for SRTP/SRTCP are by
default derived as specified in Section 4.3. default derived as specified in Section 4.3.
The values of n_a, n_tag, and SRTP_PREFIX_LENGTH MUST be fixed for The values of n_a, n_tag, and SRTP_PREFIX_LENGTH MUST be fixed for
any particular fixed value of the key. any particular fixed value of the key.
We describe the process of computing authentication tags as follows. We describe the process of computing authentication tags as follows.
The sender computes the tag of M and appends it to the packet. The The sender computes the tag of M and appends it to the packet. The
SRTP receiver verifies a message/authentication tag pair by SRTP receiver verifies a message/authentication tag pair by
computing a new authentication tag over M using the selected computing a new authentication tag over M using the selected
algorithm and key, and then compares it to the tag associated with algorithm and key, and then compares it to the tag associated with
the received message. If the two tags are equal, then the the received message. If the two tags are equal, then the
message/tag pair is valid; otherwise, it is invalid and the error message/tag pair is valid; otherwise, it is invalid and the error
audit message "AUTHENTICATION FAILURE" MUST be returned. audit message "AUTHENTICATION FAILURE" MUST be returned.
4.2.1 HMAC-SHA1 4.2.1 HMAC-SHA1
The pre-defined authentication transform for SRTP is HMAC-SHA1 The pre-defined authentication transform for SRTP is HMAC-SHA1
[RFC2104]. With HMAC-SHA1, the SRTP_PREFIX_LENGTH (Figure 3) SHALL [RFC2104]. With HMAC-SHA1, the SRTP_PREFIX_LENGTH (Figure 3) SHALL
be 0. For SRTP (respectively SRTCP), the HMAC SHALL be applied to be 0. For SRTP (respectively SRTCP), the HMAC SHALL be applied to
the session authentication key and M as specified above, i.e. the session authentication key and M as specified above, i.e.
HMAC(k_a, M). The HMAC output SHALL then be truncated to the n_tag HMAC(k_a, M). The HMAC output SHALL then be truncated to the n_tag
left-most bits. left-most bits.
4.3 Key Derivation 4.3 Key Derivation
4.3.1 Key Derivation Algorithm 4.3.1 Key Derivation Algorithm
Regardless of the encryption or message authentication transform Regardless of the encryption or message authentication transform
that is employed (it may be an SRTP pre-defined transform or newly that is employed (it may be an SRTP pre-defined transform or newly
introduced according to Section 6), interoperable SRTP introduced according to Section 6), interoperable SRTP
implementations MUST use the SRTP key derivation to generate session implementations MUST use the SRTP key derivation to generate session
keys. Once the key derivation rate is properly signaled at the start keys. Once the key derivation rate is properly signaled at the
of the session, there is no need for extra communication between the start of the session, there is no need for extra communication
parties that use SRTP key derivation. between the parties that use SRTP key derivation.
packet index ---+ packet index ---+
| |
v v
+-----------+ master +--------+ session encr_key +-----------+ master +--------+ session encr_key
| ext | key | |----------> | ext | key | |---------->
| key mgmt |-------->| key | session auth_key | key mgmt |-------->| key | session auth_key
| (optional | | deriv |----------> | (optional | | deriv |---------->
| rekey) |-------->| | session salt_key | rekey) |-------->| | session salt_key
| | master | |----------> | | master | |---------->
+-----------+ salt +--------+ +-----------+ salt +--------+
Figure 5: SRTP key derivation. Figure 5: SRTP key derivation.
At least one initial key derivation SHALL be performed by SRTP, At least one initial key derivation SHALL be performed by SRTP,
i.e., the first key derivation is REQUIRED. Further applications of i.e., the first key derivation is REQUIRED. Further applications of
the key derivation MAY be performed, according to the the key derivation MAY be performed, according to the
"key_derivation_rate" value in the cryptographic context. The key "key_derivation_rate" value in the cryptographic context. The key
derivation function SHALL be initially invoked before the first derivation function SHALL be initially invoked before the first
packet and then, if derivation rate is r > 0, further invoked on packet and then, if derivation rate is r > 0, further invoked on
every r-th packet, and produce session keys according to the non- every r-th packet, and produce session keys according to the non-
zero key derivation rate. This can be thought of as "refreshing" the zero key derivation rate. This can be thought of as "refreshing"
session keys. The value of "key_derivation_rate" MUST be kept fixed the session keys. The value of "key_derivation_rate" MUST be kept
for the lifetime of the associated master key. fixed for the lifetime of the associated master key.
Interoperable SRTP implementations MAY also derive session salting Interoperable SRTP implementations MAY also derive session salting
keys for encryption transforms, as is done in both of the pre- keys for encryption transforms, as is done in both of the pre-
defined transforms. defined transforms.
Let m and n be positive integers. A pseudo-random function family is Let m and n be positive integers. A pseudo-random function family
a set of keyed functions {PRF_n(k,x)} such that for the (secret) is a set of keyed functions {PRF_n(k,x)} such that for the (secret)
random key k, given m-bit x, PRF_n(k,x) is an n-bit string, random key k, given m-bit x, PRF_n(k,x) is an n-bit string,
computationally indistinguishable from random n-bit strings, see computationally indistinguishable from random n-bit strings, see
[HAC]. For the purpose of key derivation in SRTP, a secure PRF with [HAC]. For the purpose of key derivation in SRTP, a secure PRF with
m = 128 (or more) MUST be used, and a default PRF transform is m = 128 (or more) MUST be used, and a default PRF transform is
defined in Section 4.3.3. defined in Section 4.3.3.
Let "a DIV t" denote integer division of a by t, rounded down, and Let "a DIV t" denote integer division of a by t, rounded down, and
with the convention that "a DIV 0 = 0" for all a. We also make the with the convention that "a DIV 0 = 0" for all a. We also make the
convention of treating "a DIV t" as a bit string of the same length convention of treating "a DIV t" as a bit string of the same length
as a, and thus "a DIV t" will in general have leading zeros. as a, and thus "a DIV t" will in general have leading zeros.
Key derivation SHALL be defined as follows in terms of <label>, an Key derivation SHALL be defined as follows in terms of <label>, an
8-bit constant (see below), master_salt and key_derivation_rate, as 8-bit constant (see below), master_salt and key_derivation_rate, as
determined in the cryptographic context, and index, the packet index determined in the cryptographic context, and index, the packet index
(i.e., the 48-bit ROC || SEQ for SRTP): (i.e., the 48-bit ROC || SEQ for SRTP):
* Let r = index DIV key_derivation_rate (with DIV as defined above). * Let r = index DIV key_derivation_rate (with DIV as defined above).
* Let key_id = <label> || r. * Let key_id = <label> || r.
* Let x = key_id XOR master_salt, where key_id and master_salt are * Let x = key_id XOR master_salt, where key_id and master_salt are
aligned so that their least significant bits agree (right- aligned so that their least significant bits agree (right-
alignment). alignment).
<label> MUST be unique for each 'type' of key to be derived. We <label> MUST be unique for each type of key to be derived. We
currently define <label> 0x00 to 0x05 (see below), and future currently define <label> 0x00 to 0x05 (see below), and future
extensions MAY specify new values in the range 0x06 to 0xff for extensions MAY specify new values in the range 0x06 to 0xff for
other purposes. The n-bit SRTP key (or salt) for this packet SHALL other purposes. The n-bit SRTP key (or salt) for this packet SHALL
then be derived from the master key, k_master as follows: then be derived from the master key, k_master as follows:
PRF_n(k_master, x). PRF_n(k_master, x).
(The PRF may internally specify additional formatting and padding (The PRF may internally specify additional formatting and padding
of x, see e.g. Section 4.3.3 for the default PRF.) of x, see e.g. Section 4.3.3 for the default PRF.)
The session keys and salt SHALL now be derived using: The session keys and salt SHALL now be derived using:
- k_e (SRTP encryption): <label> = 0x00, n = n_e. - k_e (SRTP encryption): <label> = 0x00, n = n_e.
- k_a (SRTP message authentication): <label> = 0x01, n = n_a. - k_a (SRTP message authentication): <label> = 0x01, n = n_a.
- k_s (SRTP salting key) <label> = 0x02, n = n_s. - k_s (SRTP salting key) <label> = 0x02, n = n_s.
where n_e, n_s, and n_a are from the cryptographic context. where n_e, n_s, and n_a are from the cryptographic context.
The master key and master salting key MUST be random, but the master The master key and master salting key MUST be random, but the master
salt MAY be public. salt MAY be public.
Note that for a key_derivation_rate of 0, the application of the key Note that for a key_derivation_rate of 0, the application of the key
derivation SHALL take place exactly once. derivation SHALL take place exactly once.
The definition of DIV above is purely for notational convenience. The definition of DIV above is purely for notational convenience.
For a non-zero t among the set of allowed key derivation rates, "a For a non-zero t among the set of allowed key derivation rates, "a
DIV t" can be implemented as a right-shift by the base-2 logarithm DIV t" can be implemented as a right-shift by the base-2 logarithm
of t. The derivation operation is further facilitated if the rates of t. The derivation operation is further facilitated if the rates
are chosen to be powers of 256, but that granularity was considered are chosen to be powers of 256, but that granularity was considered
too coarse to be a requirement of this specification. too coarse to be a requirement of this specification.
The upper limit on the number of packets that can be secured using The upper limit on the number of packets that can be secured using
the same master key (see Section 9.2) is independent of the key the same master key (see Section 9.2) is independent of the key
derivation. derivation.
4.3.2 SRTCP Key Derivation 4.3.2 SRTCP Key Derivation
SRTCP SHALL by default use the same master key (and master salt) as SRTCP SHALL by default use the same master key (and master salt) as
SRTP. To do this securely, the following changes SHALL be done to SRTP. To do this securely, the following changes SHALL be done to
the definitions in Section 4.3.1 when applying session key the definitions in Section 4.3.1 when applying session key
derivation for SRTCP. derivation for SRTCP.
Replace the SRTP index by the 32-bit quantity: 0 || SRTCP index Replace the SRTP index by the 32-bit quantity: 0 || SRTCP index
(i.e. excluding the E-bit, replacing it with a fixed 0-bit), and use (i.e. excluding the E-bit, replacing it with a fixed 0-bit), and use
<label> = 0x03 for the SRTCP encryption key, <label> = 0x04 for the <label> = 0x03 for the SRTCP encryption key, <label> = 0x04 for the
SRTCP authentication key, and, <label> = 0x05 for the SRTCP salting SRTCP authentication key, and, <label> = 0x05 for the SRTCP salting
key. key.
4.3.3 AES-CM PRF 4.3.3 AES-CM PRF
The currently defined PRF, keyed by 128, 192, or 256 bit master key, The currently defined PRF, keyed by 128, 192, or 256 bit master key,
has input block size m = 128 and can produce n-bit outputs for n up has input block size m = 128 and can produce n-bit outputs for n up
to 2^23. PRF_n(k_master,x) SHALL be AES in Counter Mode as described to 2^23. PRF_n(k_master,x) SHALL be AES in Counter Mode as
in Section 4.1.1, applied to key k_master, and IV equal to (x*2^16), described in Section 4.1.1, applied to key k_master, and IV equal to
and with the output keystream truncated to the n first (left-most) (x*2^16), and with the output keystream truncated to the n first
bits. (Requiring n/128, rounded up, applications of AES.) (left-most) bits. (Requiring n/128, rounded up, applications of
AES.)
5. Default and mandatory-to-implement Transforms 5. Default and mandatory-to-implement Transforms
The default transforms also are mandatory-to-implement transforms in The default transforms also are mandatory-to-implement transforms in
SRTP. Of course, "mandatory-to-implement" does not imply "mandatory- SRTP. Of course, "mandatory-to-implement" does not imply
to-use". Table 1 summarizes the pre-defined transforms. The default "mandatory-to-use". Table 1 summarizes the pre-defined transforms.
values below are valid for the pre-defined transforms. The default values below are valid for the pre-defined transforms.
mandatory-to-impl. optional default mandatory-to-impl. optional default
encryption AES-CM, NULL AES-f8 AES-CM encryption AES-CM, NULL AES-f8 AES-CM
message integrity HMAC-SHA1 - HMAC-SHA1 message integrity HMAC-SHA1 - HMAC-SHA1
key derivation (PRF) AES-CM - AES-CM key derivation (PRF) AES-CM - AES-CM
Table 1: Mandatory-to-implement, optional and default transforms in Table 1: Mandatory-to-implement, optional and default transforms in
SRTP and SRTCP. SRTP and SRTCP.
5.1 Encryption: AES-CM and NULL 5.1 Encryption: AES-CM and NULL
AES running in Segmented Integer Counter Mode, as defined in Section AES running in Segmented Integer Counter Mode, as defined in Section
4.1.1, SHALL be the default encryption algorithm. The default key 4.1.1, SHALL be the default encryption algorithm. The default key
lengths SHALL be 128-bit for the session encryption key (n_e). The lengths SHALL be 128-bit for the session encryption key (n_e). The
default session salt key-length (n_s) SHALL be 112 bits. default session salt key-length (n_s) SHALL be 112 bits.
The NULL cipher SHALL also be mandatory-to-implement. The NULL cipher SHALL also be mandatory-to-implement.
5.2 Message Authentication/Integrity: HMAC-SHA1 5.2 Message Authentication/Integrity: HMAC-SHA1
HMAC-SHA1, as defined in Section 4.2.1, SHALL be the default message HMAC-SHA1, as defined in Section 4.2.1, SHALL be the default message
authentication code. The default session authentication key-length authentication code. The default session authentication key-length
(n_a) SHALL be 160 bits, the default authentication tag length (n_a) SHALL be 160 bits, the default authentication tag length
(n_tag) SHALL be 80 bits, and the SRTP_PREFIX_LENGTH SHALL be (n_tag) SHALL be 80 bits, and the SRTP_PREFIX_LENGTH SHALL be
zero for HMAC-SHA1. In addition, for SRTCP, the pre-defined HMAC- zero for HMAC-SHA1. In addition, for SRTCP, the pre-defined HMAC-
SHA1 MUST NOT be applied with a value of n_tag, nor n_a, that are SHA1 MUST NOT be applied with a value of n_tag, nor n_a, that are
smaller than these defaults. For SRTP, smaller values are NOT smaller than these defaults. For SRTP, smaller values are NOT
RECOMMENDED, but MAY be used after careful consideration of the RECOMMENDED, but MAY be used after careful consideration of the
issues in Section 7.5 and 9.5. issues in Section 7.5 and 9.5.
5.3 Key Derivation: AES-CM PRF 5.3 Key Derivation: AES-CM PRF
The AES Counter Mode based key derivation and PRF defined in The AES Counter Mode based key derivation and PRF defined in
Sections 4.3.1 to 4.3.3, using a 128-bit master key, SHALL be the Sections 4.3.1 to 4.3.3, using a 128-bit master key, SHALL be the
default method for generating session keys. The default master salt default method for generating session keys. The default master salt
length SHALL be 112 bits and the default key-derivation rate SHALL length SHALL be 112 bits and the default key-derivation rate SHALL
be zero. be zero.
6. Adding SRTP Transforms 6. Adding SRTP Transforms
Section 4 provides examples of the level of detail needed for Section 4 provides examples of the level of detail needed for
defining transforms. Whenever a new transform is to be added to defining transforms. Whenever a new transform is to be added to
SRTP, a companion standard track RFC MUST be written to exactly SRTP, a companion standard track RFC MUST be written to exactly
define how the new transform can be used with SRTP (and SRTCP). Such define how the new transform can be used with SRTP (and SRTCP).
a companion RFC SHOULD avoid overlap with the SRTP protocol Such a companion RFC SHOULD avoid overlap with the SRTP protocol
document. Note however, that it MAY be necessary to extend the SRTP document. Note however, that it MAY be necessary to extend the SRTP
or SRTCP cryptographic context definition with new parameters or SRTCP cryptographic context definition with new parameters
(including fixed or default values), add steps to the packet (including fixed or default values), add steps to the packet
processing, or even add fields to the SRTP packets. The companion processing, or even add fields to the SRTP packets. The companion
RFC SHALL explain any known issues regarding interactions between RFC SHALL explain any known issues regarding interactions between
the transform and other aspects of SRTP. the transform and other aspects of SRTP.
Each new transform document SHOULD specify its key attributes, e.g., Each new transform document SHOULD specify its key attributes, e.g.,
size of keys (minimum, maximum, recommended), format of keys, size of keys (minimum, maximum, recommended), format of keys,
recommended/required processing of input keying material, recommended/required processing of input keying material,
requirements/recommendations on key life-time, re-keying and key requirements/recommendations on key lifetime, re-keying and key
derivation, whether sharing of keys between SRTP and SRTCP is derivation, whether sharing of keys between SRTP and SRTCP is
allowed or not, etc. allowed or not, etc.
An added message integrity transform SHOULD define a minimum An added message integrity transform SHOULD define a minimum
acceptable key/tag size for SRTCP, equivalent in strength to the acceptable key/tag size for SRTCP, equivalent in strength to the
minimum values as defined in Section 5.2. minimum values as defined in Section 5.2.
7. Rationale 7. Rationale
This section explains the rationale behind several important This section explains the rationale behind several important
features of SRTP. features of SRTP.
7.1 Key derivation 7.1 Key derivation
Key derivation reduces the burden on the key establishment. As many Key derivation reduces the burden on the key establishment. As many
as six different keys are needed per crypto context (SRTP and SRTCP as six different keys are needed per crypto context (SRTP and SRTCP
encryption keys and salts, SRTP and SRTCP authentication keys), but encryption keys and salts, SRTP and SRTCP authentication keys), but
these are derived from a single master key in a cryptographically these are derived from a single master key in a cryptographically
secure way. Thus, the key management protocol needs to exchange only secure way. Thus, the key management protocol needs to exchange only
one master key (plus master salt when required), and then SRTP itself one master key (plus master salt when required), and then SRTP itself
derives all the necessary session keys (via the first, mandatory derives all the necessary session keys (via the first, mandatory
application of the key derivation function). application of the key derivation function).
Multiple applications of the key derivation function are optional, Multiple applications of the key derivation function are optional,
but will give security benefits when enabled. They prevent an but will give security benefits when enabled. They prevent an
attacker from obtaining large amounts of ciphertext produced by a attacker from obtaining large amounts of ciphertext produced by a
single fixed session key. If the attacker was able to collect a single fixed session key. If the attacker was able to collect a
large amount of ciphertext for a certain session key, he might be large amount of ciphertext for a certain session key, he might be
helped in mounting certain attacks. helped in mounting certain attacks.
Multiple applications of the key derivation function provide Multiple applications of the key derivation function provide
backwards and forward security in the sense that a compromised backwards and forward security in the sense that a compromised
session key does not compromise other session keys derived from the session key does not compromise other session keys derived from the
same master key. This means that the attacker who is able to recover same master key. This means that the attacker who is able to
a certain session key, is anyway not able to have access to messages recover a certain session key, is anyway not able to have access to
secured under previous and later session keys (derived from the same messages secured under previous and later session keys (derived from
master key). (Note that, of course, a leaked master key reveals all the same master key). (Note that, of course, a leaked master key
the session keys derived from it.) reveals all the session keys derived from it.)
Considerations arise with high-rate key-refresh, especially in large Considerations arise with high-rate key refresh, especially in large
multicast settings, see Section 11. multicast settings, see Section 11.
7.2 Salting key 7.2 Salting key
The master salt guarantees security against off-line key-collision The master salt guarantees security against off-line key-collision
attacks on the key derivation that might otherwise reduce the attacks on the key derivation that might otherwise reduce the
effective key size [MF00]. effective key size [MF00].
The derived session salting key used in the encryption, has been The derived session salting key used in the encryption, has been
introduced to protect against some attacks on additive stream introduced to protect against some attacks on additive stream
ciphers, see Section 9.2. The explicit inclusion method of the salt ciphers, see Section 9.2. The explicit inclusion method of the salt
in the IV has been selected for ease of hardware implementation. in the IV has been selected for ease of hardware implementation.
7.3 Message Integrity from Universal Hashing 7.3 Message Integrity from Universal Hashing
The particular definition of the keystream given in Section 4.1 (the The particular definition of the keystream given in Section 4.1 (the
keystream prefix) is to give provision for particular universal hash keystream prefix) is to give provision for particular universal hash
functions, suitable for message authentication in the Wegman-Carter functions, suitable for message authentication in the Wegman-Carter
paradigm [WC81]. Such functions are provably secure, simple, quick, paradigm [WC81]. Such functions are provably secure, simple, quick,
and especially appropriate for Digital Signal Processors and other and especially appropriate for Digital Signal Processors and other
processors with a fast multiply operation. processors with a fast multiply operation.
No authentication transforms are currently provided in SRTP other No authentication transforms are currently provided in SRTP other
than HMAC-SHA1. Future transforms, like the above mentioned than HMAC-SHA1. Future transforms, like the above mentioned
universal hash functions, MAY be added following the guidelines in universal hash functions, MAY be added following the guidelines in
Section 6. Section 6.
7.4 Data Origin Authentication Considerations 7.4 Data Origin Authentication Considerations
Note that in pair-wise communications, integrity and data origin Note that in pair-wise communications, integrity and data origin
authentication are provided together. However, in group scenarios authentication are provided together. However, in group scenarios
where the keys are shared between members, the MAC tag only proves where the keys are shared between members, the MAC tag only proves
that a member of the group sent the packet, but does not prevent that a member of the group sent the packet, but does not prevent
against a member impersonating another. Data origin authentication against a member impersonating another. Data origin authentication
(DOA) for multicast and group RTP sessions is a hard problem that (DOA) for multicast and group RTP sessions is a hard problem that
needs a solution; while some promising proposals are being needs a solution; while some promising proposals are being
investigated [PCST1, PCST2], more work is needed to rigorously investigated [PCST1, PCST2], more work is needed to rigorously
specify these technologies. Thus SRTP data origin authentication in specify these technologies. Thus SRTP data origin authentication in
groups is for further study. groups is for further study.
DOA can be done otherwise using signatures. However, this has high DOA can be done otherwise using signatures. However, this has high
impact in terms of bandwidth and processing time, therefore we do impact in terms of bandwidth and processing time, therefore we do
not offer this form of authentication in the pre-defined packet- not offer this form of authentication in the pre-defined packet-
integrity transform. integrity transform.
The presence of mixers and translators does not allow data origin The presence of mixers and translators does not allow data origin
authentication in case the RTP payload and/or the RTP header are authentication in case the RTP payload and/or the RTP header are
manipulated. Note that these types of middle entities also disrupt manipulated. Note that these types of middle entities also disrupt
end-to-end confidentiality (as the IV formation depends e.g. on the end-to-end confidentiality (as the IV formation depends e.g. on the
RTP header preservation). A certain trust model may choose to trust RTP header preservation). A certain trust model may choose to trust
the mixers/translators to decrypt/re-encrypt the media (this would the mixers/translators to decrypt/re-encrypt the media (this would
imply breaking the end-to-end security, with related security imply breaking the end-to-end security, with related security
implications). implications).
7.5 Short and Zero-length Message Authentication 7.5 Short and Zero-length Message Authentication
As shown in Figure 1, the authentication tag is RECOMMENDED in SRTP. As shown in Figure 1, the authentication tag is RECOMMENDED in SRTP.
A full 80-bit authentication-tag SHOULD be used, but a shorter tag A full 80-bit authentication-tag SHOULD be used, but a shorter tag
or even a zero-length tag (i.e. no message authentication) MAY be or even a zero-length tag (i.e. no message authentication) MAY be
used under certain conditions to support either of the following two used under certain conditions to support either of the following two
application environments. application environments.
1. Strong authentication can be impractical in environments where 1. Strong authentication can be impractical in environments where
bandwidth preservation is imperative. An important special bandwidth preservation is imperative. An important special
case is wireless communication systems, in which bandwidth is a case is wireless communication systems, in which bandwidth is a
scarce and expensive resource. Studies have shown that for scarce and expensive resource. Studies have shown that for
certain applications and link technologies, additional bytes certain applications and link technologies, additional bytes
may result in a significant decrease in spectrum efficiency may result in a significant decrease in spectrum efficiency
[SWO]. Considerable effort has been made to design IP header [SWO]. Considerable effort has been made to design IP header
compression techniques to improve spectrum efficiency [ROHC]. A compression techniques to improve spectrum efficiency [ROHC].
typical voice application produces 20 byte samples, and the A typical voice application produces 20 byte samples, and the
RTP, UDP and IP headers need to be jointly compressed to one or RTP, UDP and IP headers need to be jointly compressed to one or
two bytes on average in order to obtain acceptable wireless two bytes on average in order to obtain acceptable wireless
bandwidth economy [RFC3095]. In this case, strong bandwidth economy [RFC3095]. In this case, strong
authentication would impose nearly fifty percent overhead. authentication would impose nearly fifty percent overhead.
2. Authentication is impractical for applications that use data 2. Authentication is impractical for applications that use data
links with fixed-width fields that cannot accommodate the links with fixed-width fields that cannot accommodate the
expansion due to the authentication tag. This is the case for expansion due to the authentication tag. This is the case for
some important existing wireless channels. For example, zero- some important existing wireless channels. For example, zero-
byte header compression is used to adapt EVRC/SMV voice with byte header compression is used to adapt EVRC/SMV voice with
the legacy IS-95 bearer channel in CDMA2000 VoIP services. It the legacy IS-95 bearer channel in CDMA2000 VoIP services. It
was found that a not a single additional octet could be added was found that a not a single additional octet could be added
to the data, which motivated the creation of a zero-byte to the data, which motivated the creation of a zero-byte
profile for ROHC [RFC3242]. profile for ROHC [RFC3242].
A short tag is secure for a restricted set of applications. Consider A short tag is secure for a restricted set of applications.
a voice telephony application, for example, such as a G.729 audio Consider a voice telephony application, for example, such as a G.729
codec with a 20-millisecond packetization interval, protected by a audio codec with a 20-millisecond packetization interval, protected
32-bit message authentication tag. The likelihood of any given by a 32-bit message authentication tag. The likelihood of any given
packet being successfully forged is only one in 2^32. Thus an packet being successfully forged is only one in 2^32. Thus an
adversary can control no more than 20 milliseconds of audio output adversary can control no more than 20 milliseconds of audio output
during a 994-day period, on average. In contrast, the effect of a during a 994-day period, on average. In contrast, the effect of a
single forged packet can be much larger if the application is single forged packet can be much larger if the application is
stateful. A codec that uses relative or predictive compression stateful. A codec that uses relative or predictive compression
across packets will propagate the maliciously generated state, across packets will propagate the maliciously generated state,
affecting a longer duration of output. affecting a longer duration of output.
Certainly not all SRTP or telephony applications meet the criteria Certainly not all SRTP or telephony applications meet the criteria
for short or zero-length authentication tags. Section 9.5.1 for short or zero-length authentication tags. Section 9.5.1
skipping to change at page 32, line 25 skipping to change at page 34, line 25
There are emerging key management standards [MIKEY, KEYMGT, SDMS] There are emerging key management standards [MIKEY, KEYMGT, SDMS]
for establishing an SRTP cryptographic context (e.g. an SRTP master for establishing an SRTP cryptographic context (e.g. an SRTP master
key). Both proprietary and open-standard key management methods are key). Both proprietary and open-standard key management methods are
likely to be used for telephony applications [MIKEY, KINK] and likely to be used for telephony applications [MIKEY, KINK] and
multicast applications [GDOI]. This section provides guidance for multicast applications [GDOI]. This section provides guidance for
key management systems that service SRTP session. key management systems that service SRTP session.
For initialization, an interoperable SRTP implementation SHOULD be For initialization, an interoperable SRTP implementation SHOULD be
given the SSRC and MAY be given the initial RTP sequence number for given the SSRC and MAY be given the initial RTP sequence number for
the RTP stream by key management (thus, key management has a the RTP stream by key management (thus, key management has a
dependency on RTP operational parameters). Sending the RTP sequence dependency on RTP operational parameters). Sending the RTP sequence
number in the key management may be useful e.g. when the initial number in the key management may be useful e.g. when the initial
sequence number is close to wrapping (to avoid synchronization sequence number is close to wrapping (to avoid synchronization
problems), and to communicate the current sequence number to a problems), and to communicate the current sequence number to a
joining endpoint (to properly initialize its replay list). joining endpoint (to properly initialize its replay list).
If the pre-defined transforms are used, SRTP allows sharing of the If the pre-defined transforms are used, SRTP allows sharing of the
same master key between SRTP/SRTCP streams belonging to the same RTP same master key between SRTP/SRTCP streams belonging to the same RTP
session. session.
First, sharing between SRTP streams belonging to the same RTP First, sharing between SRTP streams belonging to the same RTP
session is secure if the design of the synchronization mechanism, session is secure if the design of the synchronization mechanism,
i.e., the IV, avoids keystream re-use (the two-time pad, Section i.e., the IV, avoids keystream re-use (the two-time pad, Section
9.1). This is taken care of by the fact that RTP provides for unique 9.1). This is taken care of by the fact that RTP provides for
SSRCs for streams belonging to the same RTP session. See Section 9.1 unique SSRCs for streams belonging to the same RTP session. See
for further discussion. Section 9.1 for further discussion.
Second, sharing between SRTP and the corresponding SRTCP is secure: Second, sharing between SRTP and the corresponding SRTCP is secure.
The fact that an SRTP stream and its associated SRTCP stream both The fact that an SRTP stream and its associated SRTCP stream both
carry the same SSRC does not constitute a problem for the two time carry the same SSRC does not constitute a problem for the two time
pad due to the key derivation. Thus, SRTP and SRTCP corresponding to pad due to the key derivation. Thus, SRTP and SRTCP corresponding
one RTP session MAY share master keys (as they do by default). to one RTP session MAY share master keys (as they do by default).
Note that also message authentication has a dependency on SSRC Note that also message authentication has a dependency on SSRC
uniqueness that is unrelated to the problem of keystream reuse: SRTP uniqueness that is unrelated to the problem of keystream reuse: SRTP
streams authenticated under the same key MUST have a distinct SSRC streams authenticated under the same key MUST have a distinct SSRC
in order to identify the sender of the message. This requirement is in order to identify the sender of the message. This requirement is
needed because the SSRC is the cryptographically authenticated field needed because the SSRC is the cryptographically authenticated field
used to distinguish between different SRTP streams. Were two used to distinguish between different SRTP streams. Were two
streams to use identical SSRC values, then an adversary could streams to use identical SSRC values, then an adversary could
substitute messages from one stream into the other without substitute messages from one stream into the other without
detection. detection.
skipping to change at page 33, line 22 skipping to change at page 35, line 22
SRTCP, and, between streams belonging to the same RTP session. SRTCP, and, between streams belonging to the same RTP session.
8.1. Re-keying 8.1. Re-keying
The recommended way for a particular key management system to The recommended way for a particular key management system to
provide re-key within SRTP is by associating a master key in a provide re-key within SRTP is by associating a master key in a
crypto context with an MKI. crypto context with an MKI.
This provides for easy master key retrieval (see Scenarios in This provides for easy master key retrieval (see Scenarios in
Section 11), but has the disadvantage of adding extra bits to each Section 11), but has the disadvantage of adding extra bits to each
packet. As noted in Section 7.5, some wireless links do not cater packet. As noted in Section 7.5, some wireless links do not cater
for added bits, therefore SRTP also defines a more economic way of for added bits, therefore SRTP also defines a more economic way of
triggering re-keying, via use of <From, To>, which works in some triggering re-keying, via use of <From, To>, which works in some
specific, simple scenarios (see Section 8.1.1). specific, simple scenarios (see Section 8.1.1).
SRTP senders SHALL count the amount of SRTP and SRTCP traffic being SRTP senders SHALL count the amount of SRTP and SRTCP traffic being
used for a master key and invoke key management to re-key if needed used for a master key and invoke key management to re-key if needed
(Section 9.2). These interactions are defined by the key management (Section 9.2). These interactions are defined by the key management
interface to SRTP and are not defined by this protocol interface to SRTP and are not defined by this protocol
specification. specification.
8.1.1 Use of the <From, To> for re-keying 8.1.1 Use of the <From, To> for re-keying
In addition to the use of the MKI, SRTP defines another optional In addition to the use of the MKI, SRTP defines another optional
mechanism for master key retrieval, the <From, To>. The <From, To> mechanism for master key retrieval, the <From, To>. The <From, To>
specifies the range of SRTP indices (a pair of sequence number and specifies the range of SRTP indices (a pair of sequence number and
ROC) within which a certain master key is valid, and is (when used) ROC) within which a certain master key is valid, and is (when used)
part of the crypto context. By looking at the 48-bit SRTP index of part of the crypto context. By looking at the 48-bit SRTP index of
the current SRTP packet, the corresponding master key can be found the current SRTP packet, the corresponding master key can be found
by determining which From-To interval it belongs to. For SRTCP, the by determining which From-To interval it belongs to. For SRTCP, the
most recently observed/used SRTP index (which can be obtained from most recently observed/used SRTP index (which can be obtained from
the cryptographic context) is used for this purpose, even though the cryptographic context) is used for this purpose, even though
SRTCP has its own (31-bit) index (see caveat below). SRTCP has its own (31-bit) index (see caveat below).
This method, compared to the MKI, has the advantage of identifying This method, compared to the MKI, has the advantage of identifying
the master key and defining its lifetime without adding extra bits the master key and defining its lifetime without adding extra bits
to each packet. This could be useful, as already noted, for some to each packet. This could be useful, as already noted, for some
wireless links that do not cater for added bits. However, its use wireless links that do not cater for added bits. However, its use
SHOULD be limited to specific, very simple scenarios. We recommend SHOULD be limited to specific, very simple scenarios. We recommend
to limit its use when the RTP session is a simple unidirectional or to limit its use when the RTP session is a simple unidirectional or
bi-directional stream. This is because in case of multiple streams, bi-directional stream. This is because in case of multiple streams,
it is difficult to trigger the re-key based on the <From, To> of a it is difficult to trigger the re-key based on the <From, To> of a
single RTP stream. E.g., if several streams share a master key, single RTP stream. E.g., if several streams share a master key,
there is no simple one-to-one correspondence between the index there is no simple one-to-one correspondence between the index
sequence space of a certain stream, and the index sequence space on sequence space of a certain stream, and the index sequence space on
which the <From, To> values are based. Consequently, when a master which the <From, To> values are based. Consequently, when a master
key is shared between streams, one of these streams MUST be key is shared between streams, one of these streams MUST be
designated by key management as the one whose index space defines designated by key management as the one whose index space defines
the re-keying points. Also, the re-key triggering on SRTCP is based the re-keying points. Also, the re-key triggering on SRTCP is based
on the correspondent SRTP stream, i.e. when the SRTP stream changes on the correspondent SRTP stream, i.e. when the SRTP stream changes
the master key, so does the correspondent SRTCP. This becomes the master key, so does the correspondent SRTCP. This becomes
obviously more and more complex with multiple streams. obviously more and more complex with multiple streams.
The default values for the <From, To> are "from the first observed The default values for the <From, To> are "from the first observed
packet" and "until further notice". However, the maximum limit of packet" and "until further notice". However, the maximum limit of
SRTP/SRTCP packets that are sent under each given master/session key SRTP/SRTCP packets that are sent under each given master/session key
(Section 9.2) MUST NOT be exceeded. (Section 9.2) MUST NOT be exceeded.
In case the <From, To> is used as key retrieval, then the MKI is not In case the <From, To> is used as key retrieval, then the MKI is not
inserted in the packet (and its indicator in the crypto context is inserted in the packet (and its indicator in the crypto context is
zero). However, using the MKI does not exclude using <"From", "To"> zero). However, using the MKI does not exclude using <"From", "To">
key lifetime simultaneously. This can for instance be useful to key lifetime simultaneously. This can for instance be useful to
signal at the sender side at which point in time an MKI is to be signal at the sender side at which point in time an MKI is to be
made active. made active.
8.2. Key Management parameters 8.2. Key Management parameters
The table below lists all SRTP parameters that key management can The table below lists all SRTP parameters that key management can
supply. For reference, it also provides a summary of the default and supply. For reference, it also provides a summary of the default
mandatory-to-support values for an SRTP implementation as described and mandatory-to-support values for an SRTP implementation as
in Section 5. described in Section 5.
Parameter Mandatory-to-support Default Parameter Mandatory-to-support Default
--------- -------------------- ------- --------- -------------------- -------
SRTP and SRTCP encr transf. AES_CM, NULL AES_CM SRTP and SRTCP encr transf. AES_CM, NULL AES_CM
(Other possible values: AES_f8) (Other possible values: AES_f8)
SRTP and SRTCP auth transf. HMAC-SHA1 HMAC-SHA1 SRTP and SRTCP auth transf. HMAC-SHA1 HMAC-SHA1
SRTP and SRTCP auth params: SRTP and SRTCP auth params:
n_tag (tag length) 80 80 n_tag (tag length) 80 80
SRTP prefix_length 0 0 SRTP prefix_length 0 0
Key derivation PRF AES_CM AES_CM Key derivation PRF AES_CM AES_CM
Key material params Key material params
(for each master key): (for each master key):
master key length 128 128 master key length 128 128
n_e (encr session key length) 128 128 n_e (encr session key length) 128 128
n_a (auth session key length) 160 160 n_a (auth session key length) 160 160
master salt key master salt key
length of the master salt 112 112 length of the master salt 112 112
n_s (session salt key length) 112 112 n_s (session salt key length) 112 112
skipping to change at page 35, line 38 skipping to change at page 37, line 38
Relation to other RTP profiles: Relation to other RTP profiles:
sender's order between FEC and SRTP FEC-SRTP FEC-SRTP sender's order between FEC and SRTP FEC-SRTP FEC-SRTP
(see Section 10) (see Section 10)
9. Security Considerations 9. Security Considerations
9.1 SSRC collision and two-time pad 9.1 SSRC collision and two-time pad
Any fixed keystream output, generated from the same key and index Any fixed keystream output, generated from the same key and index
MUST only be used to encrypt once. Re-using such keystream (jokingly MUST only be used to encrypt once. Re-using such keystream
called a "two-time pad" system by cryptographers), can seriously (jokingly called a "two-time pad" system by cryptographers), can
compromise security. The NSA's VENONA project [C99] provides a seriously compromise security. The NSA's VENONA project [C99]
historical example of such a compromise. It is REQUIRED that provides a historical example of such a compromise. It is REQUIRED
automatic key management be used for establishing and maintaining that automatic key management be used for establishing and
SRTP and SRTCP keying material; this requirement is to avoid maintaining SRTP and SRTCP keying material; this requirement is to
keystream reuse, which is more likely to occur with manual key avoid keystream reuse, which is more likely to occur with manual key
management. Furthermore, in SRTP, a "two-time pad" is avoided by management. Furthermore, in SRTP, a "two-time pad" is avoided by
requiring the key, or some other parameter of cryptographic requiring the key, or some other parameter of cryptographic
significance, to be unique per RTP/RTCP stream and packet. The pre- significance, to be unique per RTP/RTCP stream and packet. The pre-
defined SRTP transforms accomplish packet-uniqueness by including defined SRTP transforms accomplish packet-uniqueness by including
the packet index and stream-uniqueness by inclusion of the SSRC. the packet index and stream-uniqueness by inclusion of the SSRC.
The pre-defined transforms (AES-CM and AES-f8) allow master keys to The pre-defined transforms (AES-CM and AES-f8) allow master keys to
be shared across streams belonging to the same RTP session by the be shared across streams belonging to the same RTP session by the
inclusion of the SSRC in the IV. A master key MUST NOT be shared inclusion of the SSRC in the IV. A master key MUST NOT be shared
among different RTP sessions. among different RTP sessions.
Thus, the SSRC MUST be unique between all the RTP streams within the Thus, the SSRC MUST be unique between all the RTP streams within the
same RTP session that share the same master key. RTP itself same RTP session that share the same master key. RTP itself
provides an algorithm for detecting SSRC collisions within the same provides an algorithm for detecting SSRC collisions within the same
RTP session. Thus, temporary collisions could lead to temporary two- RTP session. Thus, temporary collisions could lead to temporary
time pad, in the unfortunate event that SSRCs collide at a point in two-time pad, in the unfortunate event that SSRCs collide at a point
time when the streams also have identical sequence numbers in time when the streams also have identical sequence numbers
(occurring with probability roughly 2^(-48)). Therefore, the key (occurring with probability roughly 2^(-48)). Therefore, the key
management SHOULD take care of avoiding such SSRC collisions by management SHOULD take care of avoiding such SSRC collisions by
including the SSRCs to be used in the session as negotiation including the SSRCs to be used in the session as negotiation
parameters, proactively assuring their uniqueness. This is a strong parameters, proactively assuring their uniqueness. This is a strong
requirements in scenarios where for example, there are multiple requirements in scenarios where for example, there are multiple
senders that can start to transmit simultaneously, before SSRC senders that can start to transmit simultaneously, before SSRC
collision are detected at the RTP level. collision are detected at the RTP level.
Note also that even with distinct SSRCs, extensive use of the same Note also that even with distinct SSRCs, extensive use of the same
key might improve chances of probabilistic collision and time- key might improve chances of probabilistic collision and time-
memory-tradeoff attacks succeeding. memory-tradeoff attacks succeeding.
As described, master keys MAY be shared between streams belonging As described, master keys MAY be shared between streams belonging
to the same RTP session, but it is RECOMMENDED that each SSRC have to the same RTP session, but it is RECOMMENDED that each SSRC have
its own master key. When master keys are shared among SSRC its own master key. When master keys are shared among SSRC
participants and SSRCs are managed by a key management module as participants and SSRCs are managed by a key management module as
recommended above, the RECOMMENDED policy for an SSRC collision recommended above, the RECOMMENDED policy for an SSRC collision
error is for the participant to leave the SRTP session as it is a error is for the participant to leave the SRTP session as it is a
sign of malfunction. sign of malfunction.
9.2 Key Usage 9.2 Key Usage
The effective key size is determined (upper bounded) by the size of The effective key size is determined (upper bounded) by the size of
the master key and, for encryption, the size of the salting key. Any the master key and, for encryption, the size of the salting key.
additive stream cipher is vulnerable to attacks that use statistical Any additive stream cipher is vulnerable to attacks that use
knowledge about the plaintext source to enable key collision and statistical knowledge about the plaintext source to enable key
time-memory tradeoff attacks [MF00,H80,BS00]. These attacks take collision and time-memory tradeoff attacks [MF00,H80,BS00]. These
advantage of commonalities among plaintexts, and provide a way for a attacks take advantage of commonalities among plaintexts, and
cryptanalyst to amortize the computational effort of decryption over provide a way for a cryptanalyst to amortize the computational
many keys, or over many bytes of output, thus reducing the effective effort of decryption over many keys, or over many bytes of output,
key size of the cipher. A detailed analysis of these attacks and thus reducing the effective key size of the cipher. A detailed
their applicability to the encryption of Internet traffic is analysis of these attacks and their applicability to the encryption
provided in [MF00]. In summary, the effective key size of SRTP when of Internet traffic is provided in [MF00]. In summary, the
used in a security system in which m distinct keys are used, is effective key size of SRTP when used in a security system in which m
equal to the key size of the cipher less the logarithm (base two) of distinct keys are used, is equal to the key size of the cipher less
m. Protection against such attacks can be provided simply by the logarithm (base two) of m. Protection against such attacks can
increasing the size of the keys used, which here can be accomplished be provided simply by increasing the size of the keys used, which
by the use of the salting key. Note that the salting key MUST be here can be accomplished by the use of the salting key. Note that
random but MAY be public. A salt size of (the suggested) size 112 the salting key MUST be random but MAY be public. A salt size of
bits protects against attacks in scenarios where at most 2^112 keys (the suggested) size 112 bits protects against attacks in scenarios
are in use. This is sufficient for all practical purposes. where at most 2^112 keys are in use. This is sufficient for all
practical purposes.
Implementations SHOULD use keys that are as large as possible. Implementations SHOULD use keys that are as large as possible.
Please note that in many cases increasing the key size of a cipher Please note that in many cases increasing the key size of a cipher
does not affect the throughput of that cipher. does not affect the throughput of that cipher.
The use of the SRTP and SRTCP indexes in the pre-defined transforms The use of the SRTP and SRTCP indexes in the pre-defined transforms
fixes the maximum number of packets that can be secured with the fixes the maximum number of packets that can be secured with the
same key. This limit is fixed to 2^48 SRTP packets for an SRTP same key. This limit is fixed to 2^48 SRTP packets for an SRTP
stream, and 2^31 SRTCP packets, when SRTP and SRTCP are considered stream, and 2^31 SRTCP packets, when SRTP and SRTCP are considered
independently. Due to for example re-keying, reaching this limit may independently. Due to for example re-keying, reaching this limit
or may not coincide with wrapping of the indices, and thus the may or may not coincide with wrapping of the indices, and thus the
sender MUST keep packet counts. However, when the session keys for sender MUST keep packet counts. However, when the session keys for
related SRTP and SRTCP streams are derived from the same master key related SRTP and SRTCP streams are derived from the same master key
(the default behavior, Section 4.3), the upper bound that has to be (the default behavior, Section 4.3), the upper bound that has to be
considered is in practice the minimum of the two quantities. That considered is in practice the minimum of the two quantities. That
is, when 2^48 SRTP packets or 2^31 SRTCP packets have been secured is, when 2^48 SRTP packets or 2^31 SRTCP packets have been secured
with the same key (whichever occurs before), the key management MUST with the same key (whichever occurs before), the key management MUST
be called to provide new master key(s) (previously stored and used be called to provide new master key(s) (previously stored and used
keys MUST NOT be used again), or the session MUST be terminated. If keys MUST NOT be used again), or the session MUST be terminated. If
a sender of RTCP discovers that the sender of SRTP (or SRTCP) has a sender of RTCP discovers that the sender of SRTP (or SRTCP) has
not updated the master or session key prior to sending 2^48 SRTP (or not updated the master or session key prior to sending 2^48 SRTP (or
2^31 SRTCP) packets belonging to the same SRTP (SRTCP) stream, it is 2^31 SRTCP) packets belonging to the same SRTP (SRTCP) stream, it is
up to the security policy of the RTCP sender how to behave, e.g. up to the security policy of the RTCP sender how to behave, e.g.
whether an RTCP BYE-packet should be sent and/or if the event should whether an RTCP BYE-packet should be sent and/or if the event should
be logged. be logged.
Note: in most typical applications (assuming at least one RTCP Note: in most typical applications (assuming at least one RTCP
packet for every 128,000 RTP packets), it will be the SRTCP index packet for every 128,000 RTP packets), it will be the SRTCP index
that first reaches the upper limit, although the time until this that first reaches the upper limit, although the time until this
skipping to change at page 37, line 49 skipping to change at page 39, line 50
communication. communication.
Note that if the master key is to be shared between SRTP streams Note that if the master key is to be shared between SRTP streams
within the same RTP session (Section 9.1), although the above bounds within the same RTP session (Section 9.1), although the above bounds
are on a per stream (i.e. per SSRC) basis, the sender MUST base re- are on a per stream (i.e. per SSRC) basis, the sender MUST base re-
key decision on the stream whose sequence number space is the first key decision on the stream whose sequence number space is the first
to be exhausted. to be exhausted.
Key derivation limits the amount of plaintext that is encrypted with Key derivation limits the amount of plaintext that is encrypted with
a fixed session key, and made available to an attacker for analysis, a fixed session key, and made available to an attacker for analysis,
but key derivation does not extend the master key's lifetime. To see but key derivation does not extend the master key's lifetime. To
this, simply consider our requirements to avoid two-time pad: two see this, simply consider our requirements to avoid two-time pad:
distinct packets MUST either be processed with distinct IVs, or with
distinct session keys, and both the distinctness of IV and of the
session keys are (for the pre-defined transforms) dependent on the
distinctness of the packet indices.
Note that with the key derivation, the effective key size is at most two distinct packets MUST either be processed with distinct IVs, or
that of the master key, even if the derived session key is with distinct session keys, and both the distinctness of IV and of
considerably longer. With the pre-defined authentication transform, the session keys are (for the pre-defined transforms) dependent on
the session authentication key is 160 bits, but the master key by the distinctness of the packet indices.
default is only 128 bits. This design choice was made to comply with
certain recommendations in [RFC2104] so that an existing HMAC Note that with the key derivation, the effective key size is at most
implementation can be plugged into SRTP without problems. Since the that of the master key, even if the derived session key is
default tag size is 80 bits, it is, for the applications in mind, considerably longer. With the pre-defined authentication transform,
also considered acceptable from security point of view. Users having the session authentication key is 160 bits, but the master key by
concerns about this are RECOMMENDED to instead use a 192 bit master default is only 128 bits. This design choice was made to comply with
key in the key derivation. It was, however, chosen not to mandate certain recommendations in [RFC2104] so that an existing HMAC
192-bit keys since existing AES implementations to be used in the implementation can be plugged into SRTP without problems. Since the
key-derivation may not always support key-lengths other than 128 default tag size is 80 bits, it is, for the applications in mind,
bits. Since AES is not defined (or properly analyzed) for use with also considered acceptable from security point of view. Users having
160 bit keys it is NOT RECOMMENDED that ad-hoc key-padding schemes concerns about this are RECOMMENDED to instead use a 192 bit master
are used to pad shorter keys to 192 or 256 bits. key in the key derivation. It was, however, chosen not to mandate
192-bit keys since existing AES implementations to be used in the
key-derivation may not always support key-lengths other than 128
bits. Since AES is not defined (or properly analyzed) for use with
160 bit keys it is NOT RECOMMENDED that ad-hoc key-padding schemes
are used to pad shorter keys to 192 or 256 bits.
9.3 Confidentiality of the RTP Payload 9.3 Confidentiality of the RTP Payload
SRTP's pre-defined ciphers are "seekable" stream ciphers, i.e. SRTP's pre-defined ciphers are "seekable" stream ciphers, i.e.
ciphers able to efficiently seek to arbitrary locations in their ciphers able to efficiently seek to arbitrary locations in their
keystream (so that the encryption or decryption of one packet does keystream (so that the encryption or decryption of one packet does
not depend on preceding packets). By using "seekable" stream not depend on preceding packets). By using seekable stream ciphers,
ciphers, SRTP avoids the denial of service attacks that are possible SRTP avoids the denial of service attacks that are possible on
on stream ciphers that lack this property. It is important to be stream ciphers that lack this property. It is important to be aware
aware that, as with any stream cipher, the exact length of the that, as with any stream cipher, the exact length of the payload is
payload is revealed by the encryption. This means that it may be revealed by the encryption. This means that it may be possible to
possible to deduce certain "formatting bits" of the payload, as the deduce certain "formatting bits" of the payload, as the length of
length of the codec output might vary due to certain parameter the codec output might vary due to certain parameter settings etc.
settings etc. This, in turn, implies that the corresponding bit of This, in turn, implies that the corresponding bit of the keystream
the keystream can be deduced. However, if the stream cipher is can be deduced. However, if the stream cipher is secure (counter
secure (counter mode and f8 are provably secure under certain mode and f8 are provably secure under certain assumptions
assumptions [BDJR,KSYH]), knowledge of a few bits of the keystream [BDJR,KSYH]), knowledge of a few bits of the keystream will not aid
will not aid an attacker in predicting subsequent keystream bits. an attacker in predicting subsequent keystream bits. Thus, the
Thus, the payload length (and information deducible from this) will payload length (and information deducible from this) will leak, but
leak, but nothing else. nothing else.
As some RTP packet could contain highly predictable data, e.g. SID, As some RTP packet could contain highly predictable data, e.g. SID,
it is important to use a cipher designed to resist known plaintext it is important to use a cipher designed to resist known plaintext
attacks (which is the current practice). attacks (which is the current practice).
9.4 Confidentiality of the RTP Header 9.4 Confidentiality of the RTP Header
In SRTP, RTP headers are sent in the clear to allow for header In SRTP, RTP headers are sent in the clear to allow for header
compression. This means that data such as payload type, compression. This means that data such as payload type,
synchronization source identifier, and timestamp are available to an synchronization source identifier, and timestamp are available to an
eavesdropper. Moreover, since RTP allows for future extensions of eavesdropper. Moreover, since RTP allows for future extensions of
headers, we cannot foresee what kind of possibly sensitive headers, we cannot foresee what kind of possibly sensitive
information might also be "leaked". information might also be "leaked".
SRTP is a low-cost method, which allows header compression to reduce SRTP is a low-cost method, which allows header compression to reduce
bandwidth. It is up to the endpoints' policies to decide about the bandwidth. It is up to the endpoints' policies to decide about the
security protocol to employ. If one really needs to protect headers, security protocol to employ. If one really needs to protect
and is allowed to do so by the surrounding environment, then one headers, and is allowed to do so by the surrounding environment,
should also look at alternatives, e.g., IPsec [RFC2401]. then one should also look at alternatives, e.g., IPsec [RFC2401].
9.5 Integrity of the RTP payload and header 9.5 Integrity of the RTP payload and header
SRTP messages are subject to attacks on their integrity and source SRTP messages are subject to attacks on their integrity and source
identification, and these risks are discussed in Section 9.5.1. To identification, and these risks are discussed in Section 9.5.1. To
protect against these attacks, each SRTP stream SHOULD be protected protect against these attacks, each SRTP stream SHOULD be protected
by HMAC-SHA1 [RFC2104] with an 80-bit output tag and a 160-bit key, by HMAC-SHA1 [RFC2104] with an 80-bit output tag and a 160-bit key,
or a message authentication code with equivalent strength. Secure or a message authentication code with equivalent strength. Secure
RTP SHOULD NOT be used without message authentication, except under RTP SHOULD NOT be used without message authentication, except under
the circumstances described in this section. It is important to the circumstances described in this section. It is important to
note that encryption algorithms, including AES Counter Mode and f8, note that encryption algorithms, including AES Counter Mode and f8,
do not provide message authentication. SRTCP MUST NOT be used with do not provide message authentication. SRTCP MUST NOT be used with
weak (or NULL) authentication. weak (or NULL) authentication.
SRTP MAY be used with weak authentication (e.g. a 32-bit SRTP MAY be used with weak authentication (e.g. a 32-bit
authentication tag), or with no authentication (the NULL authentication tag), or with no authentication (the NULL
authentication algorithm). These options allow SRTP to be used to authentication algorithm). These options allow SRTP to be used to
provide confidentiality in situations where provide confidentiality in situations where
* weak or null authentication is an acceptable security risk, and * weak or null authentication is an acceptable security risk, and
* it is impractical to provide strong message authentication. * it is impractical to provide strong message authentication.
skipping to change at page 41, line 9 skipping to change at page 43, line 9
During a security audit considering the use of weak or null During a security audit considering the use of weak or null
authentication, it is important to keep in mind the following authentication, it is important to keep in mind the following
attacks which are possible when no message authentication algorithm attacks which are possible when no message authentication algorithm
is used. is used.
An attacker who cannot predict the plaintext is still always able to An attacker who cannot predict the plaintext is still always able to
modify the message sent between the sender and the receiver so that modify the message sent between the sender and the receiver so that
it decrypts to a random plaintext value, or to send a stream of it decrypts to a random plaintext value, or to send a stream of
bogus packets to the receiver that will decrypt to random plaintext bogus packets to the receiver that will decrypt to random plaintext
values. This attack is essentially a denial of service attack, values. This attack is essentially a denial of service attack,
though in the absence of message authentication, the RTP application though in the absence of message authentication, the RTP application
will have inputs that are bit-wise correlated with the true value. will have inputs that are bit-wise correlated with the true value.
Some multimedia codecs and common operating systems will crash when Some multimedia codecs and common operating systems will crash when
such data are accepted as valid video data. This denial of service such data are accepted as valid video data. This denial of service
attack may be a much larger threat than that due to an attacker attack may be a much larger threat than that due to an attacker
dropping, delaying, or re-ordering packets. dropping, delaying, or re-ordering packets.
An attacker who cannot predict the plaintext can still replay a An attacker who cannot predict the plaintext can still replay a
previous message with certainty that the receiver will accept it. previous message with certainty that the receiver will accept it.
Applications with stateless codecs might be robust against this type Applications with stateless codecs might be robust against this type
of attack, but for other, more complex applications these attacks of attack, but for other, more complex applications these attacks
may be far more grave. may be far more grave.
An attacker who can predict the plaintext can modify the ciphertext An attacker who can predict the plaintext can modify the ciphertext
so that it will decrypt to any value of her choosing. so that it will decrypt to any value of her choosing. With an
With an additive stream cipher, an attacker will always be able to additive stream cipher, an attacker will always be able to change
change individual bits. individual bits.
An attacker may be able to subvert confidentiality due to the lack An attacker may be able to subvert confidentiality due to the lack
of authentication when a data forwarding or access control decision of authentication when a data forwarding or access control decision
is made on decrypted but unauthenticated plaintext. This is because is made on decrypted but unauthenticated plaintext. This is because
the receiver may be fooled into forwarding data to an attacker, the receiver may be fooled into forwarding data to an attacker,
leading to an indirect breach of confidentiality (see Section 3 of leading to an indirect breach of confidentiality (see Section 3 of
[B96]). This is because data-forwarding decisions are made on the [B96]). This is because data-forwarding decisions are made on the
decrypted plaintext; information in the plaintext will determine to decrypted plaintext; information in the plaintext will determine to
what subnet (or process) the plaintext is forwarded in ESP [RFC2401] what subnet (or process) the plaintext is forwarded in ESP [RFC2401]
tunnel mode (respectively, transport mode). When Secure RTP is used tunnel mode (respectively, transport mode). When Secure RTP is used
without message authentication, it should be verified that the without message authentication, it should be verified that the
application does not make data forwarding or access control application does not make data forwarding or access control
decisions based on the decrypted plaintext. decisions based on the decrypted plaintext.
Some cipher modes of operation that require padding, e.g. standard
cipher block chaining (CBC) are very sensitive to attacks on
confidentiality if certain padding types are used in the absence of
integrity. The attack [V02] shows that this is indeed the case for
the standard RTP padding as discussed in reference to Figure 1, when
used together with CBC mode. Later transform additions to SRTP MUST
therefore carefully consider the risk of using this padding without
proper integrity protection.
9.5.2 Implicit Header Authentication 9.5.2 Implicit Header Authentication
The IV formation of the f8-mode gives implicit authentication (IHA) The IV formation of the f8-mode gives implicit authentication (IHA)
of the RTP header, even when message authentication is not used. of the RTP header, even when message authentication is not used.
When IHA is used, an attacker that modifies the value of the RTP When IHA is used, an attacker that modifies the value of the RTP
header will cause the decryption process at the receiver to produce header will cause the decryption process at the receiver to produce
random plaintext values. While this protection is not equivalent to random plaintext values. While this protection is not equivalent to
message authentication, it may be useful for some applications. message authentication, it may be useful for some applications.
10. Interaction with Forward Error Correction mechanisms 10. Interaction with Forward Error Correction mechanisms
skipping to change at page 42, line 4 skipping to change at page 44, line 15
9.5.2 Implicit Header Authentication 9.5.2 Implicit Header Authentication
The IV formation of the f8-mode gives implicit authentication (IHA) The IV formation of the f8-mode gives implicit authentication (IHA)
of the RTP header, even when message authentication is not used. of the RTP header, even when message authentication is not used.
When IHA is used, an attacker that modifies the value of the RTP When IHA is used, an attacker that modifies the value of the RTP
header will cause the decryption process at the receiver to produce header will cause the decryption process at the receiver to produce
random plaintext values. While this protection is not equivalent to random plaintext values. While this protection is not equivalent to
message authentication, it may be useful for some applications. message authentication, it may be useful for some applications.
10. Interaction with Forward Error Correction mechanisms 10. Interaction with Forward Error Correction mechanisms
The default processing when using Forward Error Correction (e.g. RFC The default processing when using Forward Error Correction (e.g. RFC
2733) processing with SRTP SHALL be to perform FEC processing prior 2733) processing with SRTP SHALL be to perform FEC processing prior
to SRTP processing on the sender side and to perform SRTP processing to SRTP processing on the sender side and to perform SRTP processing
prior to FEC processing on the receiver side. Any change to this prior to FEC processing on the receiver side. Any change to this
ordering (reversing it, or, placing FEC between SRTP encryption and ordering (reversing it, or, placing FEC between SRTP encryption and
SRTP authentication) SHALL be signaled out of band. SRTP authentication) SHALL be signaled out of band.
11. Scenarios 11. Scenarios
SRTP can be used as security protocol for the RTP/RTCP traffic in SRTP can be used as security protocol for the RTP/RTCP traffic in
many different scenarios. SRTP has a number of configuration many different scenarios. SRTP has a number of configuration
options, in particular regarding key usage, and can have impact on options, in particular regarding key usage, and can have impact on
the total performance of the application according to the way it is the total performance of the application according to the way it is
used. Hence, the use of SRTP is dependent on the kind of scenario used. Hence, the use of SRTP is dependent on the kind of scenario
and application it is used with. In the following, we briefly and application it is used with. In the following, we briefly
illustrate some use cases for SRTP, and give some guidelines for illustrate some use cases for SRTP, and give some guidelines for
recommended setting of its options. recommended setting of its options.
11.1 Unicast 11.1 Unicast
A typical example would be a voice call or video-on-demand A typical example would be a voice call or video-on-demand
application. application.
Consider one bi-directional RTP stream, as one RTP session. It is Consider one bi-directional RTP stream, as one RTP session. It is
possible for the two parties to share the same master key in the two possible for the two parties to share the same master key in the two
directions according to the principles of Section 9.1. The first directions according to the principles of Section 9.1. The first
round of the key derivation splits the master key into any or all of round of the key derivation splits the master key into any or all of
the following session keys (according to the provided security the following session keys (according to the provided security
functions): functions):
SRTP_encr_key, SRTP_auth_key, SRTCP_encr_key, and SRTCP_auth key. SRTP_encr_key, SRTP_auth_key, SRTCP_encr_key, and SRTCP_auth key.
(For simplicity, we omit discussion of the salts, which are also (For simplicity, we omit discussion of the salts, which are also
derived.) In this scenario, it will in most cases suffice to have a derived.) In this scenario, it will in most cases suffice to have a
single master key with the default lifetime. This guarantees single master key with the default lifetime. This guarantees
sufficiently long lifetime of the keys and a minimum set of keys in sufficiently long lifetime of the keys and a minimum set of keys in
place for most practical purposes. Also, in this case RTCP place for most practical purposes. Also, in this case RTCP
protection can be applied smoothly. Under these assumptions, use of protection can be applied smoothly. Under these assumptions, use of
the MKI can be omitted. As the key-derivation in combination with the MKI can be omitted. As the key-derivation in combination with
large difference in the packet rate in the respective directions may large difference in the packet rate in the respective directions may
require simultaneous storage of several session keys, if storage is require simultaneous storage of several session keys, if storage is
an issue, we recommended to use low-rate key derivation. an issue, we recommended to use low-rate key derivation.
The same considerations can be extended to the unicast scenario with The same considerations can be extended to the unicast scenario with
multiple RTP sessions, where each session would have a distinct multiple RTP sessions, where each session would have a distinct
master key. master key.
11.2 Multicast (one sender) 11.2 Multicast (one sender)
Just as with (unprotected) RTP, a scalability issue arises in big Just as with (unprotected) RTP, a scalability issue arises in big
groups due to the possibly very large amount of SRTCP Receiver groups due to the possibly very large amount of SRTCP Receiver
Reports that the sender might need to process. In SRTP, the sender Reports that the sender might need to process. In SRTP, the sender
may have to keep state (the cryptographic context) for each may have to keep state (the cryptographic context) for each
receiver, or more precisely, for the SRTCP used to protect Receiver receiver, or more precisely, for the SRTCP used to protect Receiver
Reports. The overhead increases proportionally to the size of the Reports. The overhead increases proportionally to the size of the
group. In particular, re-keying requires special concern, see below. group. In particular, re-keying requires special concern, see
below.
Consider first a small group of receivers. There are a few possible Consider first a small group of receivers. There are a few possible
setups with the distribution of master keys among the receivers. setups with the distribution of master keys among the receivers.
Given a single RTP session, one possibility is that the receivers Given a single RTP session, one possibility is that the receivers
share the same master key as per Section 9.1 to secure all their share the same master key as per Section 9.1 to secure all their
respective RTCP traffic. This shared master key could then be the respective RTCP traffic. This shared master key could then be the
same one used by the sender to protect its outbound SRTP traffic. same one used by the sender to protect its outbound SRTP traffic.
Alternatively, it could be a master key shared only among the Alternatively, it could be a master key shared only among the
receivers and used solely for their SRTCP traffic. Both alternatives receivers and used solely for their SRTCP traffic. Both alternatives
requires the receivers to trust each other. requires the receivers to trust each other.
Considering SRTCP and key storage, it is recommended to use low-rate Considering SRTCP and key storage, it is recommended to use low-rate
(or zero) key_derivation (except the mandatory initial one), so that (or zero) key_derivation (except the mandatory initial one), so that
the sender does not need to store too many session keys (each SRTCP the sender does not need to store too many session keys (each SRTCP
stream might otherwise have a different session key at a given point stream might otherwise have a different session key at a given point
in time, as the SRTCP sources send at different times). Thus, in in time, as the SRTCP sources send at different times). Thus, in
case key derivation is wanted for SRTP, the cryptographic context case key derivation is wanted for SRTP, the cryptographic context
for SRTP can be kept separate from the SRTCP crypto context, so that for SRTP can be kept separate from the SRTCP crypto context, so that
it is possible to have a key_derivation_rate of 0 for SRTCP and a it is possible to have a key_derivation_rate of 0 for SRTCP and a
non-zero value for SRTP. non-zero value for SRTP.
Use of the MKI for re-keying is RECOMMENDED for most applications Use of the MKI for re-keying is RECOMMENDED for most applications
(see Section 8.1). (see Section 8.1).
If there are more than one SRTP/SRTCP stream (within the same RTP If there are more than one SRTP/SRTCP stream (within the same RTP
session) that share the master key, the upper limit of 2^48 SRTP session) that share the master key, the upper limit of 2^48 SRTP
packets / 2^31 SRTCP packets means that, before one of the streams packets / 2^31 SRTCP packets means that, before one of the streams
reaches its maximum number of packets, re-keying MUST be triggered reaches its maximum number of packets, re-keying MUST be triggered
on ALL streams sharing the master key. (From strict security point on ALL streams sharing the master key. (From strict security point
of view, only the stream reaching the maximum would need to be re- of view, only the stream reaching the maximum would need to be re-
keyed, but then the streams would no longer be sharing master key, keyed, but then the streams would no longer be sharing master key,
which is the intention.) A local policy at the sender side should which is the intention.) A local policy at the sender side should
force rekeying in a way that the maximum packet limit is not reached force rekeying in a way that the maximum packet limit is not reached
on any of the streams. Use of the MKI for re-keying is RECOMMENDED. on any of the streams. Use of the MKI for re-keying is RECOMMENDED.
In large multicast with one sender, the same considerations as for In large multicast with one sender, the same considerations as for
the small group multicast hold. The biggest issue in this scenario the small group multicast hold. The biggest issue in this scenario
is the additional load placed at the sender side, due to the state is the additional load placed at the sender side, due to the state
(cryptographic contexts) that has to be maintained for each (cryptographic contexts) that has to be maintained for each
receiver, sending back RTCP Receiver Reports. At minimum, a replay receiver, sending back RTCP Receiver Reports. At minimum, a replay
window might need to be maintained for each RTCP source. window might need to be maintained for each RTCP source.
11.3 Re-keying and access control 11.3 Re-keying and access control
Re-keying may occur due to access control (e.g., when a member is Re-keying may occur due to access control (e.g., when a member is
removed during a multicast RTP session), or for pure cryptographic removed during a multicast RTP session), or for pure cryptographic
reasons (e.g. the key is at the end of its lifetime). When using reasons (e.g. the key is at the end of its lifetime). When using
SRTP default transforms, the master key MUST be replaced before any SRTP default transforms, the master key MUST be replaced before any
of the index spaces are exhausted for any of the streams protected of the index spaces are exhausted for any of the streams protected
by one and the same master key. by one and the same master key.
How key management rekeys SRTP implementations is out of our scope, How key management re-keys SRTP implementations is out of scope, but
but it is clear that there are straightforward ways to manage keys it is clear that there are straightforward ways to manage keys for a
for a multicast group. In one-sender multicast, for example, it is multicast group. In one-sender multicast, for example, it is
typically the responsibility of the sender to determine when a new typically the responsibility of the sender to determine when a new
key is needed. The sender is the one entity that can keep track of key is needed. The sender is the one entity that can keep track of
when the maximum number of packets has been sent, as receivers may when the maximum number of packets has been sent, as receivers may
join and leave the session at any time, there may be packet loss and join and leave the session at any time, there may be packet loss and
delay etc. In scenarios other than one-sender multicast, other delay etc. In scenarios other than one-sender multicast, other
methods can be used. Here, one must take into consideration that key methods can be used. Here, one must take into consideration that
exchange can be a costly operation, taking several seconds for a key exchange can be a costly operation, taking several seconds for a
single exchange. Hence, some time before the master key is single exchange. Hence, some time before the master key is
exhausted/expires, out-of-band key management is initiated, exhausted/expires, out-of-band key management is initiated,
resulting in a new master key that is shared with the receiver(s). resulting in a new master key that is shared with the receiver(s).
In any event, to maintain synchronization when switching to the new In any event, to maintain synchronization when switching to the new
key, group policy might choose between using the MKI and the key, group policy might choose between using the MKI and the
<"From", "To">, as described in Section 8.1. <"From", "To">, as described in Section 8.1.
For access control purposes, the <"From", "To"> periods are set at For access control purposes, the <"From", "To"> periods are set at
the desired granularity, dependent on the packet rate. High rate re- the desired granularity, dependent on the packet rate. High rate
keying can be problematic for SRTCP in some large-group scenarios. re-keying can be problematic for SRTCP in some large-group
As mentioned, there are potential problems in using the SRTP index, scenarios. As mentioned, there are potential problems in using the
rather than the SRTCP index, for determining the master key. In SRTP index, rather than the SRTCP index, for determining the master
particular, for short periods during switching of master keys, it key. In particular, for short periods during switching of master
may be the case that SRTCP packets are not under the current master keys, it may be the case that SRTCP packets are not under the
key of the correspondent SRTP. Therefore, using the MKI for re- current master key of the correspondent SRTP. Therefore, using the
keying in such scenarios will produce better results. MKI for re-keying in such scenarios will produce better results.
11.4 Summary of basic scenarios 11.4 Summary of basic scenarios
The description of these scenarios highlights some recommendations The description of these scenarios highlights some recommendations
on the use of SRTP, mainly related to re-keying and large scale on the use of SRTP, mainly related to re-keying and large scale
multicast: multicast:
- Do not use fast re-keying with the <"From", "To"> - Do not use fast re-keying with the <"From", "To"> feature. It
feature. It may, in particular, give problems in retrieving the may, in particular, give problems in retrieving the correct SRTCP
correct SRTCP key, if an SRTCP packet arrives close to the re- key, if an SRTCP packet arrives close to the re-keying time. The
keying time. The MKI SHOULD be used in this case. MKI SHOULD be used in this case.
- If multiple SRTP streams in the same RTP session share the same - If multiple SRTP streams in the same RTP session share the same
master key, also moderate rate re-keying MAY have the same master key, also moderate rate re-keying MAY have the same
problems, and the MKI SHOULD be used. problems, and the MKI SHOULD be used.
- Though offering increased security, a non-zero key_derivation_rate - Though offering increased security, a non-zero key_derivation_rate
is NOT RECOMMENDED when trying to minimize the number of keys in is NOT RECOMMENDED when trying to minimize the number of keys in
use with multiple streams. use with multiple streams.
12. IANA Considerations 12. IANA Considerations
The RTP specification establishes a registry of profile names for The RTP specification establishes a registry of profile names for
use by higher-level control protocols, such as the Session use by higher-level control protocols, such as the Session
Description Protocol (SDP), to refer to transport methods. This Description Protocol (SDP), to refer to transport methods. This
profile registers the name "RTP/SAVP". profile registers the name "RTP/SAVP".
SRTP uses cryptographic transforms, which a key management protocol SRTP uses cryptographic transforms, which a key management protocol
signals. It is the task of each particular key management protocol signals. It is the task of each particular key management protocol
to register the cryptographic transforms or suites of transforms to register the cryptographic transforms or suites of transforms
with IANA. The key management protocol conveys these protocol with IANA. The key management protocol conveys these protocol
numbers, not SRTP, and each key management protocol chooses the numbers, not SRTP, and each key management protocol chooses the
numbering scheme and syntax that it requires. numbering scheme and syntax that it requires.
Specification of a key management protocol for SRTP is out of scope Specification of a key management protocol for SRTP is out of scope
here. Section 8.2, however, provides guidance on the parameters that here. Section 8.2, however, provides guidance on the parameters
need to be defined for the default and mandatory transforms. that need to be defined for the default and mandatory transforms.
13. Acknowledgements 13. Acknowledgements
The authors would like to thank Magnus Westerlund, Brian Weis, David Oran (Cisco) and Rolf Blom (Ericsson) are co-authors of this
Ghyslain Pelletier, Morgan Lindqvist, Robert Fairlie-Cuninghame, document but their valuable contributions are acknowledged here to
Adrian Perrig, the AVT WG, the Transport and Security Area keep the length of the author list down.
Directors, and Eric Rescorla for their reviews and comments.
The authors would in addition like to thank Magnus Westerlund, Brian
Weis, Ghyslain Pelletier, Morgan Lindqvist, Robert Fairlie-
Cuninghame, Adrian Perrig, the AVT WG and in particular the chairmen
Colin Perkins and Stephen Casner, the Transport and Security Area
Directors, and Eric Rescorla for their reviews and support.
14. Author's Addresses 14. Author's Addresses
Questions and comments should be directed to the authors and Questions and comments should be directed to the authors and
avt@ietf.org: avt@ietf.org:
Mark Baugher Mark Baugher
Cisco Systems, Inc. Cisco Systems, Inc.
5510 SW Orchid Street Phone: +1 408-853-4418 5510 SW Orchid Street Phone: +1 408-853-4418
Portland, OR 97219 USA Email: mbaugher@cisco.com Portland, OR 97219 USA Email: mbaugher@cisco.com
Rolf Blom
Ericsson Research
SE-16480 Stockholm Phone: +46 8 58531707
Sweden EMail: rolf.blom@era.ericsson.se
Elisabetta Carrara Elisabetta Carrara
Ericsson Research Ericsson Research
SE-16480 Stockholm Phone: +46 8 50877040 SE-16480 Stockholm Phone: +46 8 50877040
Sweden EMail: elisabetta.carrara@era.ericsson.se Sweden EMail: elisabetta.carrara@ericsson.com
David A. McGrew David A. McGrew
Cisco Systems, Inc. Cisco Systems, Inc.
San Jose, CA 95134-1706 Phone: +1 301-349-5815 San Jose, CA 95134-1706 Phone: +1 301-349-5815
USA EMail: mcgrew@cisco.com USA EMail: mcgrew@cisco.com
Mats Naslund Mats Naslund
Ericsson Research Ericsson Research
SE-16480 Stockholm Phone: +46 8 58533739 SE-16480 Stockholm Phone: +46 8 58533739
Sweden EMail: mats.naslund@era.ericsson.se Sweden EMail: mats.naslund@ericsson.com
Karl Norrman Karl Norrman
Ericsson Research Ericsson Research
SE-16480 Stockholm Phone: +46 8 4044502 SE-16480 Stockholm Phone: +46 8 4044502
Sweden EMail: karl.norrman@era.ericsson.se Sweden EMail: karl.norrman@ericsson.com
David Oran
Cisco Systems, Inc.
San Jose, CA 95134-1706
USA EMail: oran@cisco.com
15. References 15. References
Normative Normative
[AES] NIST, "Advanced Encryption Standard (AES)", FIPS PUB 197, [AES] NIST, "Advanced Encryption Standard (AES)", FIPS PUB 197,
http://www.nist.gov/aes/ http://www.nist.gov/aes/
[AVPNEW] Schulzrinne, H., Casner, S., RTP Profile for Audio and [AVPNEW] Schulzrinne, H., Casner, S., RTP Profile for Audio and
Video Conferences with Minimal Control, IETF Work in Progress, Video Conferences with Minimal Control, IETF Work in Progress,
March 2003. March 2003.
[RFC2104] Krawczyk, H., Bellare, M., and Canetti, R.: "HMAC: Keyed- [RFC2104] Krawczyk, H., Bellare, M., and Canetti, R.: "HMAC: Keyed-
hashing for message authentication". IETF RFC 2104, hashing for message authentication". IETF RFC 2104,
February 1997. February 1997.
[RTPNEW] Schulzrinne, H., Casner, S., Frederick, R., Jacobson,V., [RTPNEW] Schulzrinne, H., Casner, S., Frederick, R., Jacobson,V.,
"RTP: A Transport Protocol for Real-time Applications", "RTP: A Transport Protocol for Real-time Applications",
IETF Work in Progress, http://www.ietf.org/internet- IETF Work in Progress, http://www.ietf.org/internet-drafts/
drafts/draft-ietf-avt-rtp-new-12.txt, March 2003. draft-ietf-avt-rtp-new-12.txt, March 2003.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", IETF RFC 2119, March 1997. Requirement Levels", IETF RFC 2119, March 1997.
[RFC2401] Kent, S., and R. Atkinson, "Security Architecture for IP", [RFC2401] Kent, S., and R. Atkinson, "Security Architecture for IP",
IETF RFC 2401, November 1998. IETF RFC 2401, November 1998.
[RFC2675] Borman, D., Deering, S., Hinden, R., "IPv6 Jumbograms", [RFC2675] Borman, D., Deering, S., Hinden, R., "IPv6 Jumbograms",
IETF RFC 2675, August 1999. IETF RFC 2675, August 1999.
[RFC2828] Shirey, R., "Internet Security Glossary", IETF RFC 2828, [RFC2828] Shirey, R., "Internet Security Glossary", IETF RFC 2828,
May 2000. May 2000.
Informative Informative
[AES-CTR] Lipmaa, H., Rogaway, P., Wagner, D., "CTR-Mode [AES-CTR] Lipmaa, H., Rogaway, P., Wagner, D., "CTR-Mode
Encryption", NIST, Encryption", NIST, http://csrc.nist.gov/encryption/modes/
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[B96] Bellovin, S., "Problem Areas for the IP Security Protocols," [B96] Bellovin, S., "Problem Areas for the IP Security Protocols,"
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pp. 1-16, San Jose, CA, July 1996 pp. 1-16, San Jose, CA, July 1996
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Concrete Treatment of Symmetric Encryption: Analysis of DES Concrete Treatment of Symmetric Encryption: Analysis of DES
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ASIACRYPT 2000, LNCS 1976, pp. 1-13, Springer Verlag. ASIACRYPT 2000, LNCS 1976, pp. 1-13, Springer Verlag.
[C99] Crowell, W. P., "Introduction to the VENONA Project", [C99] Crowell, W. P., "Introduction to the VENONA Project",
http://www.nsa.gov:8080/docs/venona/index.html. http://www.nsa.gov:8080/docs/venona/index.html.
[CTR] Dworkin, M., NIST Special Publication 800-38A, "Recommendation [CTR] Dworkin, M., NIST Special Publication 800-38A, "Recommendation
for Block Cipher Modes of Operation: Methods and Techniques", for Block Cipher Modes of Operation: Methods and Techniques",
2001. http://csrc.nist.gov/publications/nistpubs/800-38a/ 2001. http://csrc.nist.gov/publications/nistpubs/800-38a/
sp800-38a.pdf. sp800-38a.pdf.
[f8-a] 3GPP TS 35.201 V4.1.0 (2001-12) Technical Specification 3rd [f8-a] 3GPP TS 35.201 V4.1.0 (2001-12) Technical Specification 3rd
Generation Partnership Project; Technical Specification Group Generation Partnership Project; Technical Specification Group
Services and System Aspects; 3G Security; Specification of the Services and System Aspects; 3G Security; Specification of the
3GPP Confidentiality and Integrity Algorithms; Document 1: f8 3GPP Confidentiality and Integrity Algorithms; Document 1: f8
and f9 Specification(Release 4). and f9 Specification(Release 4).
[f8-b] 3GPP TR 33.908 V4.0.0 (2001-09) Technical Report 3rd [f8-b] 3GPP TR 33.908 V4.0.0 (2001-09) Technical Report 3rd
Generation Partnership Project; Technical Specification Group Generation Partnership Project; Technical Specification Group
Services and System Aspects; 3G Security; General Report on the Services and System Aspects; 3G Security; General Report on
Design, Specification and Evaluation of 3GPP Standard the Design, Specification and Evaluation of 3GPP Standard
Confidentiality and Integrity Algorithms (Release 4). Confidentiality and Integrity Algorithms (Release 4).
[GDOI] Baugher, M., Hardjono, T., Harney, H., and Weis, B., "The [GDOI] Baugher, M., Hardjono, T., Harney, H., and Weis, B., "The
Group Domain of Interpretation, Accepted as IETF Proposed Group Domain of Interpretation, Accepted as IETF Proposed
Standard, http://www.ietf.org/internet-drafts/draft-ietf- Standard, http://www.ietf.org/internet-drafts/draft-ietf-
msec-gdoi-07.txt, 2003 msec-gdoi-07.txt, 2003
[HAC] Menezes, A., Van Oorschot, P., and Vanstone, S., "Handbook of [HAC] Menezes, A., Van Oorschot, P., and Vanstone, S., "Handbook of
Applied Cryptography", CRC Press, 1997, ISBN 0-8493-8523-7. Applied Cryptography", CRC Press, 1997, ISBN 0-8493-8523-7.
[H80] Hellman, M. E., "A cryptanalytic time-memory trade-off", IEEE [H80] Hellman, M. E., "A cryptanalytic time-memory trade-off", IEEE
Transactions on Information Theory, July 1980, pp. 401-406. Transactions on Information Theory, July 1980, pp. 401-406.
skipping to change at page 48, line 35 skipping to change at page 50, line 40
[KEYMGT] Arrko, J. et. al. Key Management Extensions for Session [KEYMGT] Arrko, J. et. al. Key Management Extensions for Session
Description Protocol (SDP) and Real Time Streaming Protocol Description Protocol (SDP) and Real Time Streaming Protocol
(RTSP), IETF Work in Progress, (RTSP), IETF Work in Progress,
http://www.ietf.org/internet-drafts/draft-ietf-mmusic- http://www.ietf.org/internet-drafts/draft-ietf-mmusic-
kmgmt-ext-07.txt, February 2003 kmgmt-ext-07.txt, February 2003
[KSYH] Kang, J-S., Shin, S-U., Hong, D., and Yi, O., "Provable [KSYH] Kang, J-S., Shin, S-U., Hong, D., and Yi, O., "Provable
Security of KASUMI and 3GPP Encryption Mode f8", Proceedings Security of KASUMI and 3GPP Encryption Mode f8", Proceedings
Asiacrypt 2001, Springer Verlag LNCS 2248, pp. 255-271, 2001. Asiacrypt 2001, Springer Verlag LNCS 2248, pp. 255-271, 2001.
[MIKEY] Arrko, J., et. al., "MIKEY: Multimedia Internet KEYing", [MIKEY] Arrko, J., et. al., "MIKEY: Multimedia Internet KEYing",
IETF Work in Progress, IETF Work in Progress, http://www.ietf.org/internet-drafts/
http://www.ietf.org/internet-drafts/draft-ietf-msec-mikey- draft-ietf-msec-mikey-06.txt, February 2003.
06.txt, February 2003.
[MF00] McGrew, D., and Fluhrer, S., "Attacks on Encryption of [MF00] McGrew, D., and Fluhrer, S., "Attacks on Encryption of
Redundant Plaintext and Implications on Internet Security", Redundant Plaintext and Implications on Internet Security",
the Proceedings of the Seventh Annual Workshop on Selected the Proceedings of the Seventh Annual Workshop on Selected
Areas in Cryptography (SAC 2000), Springer-Verlag. Areas in Cryptography (SAC 2000), Springer-Verlag.
[RK99] Rescorla, E., and Korver, B., "Guidelines for Writing RFC [RK99] Rescorla, E., and Korver, B., "Guidelines for Writing RFC
Text on Security Considerations," draft-rescorla-sec-cons- Text on Security Considerations," draft-rescorla-sec-cons-
00.txt 00.txt
skipping to change at page 49, line 28 skipping to change at page 51, line 31
IETF RFC 3242, April 2002. IETF RFC 3242, April 2002.
[SDMS] Baugher, M., Wing, D., "SDP Security Descriptions for Media [SDMS] Baugher, M., Wing, D., "SDP Security Descriptions for Media
Streams," IETF, Work in Progress, Streams," IETF, Work in Progress,
http://www.ietf.org/internet-drafts/draft-ietf-mmusic- http://www.ietf.org/internet-drafts/draft-ietf-mmusic-
sdescriptions-00.txt, February 2003. sdescriptions-00.txt, February 2003.
[SWO] Svanbro, K., Wiorek, J., and Olin, B., "Voice-over-IP-over- [SWO] Svanbro, K., Wiorek, J., and Olin, B., "Voice-over-IP-over-
wireless", Proc. PIMRC 2000, London, Sept. 2000. wireless", Proc. PIMRC 2000, London, Sept. 2000.
[V02] Vaudenay, S., "Security Flaws Induced by CBC Padding -
Application to SSL, IPsec, WTLS...", Advances in Cryptology,
EUROCRYPT'02, LNCS 2332, pp. 534-545.
[WC81] Wegman, M. N., and Carter, J.L, "New Hash Functions and Their [WC81] Wegman, M. N., and Carter, J.L, "New Hash Functions and Their
Use in Authentication and Set Equality", JCSS 22, 265-279, Use in Authentication and Set Equality", JCSS 22, 265-279,
1981. 1981.
16. Intellectual Property Right Considerations 16. Intellectual Property Right Considerations
The IETF takes no position regarding the validity or scope of any The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances claims of rights made available for publication and any assurances
of licenses to be made available, or the result of an attempt made of licenses to be made available, or the result of an attempt made
to obtain a general license or permission for the use of such to obtain a general license or permission for the use of such
proprietary rights by implementors or users of this proprietary rights by implementors or users of this specification
specification can be obtained from the IETF Secretariat. can be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive this standard. Please address the information to the IETF Executive
Director. Director.
17. Full Copyright Statement 17. Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved. Copyright (C) The Internet Society (2003). All Rights Reserved.
skipping to change at page 50, line 36 skipping to change at page 53, line 7
This document and the information contained herein is provided on an This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Appendix A: Pseudocode for Index Determination Appendix A: Pseudocode for Index Determination
The following is an example of pseudocode for the algorithm to The following is an example of pseudo-code for the algorithm to
determine the index i of an SRTP packet with sequence number SEQ. In determine the index i of an SRTP packet with sequence number SEQ.
the following, signed arithmetic is assumed. In the following, signed arithmetic is assumed.
if (s_l < 32,768) if (s_l < 32,768)
if (SEQ - s_l > 32,768) if (SEQ - s_l > 32,768)
set v to (ROC-1) mod 2^32 set v to (ROC-1) mod 2^32
else else
set v to ROC set v to ROC
endif endif
else else
if (s_l - 32,768 > SEQ) if (s_l - 32,768 > SEQ)
set v to (ROC+1) mod 2^32 set v to (ROC+1) mod 2^32
skipping to change at page 52, line 38 skipping to change at page 54, line 46
Counter Keystream Counter Keystream
F0F1F2F3F4F5F6F7F8F9FAFBFCFD0000 E03EAD0935C95E80E166B16DD92B4EB4 F0F1F2F3F4F5F6F7F8F9FAFBFCFD0000 E03EAD0935C95E80E166B16DD92B4EB4
F0F1F2F3F4F5F6F7F8F9FAFBFCFD0001 D23513162B02D0F72A43A2FE4A5F97AB F0F1F2F3F4F5F6F7F8F9FAFBFCFD0001 D23513162B02D0F72A43A2FE4A5F97AB
F0F1F2F3F4F5F6F7F8F9FAFBFCFD0002 41E95B3BB0A2E8DD477901E4FCA894C0 F0F1F2F3F4F5F6F7F8F9FAFBFCFD0002 41E95B3BB0A2E8DD477901E4FCA894C0
... ... ... ...
F0F1F2F3F4F5F6F7F8F9FAFBFCFDFEFF EC8CDF7398607CB0F2D21675EA9EA1E4 F0F1F2F3F4F5F6F7F8F9FAFBFCFDFEFF EC8CDF7398607CB0F2D21675EA9EA1E4
F0F1F2F3F4F5F6F7F8F9FAFBFCFDFF00 362B7C3C6773516318A077D7FC5073AE F0F1F2F3F4F5F6F7F8F9FAFBFCFDFF00 362B7C3C6773516318A077D7FC5073AE
F0F1F2F3F4F5F6F7F8F9FAFBFCFDFF01 6A2CC3787889374FBEB4C81B17BA6C44 F0F1F2F3F4F5F6F7F8F9FAFBFCFDFF01 6A2CC3787889374FBEB4C81B17BA6C44
Nota Bene: this test case is contrived so that the latter part of the Nota Bene: this test case is contrived so that the latter part of the
keystream segment coincides with the test case in Section F.5.1 of keystream segment coincides with the test case in Section F.5.1 of
[CTR]. [CTR].
B.3 Key Derivation Test Vectors B.3 Key Derivation Test Vectors
This section provides test data for the default key derivation This section provides test data for the default key derivation
function, which uses AES-128 in Counter Mode. In the following, we function, which uses AES-128 in Counter Mode. In the following, we
walk through the initial key derivation for the AES-128 Counter Mode walk through the initial key derivation for the AES-128 Counter Mode
cipher, which requires a 16 octet session encryption key and a 14 cipher, which requires a 16 octet session encryption key and a 14
octet session salt, and an authentication function which requires a octet session salt, and an authentication function which requires a
94-octet session authentication key. These values are called the 94-octet session authentication key. These values are called the
cipher key, the cipher salt, and the auth key in the following. cipher key, the cipher salt, and the auth key in the following.
Since this is the initial key derivation, the value of (index DIV Since this is the initial key derivation, the value of (index DIV
key_derivation_rate) is zero (actually, a six-octet string of key_derivation_rate) is zero (actually, a six-octet string of
zeros). In the following, we shorten key_derivation_rate to kdr. zeros). In the following, we shorten key_derivation_rate to kdr.
The inputs to the key derivation function are the 16 octet master The inputs to the key derivation function are the 16 octet master
key and the 14 octet master salt: key and the 14 octet master salt:
master key: E1F97A0D3E018BE0D64FA32C06DE4139 master key: E1F97A0D3E018BE0D64FA32C06DE4139
master salt: 0EC675AD498AFEEBB6960B3AABE6 master salt: 0EC675AD498AFEEBB6960B3AABE6
We first show how the cipher key is generated. The input block for We first show how the cipher key is generated. The input block for
AES-CM is generated by exclusive-oring the master salt with the AES-CM is generated by exclusive-oring the master salt with the
concatenation of the encryption key label 0x00 with (index DIV kdr), concatenation of the encryption key label 0x00 with (index DIV kdr),
then padding on the right with two null octets (which implements the then padding on the right with two null octets (which implements the
multiply-by-2^16 operation, see Section 4.3.3). The resulting value multiply-by-2^16 operation, see Section 4.3.3). The resulting value
is then AES-CM- encrypted using the master key to get the cipher is then AES-CM- encrypted using the master key to get the cipher
key. key.
index DIV kdr: 000000000000 index DIV kdr: 000000000000
label: 00 label: 00
master salt: 0EC675AD498AFEEBB6960B3AABE6 master salt: 0EC675AD498AFEEBB6960B3AABE6
----------------------------------------------- -----------------------------------------------
xor: 0EC675AD498AFEEBB6960B3AABE6 (x, PRF input) xor: 0EC675AD498AFEEBB6960B3AABE6 (x, PRF input)
x*2^16: 0EC675AD498AFEEBB6960B3AABE60000 (AES-CM input) x*2^16: 0EC675AD498AFEEBB6960B3AABE60000 (AES-CM input)
cipher key: C61E7A93744F39EE10734AFE3FF7A087 (AES-CM output) cipher key: C61E7A93744F39EE10734AFE3FF7A087 (AES-CM output)
Next, we show how the cipher salt is generated. The input block for Next, we show how the cipher salt is generated. The input block for
AES-CM is generated by exclusive-oring the master salt with the AES-CM is generated by exclusive-oring the master salt with the
concatenation of the encryption salt label. That value is padded concatenation of the encryption salt label. That value is padded
and encrypted as above. and encrypted as above.
index DIV kdr: 000000000000 index DIV kdr: 000000000000
label: 02 label: 02
master salt: 0EC675AD498AFEEBB6960B3AABE6 master salt: 0EC675AD498AFEEBB6960B3AABE6
---------------------------------------------- ----------------------------------------------
xor: 0EC675AD498AFEE9B6960B3AABE6 (x, PRF input) xor: 0EC675AD498AFEE9B6960B3AABE6 (x, PRF input)
x*2^16: 0EC675AD498AFEE9B6960B3AABE60000 (AES-CM input) x*2^16: 0EC675AD498AFEE9B6960B3AABE60000 (AES-CM input)
30CBBC08863D8C85D49DB34A9AE17AC6 (AES-CM ouptut) 30CBBC08863D8C85D49DB34A9AE17AC6 (AES-CM ouptut)
cipher salt: 30CBBC08863D8C85D49DB34A9AE1 cipher salt: 30CBBC08863D8C85D49DB34A9AE1
We now show how the auth key is generated. The input block for We now show how the auth key is generated. The input block for
AES-CM is generated as above, but using the authentication key AES-CM is generated as above, but using the authentication key
label. label.
index DIV kdr: 000000000000 index DIV kdr: 000000000000
label: 01 label: 01
master salt: 0EC675AD498AFEEBB6960B3AABE6 master salt: 0EC675AD498AFEEBB6960B3AABE6
----------------------------------------------- -----------------------------------------------
xor: 0EC675AD498AFEEAB6960B3AABE6 (x, PRF input) xor: 0EC675AD498AFEEAB6960B3AABE6 (x, PRF input)
x*2^16: 0EC675AD498AFEEAB6960B3AABE60000 (AES-CM input) x*2^16: 0EC675AD498AFEEAB6960B3AABE60000 (AES-CM input)
skipping to change at page 54, line 24 skipping to change at page 56, line 33
AES input blocks are shown on the right. AES input blocks are shown on the right.
auth key AES input blocks auth key AES input blocks
CEBE321F6FF7716B6FD4AB49AF256A15 0EC675AD498AFEEAB6960B3AABE60000 CEBE321F6FF7716B6FD4AB49AF256A15 0EC675AD498AFEEAB6960B3AABE60000
6D38BAA48F0A0ACF3C34E2359E6CDBCE 0EC675AD498AFEEAB6960B3AABE60001 6D38BAA48F0A0ACF3C34E2359E6CDBCE 0EC675AD498AFEEAB6960B3AABE60001
E049646C43D9327AD175578EF7227098 0EC675AD498AFEEAB6960B3AABE60002 E049646C43D9327AD175578EF7227098 0EC675AD498AFEEAB6960B3AABE60002
6371C10C9A369AC2F94A8C5FBCDDDC25 0EC675AD498AFEEAB6960B3AABE60003 6371C10C9A369AC2F94A8C5FBCDDDC25 0EC675AD498AFEEAB6960B3AABE60003
6D6E919A48B610EF17C2041E47403576 0EC675AD498AFEEAB6960B3AABE60004 6D6E919A48B610EF17C2041E47403576 0EC675AD498AFEEAB6960B3AABE60004
6B68642C59BBFC2F34DB60DBDFB2 0EC675AD498AFEEAB6960B3AABE60005 6B68642C59BBFC2F34DB60DBDFB2 0EC675AD498AFEEAB6960B3AABE60005
This draft expires in November 2003 This draft expires in December 2003.
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