< draft-ietf-msec-tesla-intro-01.txt   draft-ietf-msec-tesla-intro-02.txt >
Internet Engineering Task Force IETF MSEC Internet Engineering Task Force IETF MSEC
Internet Draft Perrig, Canetti, Song, Tygar, Briscoe Internet Draft Perrig, Canetti, Song, Tygar, Briscoe
draft-ietf-msec-tesla-intro-01.txt UC Berkeley/Digital Fountain/IBM/BT draft-ietf-msec-tesla-intro-02.txt UC Berkeley / Digital Fountain / IBM / BT
27 October 2002 May 2004
Expires: 27 April 2002 Expires: November 2004
TESLA: Multicast Source Authentication Transform Introduction TESLA: Multicast Source Authentication Transform Introduction
STATUS OF THIS MEMO STATUS OF THIS MEMO
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Abstract Abstract
Data authentication is an important component for many applications, Data authentication is an important component for many applications,
for example audio and video Internet broadcasts, or data distribution for example audio and video Internet broadcasts, or data distribution
by satellite. This document introduces TESLA, a secure source authen­ by satellite. This document introduces TESLA, a secure source
tication mechanism for multicast or broadcast data streams. This doc­ authentication mechanism for multicast or broadcast data streams. This
ument provides an algorithmic description of the scheme for informa­ document provides an algorithmic description of the scheme for
tional purposes, and in particular, it is intended to assist in writ­ informational purposes, and in particular, it is intended to assist
ing standardizable and secure specifications for protocols based on in writing standardizable and secure specifications for protocols
TESLA in different contexts. based on TESLA in different contexts.
The main deterrents so far for a data authentication mechanism for The main deterrents so far for a data authentication mechanism for
multicast were the seemingly conflicting requirements: loss toler­ multicast were the seemingly conflicting requirements: loss tolerance,
ance, high efficiency, no per-receiver state at the sender. The prob­ high efficiency, no per-receiver state at the sender. The problem
lem is particularly hard in settings with high packet loss rates and is particularly hard in settings with high packet loss rates and
where lost packets are not retransmitted, and where the receiver where lost packets are not retransmitted, and where the receiver
wants to authenticate each packet it receives. wants to authenticate each packet it receives.
TESLA provides authentication of individual data packets, regardless TESLA provides authentication of individual data packets, regardless
of the packet loss rate. In addition, TESLA features low overhead for of the packet loss rate. In addition, TESLA features low overhead for
both sender and receiver, and does not require per-receiver state at both sender and receiver, and does not require per-receiver state at
the sender. TESLA is secure as long as the sender and receiver are the sender. TESLA is secure as long as the sender and receiver are
loosely time synchronized. loosely time synchronized.
Table of Contents Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . 2 1 Introduction . . . . . . . . . . . . . . . . . . . . . . 2
2 Functionality . . . . . . . . . . . . . . . . . . . . . . 3 1.1 Notation . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Functionality . . . . . . . . . . . . . . . . . . . . . . 4
2.1 Threat Model and Security Guarantee . . . . . . . . . . . 4 2.1 Threat Model and Security Guarantee . . . . . . . . . . . 4
2.2 Assumptions . . . . . . . . . . . . . . . . . . . . . . . 5 2.2 Assumptions . . . . . . . . . . . . . . . . . . . . . . . 5
3 Notation . . . . . . . . . . . . . . . . . . . . . . . . 5 3 The Basic TESLA Protocol . . . . . . . . . . . . . . . . 5
4 The Basic TESLA Protocol . . . . . . . . . . . . . . . . 5 3.1 Sketch of protocol . . . . . . . . . . . . . . . . . . . 6
4.1 Sketch of protocol . . . . . . . . . . . . . . . . . . . 6 3.2 Sender Setup . . . . . . . . . . . . . . . . . . . . . . 7
4.2 Sender Setup . . . . . . . . . . . . . . . . . . . . . . 7 3.3 Bootstrapping Receivers . . . . . . . . . . . . . . . . . 7
4.3 Bootstrapping Receivers . . . . . . . . . . . . . . . . . 7 3.3.1 Time Synchronization. . . . . . . . . . . . . . . . . . . 8
4.4 Broadcasting Authenticated Messages . . . . . . . . . . . 8 3.4 Broadcasting Authenticated Messages . . . . . . . . . . . 8
4.5 Authentication at Receiver . . . . . . . . . . . . . . . 8 3.5 Authentication at Receiver . . . . . . . . . . . . . . . 8
4.6 Determining the Key Disclosure Delay . . . . . . . . . . 9 3.6 Determining the Key Disclosure Delay . . . . . . . . . . 9
4.7 Some extenstions. . . . . . . . . . . . . . . . . . . . . 10 3.7 An alternative delay description method . . . . . . . . . 10
5 Layer placement . . . . . . . . . . . . . . . . . . . . . 10 3.8 Some extensions . . . . . . . . . . . . . . . . . . . . . 11
6 Acknowledgments . . . . . . . . . . . . . . . . . . . . . 10 4 Layer placement . . . . . . . . . . . . . . . . . . . . . 11
7 Bibliography . . . . . . . . . . . . . . . . . . . . . . 10 5 Security considerations . . . . . . . . . . . . . . . . . 11
A Author Contact Information . . . . . . . . . . . . . . . 12 6 Acknowledgments . . . . . . . . . . . . . . . . . . . . . 12
B Full Copyright Statement . . . . . . . . . . . . . . . . 13 7 Bibliography . . . . . . . . . . . . . . . . . . . . . . 12
A Author Contact Information . . . . . . . . . . . . . . . 13
B Full Copyright Statement . . . . . . . . . . . . . . . . 14
1 Introduction 1 Introduction
The power of multicast is that one packet can reach millions of The power of multicast is that one packet can reach millions of
receivers. This great property is unfortunately also a great danger: receivers. This great property is unfortunately also a great danger:
an attacker that sends one malicious packet can also potentially an attacker that sends one malicious packet can also potentially
reach millions of receivers. Receivers need multicast source authen­ reach millions of receivers. Receivers need multicast source
tication to ensure that a given packet originates from the correct authentication to ensure that a given packet originates from the correct
source. source.
In unicast communication, we can achieve data authentication through In unicast communication, we can achieve data authentication through
a purely symmetric mechanism: the sender and the receiver share a a purely symmetric mechanism: the sender and the receiver share a
secret key to compute a message authentication code (MAC) of all com­ secret key to compute a message authentication code (MAC) of all
municated data. When a message with a correct MAC arrives, the communicated data. When a message with a correct MAC arrives, the
receiver is assured that the sender generated that message. Standard receiver is assured that the sender generated that message. Standard
mechanisms achieve unicast authentication this way, for example TLS mechanisms achieve unicast authentication this way, for example TLS
or IPsec [1,2]. or IPsec [1,2].
The symmetric MAC authentication is not secure in a broadcast set­ The symmetric MAC authentication is not secure in a broadcast
ting. Consider a sender that broadcasts authentic data to mutually setting. Consider a sender that broadcasts authentic data to mutually
untrusted receivers. The symmetric MAC is not secure: every receiver untrusting receivers. The symmetric MAC is not secure: every receiver
knows the MAC key, and hence could impersonate the sender and forge knows the MAC key, and hence could impersonate the sender and forge
messages to other receivers. Intuitively, we need an asymmetric mech­ messages to other receivers. Intuitively, we need an asymmetric
anism to achieve authenticated broadcast, such that every receiver mechanism to achieve authenticated broadcast, such that every receiver
can verify the authenticity of messages it receives, without being can verify the authenticity of messages it receives, without being
able to generate authentic messages. Achieving this in an efficient able to generate authentic messages. Achieving this in an efficient
way is a challenging problem [3]. way is a challenging problem [3].
The standard approach to achieve such asymmetry for authentication is The standard approach to achieve such asymmetry for authentication is
to use asymmetric cryptography, for instance a digital signature. to use asymmetric cryptography, for instance a digital signature.
Digital signatures have the required asymmetric property: the sender Digital signatures have the required asymmetric property: the sender
generates the signature with its private key, and all receivers can generates the signature with its private key, and all receivers can
verify the signature with the sender's public key, but a receiver verify the signature with the sender's public key, but a receiver
with the public key alone cannot generate a digital signature for a with the public key alone cannot generate a digital signature for a
new message. A digital signature provides non-repudiation, which is a new message. A digital signature provides non-repudiation, which is a
stronger property than authentication. Unfortunately, digital signa­ stronger property than authentication. Unfortunately, digital
tures have a high cost: they have a high computation overhead for signatures have a high cost: they have a high computation overhead for
both the sender and the receiver, as well as a high communication both the sender and the receiver, as well as a high communication
overhead. Since we assume broadcast settings where the sender does overhead. Since we assume broadcast settings where the sender does
not retransmit lost packets, and the receiver still wants to immedi­ not retransmit lost packets, and the receiver still wants to
ately authenticate each packet it receives, we would need to attach a immediately authenticate each packet it receives, we would need to
digital signature to each message. Because of the high overhead of attach a digital signature to each message. Because of the high
asymmetric cryptography, this approach would restrict us to low-rate overhead of asymmetric cryptography, this approach would restrict
streams, and to senders and receivers with powerful workstations. To us to low-rate streams, and to senders and receivers with powerful
deal with the high overhead of asymmetric cryptography, we can try to workstations. To deal with the high overhead of asymmetric cryptography,
amortize one digital signature over multiple messages. However, such we can try to amortize one digital signature over multiple messages.
an approach is still expensive in contrast to symmetric cryptography, However, such an approach is still expensive in contrast to symmetric
since symmetric cryptography is in general 3 to 5 orders of magnitude cryptography, since symmetric cryptography is in general 3 to 5 orders
more efficient than asymmetric cryptography. In addition, the of magnitude more efficient than asymmetric cryptography. In addition,
straight-forward amortization of one digital signature over multiple the straight-forward amortization of one digital signature over multiple
packets requires reliability, as the receiver needs to receive all packets requires reliability, as the receiver needs to receive all
packets to verify the signature. A number of schemes that follow this packets to verify the signature. A number of schemes that follow this
approach are [4,5,6,7,8]. See [9] for more details. approach are [4,5,6,7,8]. See [9] for more details.
This draft presents the Timed Efficient Stream Loss-tolerant Authen­ This document presents the Timed Efficient Stream Loss-tolerant
tication protocol (TESLA). TESLA uses mainly symmetric cryptography, Authentication protocol (TESLA). TESLA uses mainly symmetric
and uses time delayed key disclosure to achieve the required asymme­ cryptography, and uses time delayed key disclosure to achieve the
try property. However, TESLA requires loosely synchronized clocks required asymmetry property. However, TESLA requires loosely
between the sender and the receivers. See more details in Section 4. synchronized clocks between the sender and the receivers. See more
Other schemes that follow a similar approach to TESLA are [10,11,12]. details in Section 4. Other schemes that follow a similar approach
to TESLA are [10,11,12].
1.1 Notation
To denote the subscript or an index of a variable, we use the
underscore between the variable name and the index, e.g. the key K with
index i is K_i, the key K with index i+d is K_{i+d}. To write a
superscript we use the caret, e.g. the function F with the argument x
executed i times is F^i(x), executed j-1 times we write F^{j-1}(x).
2 Functionality 2 Functionality
TESLA provides delayed per-packet data authentication. The key idea TESLA provides delayed per-packet data authentication. The key idea
to providing both efficiency and security is a delayed disclosure of to providing both efficiency and security is a delayed disclosure of
keys. The delayed key disclosure results in an authentication delay. keys. The delayed key disclosure results in an authentication delay.
In practice, the delay is on the order of one RTT (Round-trip-time). In practice, the delay is on the order of one RTT (Round-trip-time).
TESLA has the following properties: TESLA has the following properties:
· Low computation overhead for generation and verification of ¸ Low computation overhead for generation and verification of
authentication information authentication information
· Low communication overhead ¸ Low communication overhead
· Limited buffering required for the sender and the receiver, hence ¸ Limited buffering required for the sender and the receiver, hence
timely authentication for each individual packet timely authentication for each individual packet
· Strong robustness to packet loss ¸ Strong robustness to packet loss
· Scales to a large number of receivers ¸ Scales to a large number of receivers
· Security is guaranteed as long as the sender and recipients are ¸ Security is guaranteed as long as the sender and recipients are
loosely time synchronized, where synchronization can take place loosely time synchronized, where synchronization can take place
at session set-up. at session set-up.
TESLA can be used both in the network layer or in the application TESLA can be used either in the network layer, or in the transport
layer. The delayed authentication, however, requires buffering of layer, or in the application layer. The delayed authentication,
packets until authentication is completed. however, requires buffering of packets until authentication is completed.
2.1 Threat Model and Security Guarantee 2.1 Threat Model and Security Guarantee
We design TESLA to be secure against a powerful adversary with the We design TESLA to be secure against a powerful adversary with the
following capabilities: following capabilities:
· Full control over the network. The adversary can eavesdrop, cap­ ¸ Full control over the network. The adversary can eavesdrop,
ture, drop, resend, delay, and alter packets. capture, drop, resend, delay, and alter packets.
· Access to a fast network with negligible delay. ¸ Access to a fast network with negligible delay.
· The adversary's computational resources may be very large, but ¸ The adversary's computational resources may be very large, but
not unbounded. In particular, this means that the adversary can not unbounded. In particular, this means that the adversary can
perform efficient computations, such as computing a reasonable perform efficient computations, such as computing a reasonable
number of pseudo-random function applications and MACs with neg­ number of pseudo-random function applications and MACs with
ligible delay. Nonetheless, the adversary cannot find the key of negligible delay. Nonetheless, the adversary cannot find the key
a pseudorandom function (or distinguish it from a random func­ of a pseudorandom function (or distinguish it from a random
tion) with non-negligible probability. function) with non-negligible probability.
The security property of TESLA guarantees that the receiver never The security property of TESLA guarantees that the receiver never
accepts M_i as an authentic message unless the sender really sent accepts M_i as an authentic message unless the sender really sent
M_i. A scheme that provides this guarantee is called a secure broad­ M_i. A scheme that provides this guarantee is called a secure
cast authentication scheme. broadcast authentication scheme.
Since TESLA requires the receiver to buffer packets before authenti­ Since TESLA requires the receiver to buffer packets before
cation, the receiver needs to protect itself from a potential denial- authentication, the receiver needs to protect itself from a
of-service (DOS) attack due to a flood of bogus packets. potential denial-of-service (DOS) attack due to a flood of bogus packets.
2.2 Assumptions 2.2 Assumptions
TESLA makes the following assumptions in order to provide security: TESLA makes the following assumptions in order to provide security:
1. The sender and the receiver MUST be loosely time synchronized. 1. The sender and the receiver must be be able to loosely
Loosely time synchronized means that the synchronization does synchronize. Specifically, each receiver must be able to
not need to be precise, but the receiver MUST know an upper compute an upper bound on the lag of the receiver clock
bound on the dispersion (the maximum clock offset). For the relative to the sender clock. We denote this quantity by D_t.
purposes of this draft, we assume that the receiver knows the (That is, D_t = sender time - receiver time).
maximum clock offset between its clock and the sender's clock, We note that an upper bound on D_t can be easily obtained via
which we denote with D_t. We stress that the sender and a simple two-message exchange. (Such an exchange can be
receiver's clock do not need to be synchronized a-priori. piggybacked on any session initiation protocol. Alternatively,
Instead, the receiver can easily achieve the required synchro­ standard protocols such as as NTP [16] can be used.
nization through a two-round message exchange with the sender. (The synchronization assumption of TESLA is considerably weaker
(This stands in contrast with authentication protocols based the synchronization requirements of authentication protocols based
on timestamps. In those protocols, the participants are on timestamps. In those protocols, the participants are
assumed to have the same global time a-priori.) assumed to have the same global time a-priori.)
2. TESLA MUST be bootstrapped at session set-up through a regular 2. TESLA MUST be bootstrapped at session set-up through a regular
data authentication system. We recommend to use a digital sig­ data authentication system. We recommend to use a digital
nature algorithm for this purpose, in which case the receiver signature algorithm for this purpose, in which case the receiver
is REQUIRED to have an authentic copy of either the sender's is REQUIRED to have an authentic copy of either the sender's
public key certificate or a root key certificate in case of a public key certificate or a root key certificate in case of a
PKI (public-key infrastructure). PKI (public-key infrastructure).
3. TESLA uses cryptographic MAC and PRF (pseudo-random func­ 3. TESLA uses cryptographic MAC and PRF (pseudo-random
tions). These MUST be cryptographically secure. Further functions). These MUST be cryptographically secure. Further
details on the instantiation of the MAC and PRF are in Section details on the instantiation of the MAC and PRF are in Section
4.2. 4.2.
4. We would like to emphasize that the security of TESLA does NOT 4. We would like to emphasize that the security of TESLA does NOT
rely on any assumptions on network propagation delay. rely on any assumptions on network propagation delay.
3 Notation 3 The Basic TESLA Protocol
To denote the subscript or an index of a variable, we use the under­
score between the variable name and the index, e.g. the key K with
index i is K_i, the key K with index i+d is K_{i+d}. To write a
superscript we use the caret, e.g. the function F with the argument x
executed i times is F^i(x), executed j-1 times we write F^{j-1}(x).
4 The Basic TESLA Protocol
TESLA is described in several academic publications: A book on broad­ TESLA is described in several academic publications: A book on
cast security [13], a journal paper [14], and two conference papers broadcast security [13], a journal paper [14], and two conference papers
[8,15]. Please refer to these publications for an in-depth treatment. [8,15]. Please refer to these publications for an in-depth treatment.
4.1 Sketch of protocol 3.1 Sketch of protocol
We first outline the main ideas behind TESLA. We first outline the main ideas behind TESLA.
As we argue in the introduction, broadcast authentication requires a As we argue in the introduction, broadcast authentication requires a
source of asymmetry. TESLA uses time for asymmetry. We assume that source of asymmetry. TESLA uses time for asymmetry. We first make sure
the sender and receivers are all loosely time synchronized -- up to that the sender and receivers are loosely time synchronized as described
some D_t value, all parties agree on the current time. The sender above. Next, the sender forms a one-way chain of keys, where each key in
forms a one-way chain, where each such value is associated with a chain is associated with a time interval (say, a second). Here is the
time interval (say, a second). Here is the basic approach: basic approach:
· The sender attaches a MAC to each packet. The MAC is computed ¸ The sender attaches a MAC to each packet. The MAC is computed
over the contents of the packet. For each packet, the sender uses over the contents of the packet. For each packet, the sender uses
the current value from the one-way chain as a cryptographic key the current key from the one-way chain as a cryptographic key
to compute the MAC. to compute the MAC.
· Each receiver receives the packet. Each receiver knows the sched­ ¸ The sender discloses a key from the one-way chain after some
ule for disclosing keys and, since the clocks are loosely syn­ pre-defined time delay. (e.g., the key used in time interval i
chronized, can check that the key used to compute the MAC is is disclosed at time interval i+3.)
still secret by determining that the sender could not have yet
reached the time for disclosing it. If the MAC key is still
secret, then the receiver buffers the packet.
· According to a schedule, the sender discloses the key from the
one-way chain.
· Each receiver checks that the disclosed key is correct (using ¸ Each receiver receives the packet. Each receiver knows the
previously released keys) and then checks the correctness of the schedule for disclosing keys and, since it has an upper bound on
MAC. If the MAC is correct, the receiver accepts the packet. the local time at the sender, it can check that the key used to
compute the MAC was not yet disclosed by the sender. If so, then
the receiver buffers the packet. Otherwise the packet is dropped.
(Note that we do not know for sure whether a "late packet" is a
bogus one or simply a delayed packet. We drop the packet since we
are unable to authenticate it.)
Note that one way chains have the property that if intermediate val­ ¸ Each receiver checks that the disclosed key belongs to the hash-chain
ues of the one-way chain are lost, they can be recomputed using the (by checking against previously released keys in the chain) and then
following values. So, even if some key disclosures are lost, a checks the correctness of the MAC. If the MAC is correct, the
receiver can recover the key chain and check the correctness of ear­ receiver accepts the packet.
lier packets.
The sender distributes a stream of messages {M_i}, and the sender Note that one-way chains have the property that if intermediate
sends each message M_i in a network packet P_i along with authentica­ values of the one-way chain are lost, they can be recomputed using
tion information. The broadcast channel may be lossy, but in many subsequent values in the chain . So, even if some key disclosures
broadcast applications the sender does not retransmit lost packets. are lost, a receiver can recover the corresponding keys and check
Despite packet loss, each receiver needs to authenticate every mes­ the correctness of earlier packets.
sage it receives.
We now describe the stages of the basic TESLA protocol in this order: We now describe the stages of the basic TESLA protocol in this order:
sender setup, receiver bootstrap, sender transmission of authenti­ sender setup, receiver bootstrap, sender transmission of
cated broadcast messages, and receiver authentication of broadcast authenticated broadcast messages, and receiver authentication of
messages. broadcast messages.
4.2 Sender Setup 3.2 Sender Setup
The sender divides the time into uniform intervals of duration T_int. The sender divides the time into uniform intervals of duration T_int.
The sender assigns one key from the one-way chain to each time inter­ The sender assigns one key from the one-way chain to each time
val in sequence. interval in sequence.
The sender determines the length N of the one-way chain K_0, K_1, The sender determines the length N of the one-way chain K_0, K_1,
..., K_N, and this length limits the maximum transmission duration ..., K_N, and this length limits the maximum transmission duration
before a new one-way chain must be created. The sender picks a random before a new one-way chain must be created. The sender picks a random
value for K_N. Using a pseudo-random function (PRF) f, the sender value for K_N. Using a pseudo-random function (PRF) f, the sender
constructs the one-way function F: F(k) = f_k(0). The rest of the constructs the one-way function F: F(k) = f_k(0). The rest of the
chain is computed recursively using K_i = F(K_{i+1}). Note that this chain is computed recursively using K_i = F(K_{i+1}). Note that this
gives us K_i = F^{N-i}(K_N), so the receiver can compute any value in gives us K_i = F^{N-i}(K_N), so the receiver can compute any value in
the key chain from K_N even if is does not have intermediate values. the key chain from K_N even if is does not have intermediate values.
The key K_i will be used to authenticate packets sent in time inter­ The key K_i will be used to authenticate packets sent in time
val i. interval i.
4.3 Bootstrapping Receivers 3.3 Bootstrapping Receivers
Before a receiver can authenticate messages with TESLA, it needs to Before a receiver can authenticate messages with TESLA, it needs to
be loosely time synchronized with the sender, know the disclosure have:
schedule of keys, and receive an authenticated key of the one-way key * An upper bound D_t on the lag of its own clock with respect to
chain. the clock of the sender. (That is, if the local time reading is t,
the current time reading at the sender is at most t+D_t.).
Various approaches exist for time synchronization [16,17,18,19]. * The disclosure schedule of keys. (Note that this information is not
TESLA, however, only requires loose time synchronization between the essential. See details below.)
sender and the receivers, so a simple algorithm is sufficient. The * One authenticated key of the one-way key chain. (Typically, this
time synchronization property that TESLA requires is that each will be the last key in the chain, i.e. K_0, this key will be
receiver can place an upper bound of the senders local time. TESLA signed by the sender, and all receivers will verify the signature against
offers direct, indirect, and delayed synchronization as three default the public key of the signer.
options, which we will describe in the TESLA technical draft.
The sender sends the key disclosure schedule by transmitting the fol­ The sender sends the key disclosure schedule by transmitting the
lowing information to the receivers over an authenticated channel following information to the receivers over an authenticated channel
(either via a digitally signed broadcast message, or over an authen­ (either via a digitally signed broadcast message, or over an
ticated unicast channel with each receiver): authenticated unicast channel with each receiver):
· Time interval schedule: interval duration T_int, start time and ¸ Time interval schedule: interval duration T_int, start time of
index of interval i, length of one-way key chain. interval i and index of interval i, length of one-way key chain.
· Key disclosure delay d (number of intervals). ¸ Key disclosure delay d (number of intervals).
· A key commitment to the key chain K_i (i < j - d + 1, where j is ¸ A commitment to the key chain K_i (i < j - d + 1, where j is
the current interval index). the current interval index).
The receiver can perform the time synchronization and getting the The receiver can perform the time synchronization and getting the
authenticated TESLA parameters in a two-round message exchange, which authenticated TESLA parameters in a two-round message exchange, which
we will describe in the technical TESLA draft. Time synchronization we will describe in the technical TESLA document. Time synchronization
can be performed as part of the registration protocol between member can be performed as part of the registration protocol between member
and sender. and sender.
4.4 Broadcasting Authenticated Messages 3.3.1 Time Synchronization
Various approaches exist for time synchronization [16,17,18,19].
TESLA, however, only requires the receiver to know an upper bound on
the delay of its local clock with respect to the receiver's clock,
so a simple algorithm is sufficient. TESLA can be used with direct,
indirect, and delayed synchronization as three default options.
The specific synchronization method will be part of each instantiation
of TESLA, and needs to be described in the appropriate standards-track
RFC.
For completeness we sketch a simple method for direct synchronization
between the sender and a receiver:
* The receiver sends a (sync t_r) message to the sender and records
its local time t_r.
* Upon receipt of the (sync t_r) message, the sender records its
local time t_s and sends (synch, t_r,t_s) to the receiver.
* Upon receiving (synch,t_r,t_s), the receiver sets D_t = t_s - t_r + S,
where S is an estimated bound on the clock drift throughout the
duration of the session.
Note:
* Assuming that the messages are authentic (i.e., the message received
the receiver was actually sent by the sender), and assuming that the
clock drift is at most S, then at any point throughout the session
we have that T_s < T_r + D_t, where T_s is the current time at the
sender and T_r is the current time at the receiver.
* The exchange of sync messages needs to be authenticated. This can be
done in a number of ways, for instance a secure NTP protocol, or in
conjunction with a session set-up protocol.
3.4 Broadcasting Authenticated Messages
Each key in the one-way key chain corresponds to a time interval. Each key in the one-way key chain corresponds to a time interval.
Every time a sender broadcasts a message, it appends a MAC to the Every time a sender broadcasts a message, it appends a MAC to the
message, using the key corresponding to the current time interval. message, using the key corresponding to the current time interval.
The key remains secret for the next d-1 intervals, so messages a The key remains secret for the next d-1 intervals, so messages a
sender broadcasts in interval j effectively disclose key K_j-d. We sender broadcasts in interval j effectively disclose key K_j-d. We
call d the key disclosure delay. call d the key disclosure delay.
We do not want to use the same key multiple times in different cryp­ We do not want to use the same key multiple times in different
tographic operations, that is, to use key K_j to derive the previous cryptographic operations, that is, to use key K_j to derive the previous
key of the one-way key chain K_{j-1}, and to use the same key K_j as key of the one-way key chain K_{j-1}, and to use the same key K_j as
the key to compute the MACs in time interval j may potentially lead the key to compute the MACs in time interval j may potentially lead
to a cryptographic weakness. Using a pseudo-random function (PRF) to a cryptographic weakness. Using a pseudo-random function (PRF)
f', we construct the one-way function F': F'(k) = f'_k(1). We use F' f', we construct the one-way function F': F'(k) = f'_k(1). We use F'
to derive the key to compute the MAC of messages in each interval. to derive the key to compute the MAC of messages in each interval.
The sender derives the MAC key as follows: K'_i = F'(K_i). Figure 1 The sender derives the MAC key as follows: K'_i = F'(K_i). Figure 1
depicts the one-way key chain construction and MAC key derivation. To depicts the one-way key chain construction and MAC key derivation. To
broadcast message M_j in interval i the sender constructs packet P_j broadcast message M_j in interval i the sender constructs packet
= {M_j || MAC(K'_i,M_j) || K_{i-d}}, where || denotes concatenation. P_j = {M_j || i || MAC(K'_i,M_j) || K_{i-d}}, where || denotes
concatenation.
F(K_i) F(K_{i+1}) F(K_{i+2}) F(K_i) F(K_{i+1}) F(K_{i+2})
K_{i-1} <------- K_i <--------- Ki+1 <------- K_{i-1} <------- K_i <--------- Ki+1 <------- Ki+2
| | | | | |
| F'(K_{i-1}) | F'(K_i) | F'(K_{i+1}) | F'(K_{i-1}) | F'(K_i) | F'(K_{i+1})
| | | | | |
V V V V V V
K'_{i-1} K'_i K'_{i+1} K'_{i-1} K'_i K'_{i+1}
Figure 1: At the top of the figure, we see the one-way key chain Figure 1: At the top of the figure, we see the one-way key chain
(derived using the one-way function F), and the derived MAC keys (derived using the one-way function F), and the derived MAC keys
(derived using the one-way function F'). (derived using the one-way function F').
4.5 Authentication at Receiver 3.5 Authentication at Receiver
Once a sender discloses a key, we must assume that all parties might Once a sender discloses a key, we must assume that all parties might
have access to that key. An adversary could create a bogus message have access to that key. An adversary could create a bogus message
and forge a MAC using the disclosed key. So whenever a packet and forge a MAC using the disclosed key. So whenever a packet
arrives, the receiver must verify that the MAC is based on a safe arrives, the receiver must verify that the MAC is based on a safe
key; a safe key is one that is still secret (only known by the key; a safe key is one that is still secret (only known by the
sender). We define a safe packet or safe message to be one with a MAC sender). We define a safe packet or safe message to be one with a MAC
that is computed with a safe key. that is computed with a safe key.
If the packet is not safe, the receiver must discard that packet, If the packet is not safe, the receiver must discard that packet,
because the authenticity is not assured any more. because the authenticity is not assured any more.
We now explain the TESLA authentication in more detail. When the We now explain the TESLA authentication in more detail. When the
receiver receives packet P_j sent in interval i, the receiver com­ receiver receives packet P_j sent in interval i, the receiver
putes an upper bound on the sender's clock: t_j. To test whether the computes an upper bound on the sender's clock: t_j. To test whether the
packet is safe, the receiver computes the highest interval x the packet is safe, the receiver computes the highest interval x the
sender could possibly be in, namely x = floor((t_j - T_0) / T_int). sender could possibly be in, namely x = floor((t_j - T_0) / T_int).
The receiver verifies that x < i + d (where i is the interval index), The receiver verifies that x < i + d (where i is the interval index),
which implies that the sender is not yet in the interval during which which implies that the sender is not yet in the interval during which
it discloses the key K_i. it discloses the key K_i. If the condition fails then the receiver
drops the packet.
The receiver cannot yet verify the authenticity of packets sent in The receiver cannot yet verify the authenticity of packets sent in
interval i without key K_i. Instead, it adds the triplet ( i, M_j, interval i without key K_i. Instead, it adds the triplet ( i, M_j,
MAC( K'_i, M_j) ) to a buffer, and verifies the authenticity after it MAC( K'_i, M_j) ) to a buffer, and verifies the authenticity after it
learns K'_i. learns K'_i.
What does a receiver do when it receives the disclosed key K_i? What does a receiver do when it receives the disclosed key K_i?
First, it checks whether it already knows K_i or a later key K_j First, it checks whether it already knows K_i or a later key K_j
(j>i). If K_i is the latest key received to date, the receiver checks (j>i). If K_i is the latest key received to date, the receiver checks
the legitimacy of K_i by verifying, for some earlier key K_v (v<i) the legitimacy of K_i by verifying, for some earlier key K_v (v<i)
skipping to change at page 9, line 45 skipping to change at page 10, line 21
Using a disclosed key, we can calculate all previous disclosed keys, Using a disclosed key, we can calculate all previous disclosed keys,
so even if packets are lost, we will still be able to verify so even if packets are lost, we will still be able to verify
buffered, safe packets from earlier time intervals. Thus, if i-v>1, buffered, safe packets from earlier time intervals. Thus, if i-v>1,
the receiver can also verify the authenticity of the stored packets the receiver can also verify the authenticity of the stored packets
of intervals v+1 ... i-1. of intervals v+1 ... i-1.
Note that the security of TESLA does not rely on any assumptions on Note that the security of TESLA does not rely on any assumptions on
network propagation delay. network propagation delay.
4.6 Determining the Key Disclosure Delay 3.6 Determining the Key Disclosure Delay
An important TESLA parameter is the key disclosure delay d. Although An important TESLA parameter is the key disclosure delay d. Although
the choice of the disclosure delay does not affect the security of the choice of the disclosure delay does not affect the security of
the system, it is an important performance factor. A short disclosure the system, it is an important performance factor. A short disclosure
delay will cause packets to loose their safety property, so receivers delay will cause packets to lose their safety property, so receivers
will discard them; but a long disclosure delay leads to a long will discard them; but a long disclosure delay leads to a long
authentication delay for receivers. We recommend choosing the disclo­
sure delay as follows: in direct time synchronization let the RTT be authentication delay for receivers. We recommend choosing the
a reasonable upper bound on the round trip time between the sender discloˇ sure delay as follows: in direct time synchronization let
and the receiver; then choose d = ceil( RTT / T_int) + 1. Note that the RTT be a reasonable upper bound on the round trip time between the
rounding up the quotient ensures that d >= 2. Also note that a dis­ sender and the receiver; then choose d = ceil( RTT / T_int) + 1. Note
closure delay of one time interval (d=1) does not work. Consider that rounding up the quotient ensures that d >= 2. Also note that a
disclosure delay of one time interval (d=1) does not work. Consider
packets sent close to the boundary of the time interval: after the packets sent close to the boundary of the time interval: after the
network propagation delay and the receiver time synchronization network propagation delay and the receiver time synchronization
error, a receiver will need to discard the packet, because the sender error, a receiver will need to discard the packet, because the sender
will already be in the next time interval, when it discloses the cor­ will already be in the next time interval, when it discloses the
responding key. corresponding key.
4.7 Some extenstions 3.7 An alternative delay description method
Let us mention two salient extenstions of the basic TESLA scheme. The above description instructs the sender to include the time interval
i in each packet. The receiver then uses i to determine the time at which
the key authenticating the packet is disclosed. This method limits the
the sender to a pre-determined schedule of disclosing keys.
Alternatively, the sender may directly include in each packet the time t_p
at which it is going to disclose the key for this packet. This way, the
receiver does not need to know the duration of intervals or the delay
factor d. All the receiver needs to know is the bound D_t on the clock
skew and T_0, the sender's local time at the initiation of the session.
Then the receiver records the local time T when the packet has arrived,
and verifies that
T <= T_0 + D_t + t_p.
Else the packet is dropped.
Another advantage of this method is that the sender is able to change
the duration of intervals and the key disclosure delay dynamically
throughout the session.
3.8 Some extensions
Let us mention two salient extensions of the basic TESLA scheme.
A first extension allows having multiple TESLA authentication chains A first extension allows having multiple TESLA authentication chains
for a single stream, where each chain uses a different delay for for a single stream, where each chain uses a different delay for
disclosing the keys. This extension is typically used to deal with disclosing the keys. This extension is typically used to deal with
heterogenous network delays withing a single multicast transmission. heterogeneous network delays within a single multicast transmission.
A second extension allows having most of the buffering of packets A second extension allows having most of the buffering of packets
at the sender side (rather than at the receiver side). Both at the sender side (rather than at the receiver side). Both
extensions are described in [15]. extensions are described in [15].
5 Layer placement 4 Layer placement
The TESLA authentication can be performed at any layer in the net­ The TESLA authentication can be performed at any layer in the
working stack. The two logical places are in the network or the networking stack. Three natural places are in the network, transport,
application layer. We list some considerations regarding the choice or the application layer. We list some considerations regarding the
of layer: choice of layer:
· Performing TESLA in the network layer has the advantage that the ¸ Performing TESLA in the network layer has the advantage that the
transport or application layer only receives authenticated data, transport or application layer only receives authenticated data,
potentially aiding a reliability protocol and preventing denial- potentially aiding a reliability protocol and preventing denial
of-service attacks. (Indeed, reliable multicast tools based on of service attacks. (Indeed, reliable multicast tools based on
forward error correction are highly susceptible to denial of ser­ forward error correction are highly susceptible to denial of
vice due to bogus packets.) service due to bogus packets.)
· Performing TESLA in the application layer has the advantage that ¸ Performing TESLA in either the transport or the application layer
the network layer remains unchanged; but it has the drawback that has the advantage that the network layer remains unchanged; but it
packets are obtained by the application layer only after being has the drawback that packets are obtained by the application layer
processed by the transport layer. Consequently, if TCP is used only after being processed by the transport layer. Consequently,
then this may introduce additional and unpredictable delays on if TCP is used then this may introduce additional and unpredictable
top of the unavoidable network delays. (However, if UDP is used delays on top of the unavoidable network delays. (However, if UDP
then this is not a problem.) is used then this is not a problem.)
5. Security Considerations
See the academic publications on TESLA [8,14,20] for several security
analyses. Regarding the security of implementations, by far the most
delicate point is the verification of the timing conditions. Care
should be taken to to make sure that:
(a) The value bound D_t on the clock skew is calculated according to the
spec at session set-up.
(b) The receiver records the arrival time of the packet as soon as possible
after the packet's arrival, and computes the safety condition correctly.
6 Acknowledgments 6 Acknowledgments
We would like to thank Mike Luby for his feedback and support. We would like to thank Mike Luby for his feedback and support.
7 Bibliography 7 Bibliography
[1] T. Dierks and C. Allen, "The TLS protocol version 1.0." Internet [1] T. Dierks and C. Allen, "The TLS protocol version 1.0." Internet
Request for Comments RFC 2246, January 1999. Proposed standard. Request for Comments RFC 2246, January 1999. Proposed standard.
skipping to change at page 11, line 14 skipping to change at page 12, line 26
[3] D. Boneh, G. Durfee, and M. Franklin, "Lower bounds for multicast [3] D. Boneh, G. Durfee, and M. Franklin, "Lower bounds for multicast
message authentication," in Advances in Cryptology -- EUROCRYPT '2001 message authentication," in Advances in Cryptology -- EUROCRYPT '2001
(B. Pfitzmann, ed.), vol. 2045 of Lecture Notes in Computer Science , (B. Pfitzmann, ed.), vol. 2045 of Lecture Notes in Computer Science ,
(Innsbruck, Austria), pp. 434--450, Springer-Verlag, Berlin Germany, (Innsbruck, Austria), pp. 434--450, Springer-Verlag, Berlin Germany,
2001. 2001.
[4] R. Gennaro and P. Rohatgi, "How to Sign Digital Streams," tech. [4] R. Gennaro and P. Rohatgi, "How to Sign Digital Streams," tech.
rep., IBM T.J.Watson Research Center, 1997. rep., IBM T.J.Watson Research Center, 1997.
[5] P. Rohatgi, "A compact and fast hybrid signature scheme for mul­ [5] P. Rohatgi, "A compact and fast hybrid signature scheme for mulˇ
ticast packet authentication," in 6th ACM Conference on Computer and ticast packet authentication," in 6th ACM Conference on Computer and
Communications Security , November 1999. Communications Security , November 1999.
[6] P. Rohatgi, "A hybrid signature scheme for multicast source [6] P. Rohatgi, "A hybrid signature scheme for multicast source
authentication," Internet Draft, Internet Engineering Task Force, authentication," Internet Draft, Internet Engineering Task Force,
June 1999. Work in progress. June 1999. Work in progress.
[7] C. K. Wong and S. S. Lam, "Digital signatures for flows and mul­ [7] C. K. Wong and S. S. Lam, "Digital signatures for flows and mulˇ
ticasts," in Proc. IEEE ICNP `98 , 1998. ticasts," in Proc. IEEE ICNP `98 , 1998.
[8] A. Perrig, R. Canetti, J. Tygar, and D. X. Song, "Efficient [8] A. Perrig, R. Canetti, J. Tygar, and D. X. Song, "Efficient
authentication and signing of multicast streams over lossy channels," authentication and signing of multicast streams over lossy channels,"
in IEEE Symposium on Security and Privacy , May 2000. in IEEE Symposium on Security and Privacy , May 2000.
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Pinkas, "Multicast security: A taxonomy and some efficient construc­ Pinkas, "Multicast security: A taxonomy and some efficient construcˇ
tions," in Infocom '99 , 1999. tions," in Infocom '99 , 1999.
[10] S. Cheung, "An efficient message authentication scheme for link [10] S. Cheung, "An efficient message authentication scheme for link
state routing," in 13th Annual Computer Security Applications Confer­ state routing," in 13th Annual Computer Security Applications Conferˇ
ence , 1997. ence , 1997.
[11] F. Bergadano, D. Cavagnino, and B. Crispo, "Chained stream [11] F. Bergadano, D. Cavagnino, and B. Crispo, "Chained stream
authentication," in Selected Areas in Cryptography 2000 , (Waterloo, authentication," in Selected Areas in Cryptography 2000 , (Waterloo,
Canada), August 2000. A talk describing this scheme was given at IBM Canada), August 2000. A talk describing this scheme was given at IBM
Watson in August 1998. Watson in August 1998.
[12] F. Bergadano, D. Cavalino, and B. Crispo, "Individual single [12] F. Bergadano, D. Cavalino, and B. Crispo, "Individual single
source authentication on the mbone," in ICME 2000 , Aug 2000. A talk source authentication on the mbone," in ICME 2000 , Aug 2000. A talk
containing this work was given at IBM Watson, August 1998. containing this work was given at IBM Watson, August 1998.
[13] A. Perrig and J. D. Tygar, Secure Broadcast Communication in [13] A. Perrig and J. D. Tygar, Secure Broadcast Communication in
Wired and Wireless Networks Kluwer Academic Publishers, Oct. 2002. Wired and Wireless Networks Kluwer Academic Publishers, Oct. 2002.
ISBN 0792376501. ISBN 0792376501.
[14] A. Perrig, R. Canetti, J. D. Tygar, and D. Song, "The tesla [14] A. Perrig, R. Canetti, J. D. Tygar, and D. Song, "The tesla
broadcast authentication protocol," RSA CryptoBytes , vol. 5, no. broadcast authentication protocol," RSA CryptoBytes , vol. 5, no.
Summer, 2002. Summer, 2002.
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secure source authentication for multicast," in Network and Dis­ secure source authentication for multicast," in Network and Disˇ
tributed System Security Symposium, NDSS '01 , pp. 35--46, February tributed System Security Symposium, NDSS '01 , pp. 35--46, February
2001. 2001.
[16] D. L. Mills, "Network Time Protocol (Version 3) Specification, [16] D. L. Mills, "Network Time Protocol (Version 3) Specification,
Implementation and Analysis." Internet Request for Comments, March Implementation and Analysis." Internet Request for Comments, March
1992. RFC 1305. 1992. RFC 1305.
[17] B. Simons, J. Lundelius-Welch, and N. Lynch, "An overview of [17] B. Simons, J. Lundelius-Welch, and N. Lynch, "An overview of
clock synchronization," in Fault-Tolerant Distributed Computing (B. clock synchronization," in Fault-Tolerant Distributed Computing (B.
Simons and A. Spector, eds.), no. 448 in LNCS, pp. 84--96, Springer- Simons and A. Spector, eds.), no. 448 in LNCS, pp. 84--96, Springer-
Verlag, Berlin Germany, 1990. Verlag, Berlin Germany, 1990.
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work clocks," in Proceedings of the conference on Communications work clocks," in Proceedings of the conference on Communications
architectures, protocols and applications, SIGCOMM 94 , (London, Eng­ architectures, protocols and applications, SIGCOMM 94 , (London,
land), pp. 317--327, 1994. England), pp. 317--327, 1994.
[19] L. Lamport and P. Melliar-Smith, "Synchronizing clocks in the [19] L. Lamport and P. Melliar-Smith, "Synchronizing clocks in the
presence of faults," J. ACM , vol. 32, no. 1, pp. 52--78, 1985. presence of faults," J. ACM , vol. 32, no. 1, pp. 52--78, 1985.
[20] Philippa Broadfoot and Gavin Lowe, "Analysing a Stream
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A Author Contact Information A Author Contact Information
Adrian Perrig Adrian Perrig
UC Berkeley / Digital Fountain ECE Department
102 South Hall Carnegie Mellon University
Berkeley, CA 94720 Pittsburgh, PA
US US
perrig@cs.berkeley.edu perrig@ece.cmu.edu
Ran Canetti Ran Canetti
IBM Research IBM Research
30 Saw Mill River Rd 30 Saw Mill River Rd
Hawthorne, NY 10532 Hawthorne, NY 10532
US US
canetti@watson.ibm.com canetti@watson.ibm.com
Dawn Song Dawn Song
UC Berkeley CS Department
387 Soda Hall, 1776 Carnegie Mellon University
Berkeley, CA 94720-1776 Pittsburgh, PA
US US
dawnsong@cs.berkeley.edu dawnsong@cmu.edu
Doug Tygar Doug Tygar
UC Berkeley UC Berkeley
102 South Hall, 4600 102 South Hall, 4600
Berkeley, CA 94720-4600 Berkeley, CA 94720-4600
US US
tygar@cs.berkeley.edu tygar@cs.berkeley.edu
Bob Briscoe Bob Briscoe
BT Research BT Research
B54/74, BT Labs B54/74, BT Labs
Martlesham Heath Martlesham Heath
Ipswich, IP5 3RE Ipswich, IP5 3RE
skipping to change at page 13, line 16 skipping to change at page 13, line 4
US US
tygar@cs.berkeley.edu tygar@cs.berkeley.edu
Bob Briscoe Bob Briscoe
BT Research BT Research
B54/74, BT Labs B54/74, BT Labs
Martlesham Heath Martlesham Heath
Ipswich, IP5 3RE Ipswich, IP5 3RE
UK UK
bob.briscoe@bt.com bob.briscoe@bt.com
B Full Copyright Statement B Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved. Copyright (C) The Internet Society (2000). All Rights Reserved.
This document and translations of it may be copied and furnished to This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this doc­ included on all such copies and derivative works. However, this docˇ
ument itself may not be modified in any way, such as by removing the ument itself may not be modified in any way, such as by removing the
copyright notice or references to the Internet Society or other copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of develop­ Internet organizations, except as needed for the purpose of developˇ
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MER­ HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERˇ
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