Network Working Group D. McGrew
Internet-Draft J. Foley
Intended status: Standards Track Cisco Systems
Expires: August 18, 2014 K. Paterson
Royal Holloway, University of
London
February 14, 2014
Authenticated Encryption with AES-CBC and HMAC-SHA
draft-mcgrew-aead-aes-cbc-hmac-sha2-04.txt
Abstract
This document specifies algorithms for authenticated encryption with
associated data (AEAD) that are based on the composition of the
Advanced Encryption Standard (AES) in the Cipher Block Chaining (CBC)
mode of operation for encryption, and the HMAC-SHA message
authentication code (MAC).
These are randomized encryption algorithms, and thus are suitable for
use with applications that cannot provide distinct nonces to each
invocation of the AEAD encrypt operation.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 18, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. History . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Conventions Used In This Document . . . . . . . . . . . . 4
2. CBC-HMAC algorithms . . . . . . . . . . . . . . . . . . . . . 5
2.1. Encryption . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Decryption . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3. Length . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.4. AEAD_AES_128_CBC_HMAC_SHA_256 . . . . . . . . . . . . . . 8
2.5. AEAD_AES_192_CBC_HMAC_SHA_384 . . . . . . . . . . . . . . 9
2.6. AEAD_AES_256_CBC_HMAC_SHA_384 . . . . . . . . . . . . . . 9
2.7. AEAD_AES_256_CBC_HMAC_SHA_512 . . . . . . . . . . . . . . 10
2.8. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 10
3. Randomness Requirements . . . . . . . . . . . . . . . . . . . 11
4. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5. Test Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.1. AEAD_AES_128_CBC_HMAC_SHA256 . . . . . . . . . . . . . . . 14
5.2. AEAD_AES_192_CBC_HMAC_SHA384 . . . . . . . . . . . . . . . 15
5.3. AEAD_AES_256_CBC_HMAC_SHA384 . . . . . . . . . . . . . . . 17
5.4. AEAD_AES_256_CBC_HMAC_SHA512 . . . . . . . . . . . . . . . 18
6. Security Considerations . . . . . . . . . . . . . . . . . . . 20
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
8.1. Normative References . . . . . . . . . . . . . . . . . . . 23
8.2. Informative References . . . . . . . . . . . . . . . . . . 23
Appendix A. CBC Encryption and Decryption . . . . . . . . . . . . 26
Appendix B. Alternative Interface for Legacy Encoding . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29
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1. Introduction
Authenticated Encryption (AE) [BN00] is a form of encryption that, in
addition to providing confidentiality for the plaintext that is
encrypted, provides a way to check its integrity and authenticity.
This combination of features can, when properly implemented, provide
security against adversaries who have access to full decryption
capabilities for ciphertexts of their choice, and access to full
encryption capabilities for plaintexts of their choice. The strong
form of security provided by AE is known to be robust against a large
class of adversaries for general purpose applications of AE,
including applications such as securing network communications over
untrusted networks. The strong security properties of AE stand in
contrast to the known weaknesses of "encryption only" forms of
encryption, see [B96][YHR04] [DP07] for examples.
Authenticated encryption with Associated Data, or AEAD [R02], adds
the ability to check the integrity and authenticity of some
associated data (sometimes called "additional authenticated data")
for which confidentiality is not required (or is not desirable).
While many approaches to building AEAD schemes are known, a
particularly simple, well-understood, and cryptographically strong
method is to employ an "Encrypt-then-MAC" construction. This
document defines new AEAD algorithms of this general type, using the
interface defined in [RFC5116], based on the Advanced Encryption
Standard (AES) [FIPS197] in the Cipher Block Chaining (CBC) mode of
operation [SP800-38] and HMAC using the Secure Hash Algorithm (SHA)
[FIPS186-2], with security levels of 128, 192, and 256 bits.
Comments on this version are requested and should be forwarded to the
IRTF Crypto Forum Research Group (CFRG). An earlier version of this
document benefited from some review from that group.
1.1. History
This subsection describes the revision history of this Internet
Draft. It should be removed by the RFC Editor before publication as
an RFC.
The changes of version 02 from version 01 are:
Added test cases for each of the five operational modes.
Added John as a coauthor.
Adds a legacy-style interface in Appendix B.
The changes of version 01 from version 00 are:
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MIN_LEN_A and associated logic was eliminated.
Padding String (PS) typo corrected in Section 2.1.
Decryption Step 3 refers to the appropriate step in the encryption
process.
Random IV min-entropy clarified in Section 3.
HMAC keys are now the same size as the truncated output (128 or
256 bits). Previously, the HMAC keys were the same size as the
full hash output (256, 384, or 512 bits).
An algorithm based on the combination of AES-256 and HMAC-SHA-384
has been added, for compatibility with
draft-burgin-kerberos-aes-cbc-hmac-sha2.
The test cases in the previous version are no longer valid, and
thus have been removed. New test cases have been computed (and
the authors thank John Foley for this contribution) but have not
been included, pending confirmation from a second, independent
implementation.
1.2. Conventions Used In This Document
We use the following notational conventions.
CBC-ENC(X,P) denotes the CBC encryption of P using the cipher with
the key X
MAC(Y, M) denotes the application of the Message Authentication
Code (MAC) to the message M, using the key Y
The concatenation of two octet strings A and B is denoted as
A || B
len(X) denotes the number of bits in the string X, expressed as an
unsigned integer in network byte order.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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2. CBC-HMAC algorithms
This section defines CBC-HMAC, an algorithm based on the the encrypt-
then-MAC method defined in Section 4.3 of [BN00]. That method
constructs a randomized AEAD algorithm out of a randomized cipher,
such as a block cipher mode of operation that uses a random
initialization vector, and a MAC.
Section 2.1 and Section 2.2 define the CBC-HMAC encryption and
decryption algorithms, without specifying the particular block cipher
or hash function to be used. Section 2.4, Section 2.5, and
Section 2.7 define instances of CBC-HMAC that specify those details.
2.1. Encryption
We briefly recall the encryption interface defined in Section 2 of
[RFC5116]. The AEAD encryption algorithm takes as input four octet
strings: a secret key K, a plaintext P, associated data A, and a
nonce N. An authenticated ciphertext value is provided as output.
The data in the plaintext are encrypted and authenticated, and the
associated data are authenticated, but not encrypted. The key MUST
be generated in a way that is uniformly random or pseudorandom;
guidance on random sources is provided in [RFC4086].
In CBC-HMAC, the nonce N MUST be a zero-length string; a nonce is not
needed and is not used (see Section 4 for further background).
The CBC-HMAC encryption process is as follows, or uses an equivalent
set of steps:
1. The secondary keys MAC_KEY and ENC_KEY are generated from the
input key K as follows. Each of these two keys is an octet
string.
MAC_KEY consists of the initial MAC_KEY_LEN octets of K, in
order.
ENC_KEY consists of the final ENC_KEY_LEN octets of K, in
order.
Here we denote the number of octets in the MAC_KEY as
MAC_KEY_LEN, and the number of octets in ENC_KEY as ENC_KEY_LEN;
the values of these parameters are specified by the AEAD
algorithms (in Section 2.4 and Section 2.5). The number of
octets in the input key K is the sum of MAC_KEY_LEN and
ENC_KEY_LEN. When generating the secondary keys from K, MAC_KEY
and ENC_KEY MUST NOT overlap.
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2. Prior to CBC encryption, the plaintext P is padded by appending a
padding string PS to that data, to ensure that len(P || PS) is a
multiple of 128, as is needed for the CBC operation. The value
of PS is as follows:
PS = 01 if len(P) mod 128 = 120,
PS = 0202 if len(P) mod 128 = 112,
PS = 030303 if len(P) mod 128 = 104,
PS = 04040404 if len(P) mod 128 = 96,
PS = 0505050505 if len(P) mod 128 = 88,
PS = 060606060606 if len(P) mod 128 = 80,
PS = 07070707070707 if len(P) mod 128 = 72,
PS = 0808080808080808 if len(P) mod 128 = 64,
PS = 090909090909090909 if len(P) mod 128 = 56,
PS = 0A0A0A0A0A0A0A0A0A0A if len(P) mod 128 = 48,
PS = 0B0B0B0B0B0B0B0B0B0B0B if len(P) mod 128 = 40,
PS = 0C0C0C0C0C0C0C0C0C0C0C0C if len(P) mod 128 = 32,
PS = 0D0D0D0D0D0D0D0D0D0D0D0D0D if len(P) mod 128 = 24,
PS = 0E0E0E0E0E0E0E0E0E0E0E0E0E0E if len(P) mod 128 = 16,
PS = 0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F if len(P) mod 128 = 8,
PS = 10101010101010101010101010101010 if len(P) mod 128 = 0.
Note that padding MUST be added to the plaintext; if the number
of bits in P is a multiple of 128, then 128 bits of padding will
be added.
3. The plaintext and appended padding P || PS is CBC encrypted using
ENC_KEY as the key, as described in Appendix A. We denote the
ciphertext output from this step as S.
4. The octet string AL is equal to the number of bits in A expressed
as a 64-bit unsigned integer in network byte order.
5. A message authentication tag T is computed by applying HMAC
[RFC2104] to the following data, in order:
the associated data A,
the ciphertext S computed in the previous step, and
the octet string AL defined above.
The string MAC_KEY is used as the MAC key. We denote the output
of the MAC computed in this step as T.
6. The AEAD Ciphertext consists of the string S, with the string T
appended to it. This Ciphertext is returned as the output of the
AEAD encryption operation.
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The encryption process can be illustrated as follows. Here P, A, and
C denote the AEAD plaintext, associated data, and ciphertext,
respectively.
MAC_KEY = initial MAC_KEY_LEN bytes of K
ENC_KEY = final ENC_KEY_LEN bytes of K
S = CBC-ENC(ENC_KEY, P || PS),
T = MAC(MAC_KEY, A || S || AL),
C = S || T.
2.2. Decryption
The authenticated decryption operation has four inputs: K, N, and A,
as defined above, and the Ciphertext C. As discussed above, N is an
empty string in AES-CBC and is not used below. It has only a single
output, either a plaintext value P or a special symbol FAIL that
indicates that the inputs are not authentic. The authenticated
decryption algorithm takes is as follows, or uses an equivalent set
of steps:
1. The secondary keys MAC_KEY and ENC_KEY are generated from the
input key K as in Step 1 of Section 2.1.
2. The final T_LEN octets are stripped from C. Here T_LEN denotes
the number of octets in the MAC, which is a fixed parameter of
the AEAD algorithm. We denote the initial octets of C as S, and
denote the final T_LEN octets as T.
3. The integrity and authenticity of A and C are checked by
computing HMAC with the inputs as in Step 6 of Section 2.1. The
value T, from the previous step, is compared to the HMAC output,
using a comparison routine that takes constant time to execute.
If those values are identical, then A and C are considered valid,
and the processing continues. Otherwise, all of the data used in
the MAC validation are discarded, and the AEAD decryption
operation returns an indication that it failed, and the operation
halts.
4. The value S is CBC decrypted, as described in Appendix A, using
the value ENC_KEY is as the decryption key.
5. The padding string is stripped from the resulting plaintext.
Note that the length of PS can be inferred from the value of the
final octet of P || PS, if that value is between 01 and 10
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(hexadecimal). If the final octet has a value outside that
range, then all of the data used in the processing of the message
is zeroized and discarded, and the AEAD decryption operation
returns an indication that it failed, and the operation halts.
6. The plaintext value is returned.
2.3. Length
The length of the ciphertext can be inferred from that of the
plaintext. The number L of octets in the ciphertext is given by
L = 16 * ( floor(M / 16) + 2)
where M denotes the number of octets in the plaintext, and the
function floor() rounds its argument down to the nearest integer.
This fact is useful to applications that need to reserve space for a
ciphertext within a message or data structure.
2.4. AEAD_AES_128_CBC_HMAC_SHA_256
This algorithm is randomized; each invocation of the encrypt
operation makes use of a random value (the IV described in
Appendix A). It is based on the CBC-HMAC algorithm detailed above,
and uses the HMAC message authentication code [RFC2104] with the SHA-
256 hash function [FIPS186-2] to provide message authentication, with
the HMAC output truncated to 128 bits, corresponding to the HMAC-SHA-
256-128 algorithm defined in [RFC4868]. For encryption, it uses the
Advanced Encryption Standard (AES) [FIPS197] block cipher in CBC
mode.
The input key K is 32 octets long.
ENC_KEY_LEN is 16 octets.
The SHA-256 hash algorithm is used in HMAC. MAC_KEY_LEN is 16
octets. The HMAC-SHA-256 output is truncated to T_LEN=16 octets, by
stripping off the final 16 octets. Test cases for HMAC-SHA-256 are
provided in [RFC4231].
The lengths of the inputs are restricted as follows:
K_LEN is 32 octets,
P_MAX is 2^64 - 1 octets,
A_MAX is 2^64 - 1 octets,
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N_MIN and N_MAX are zero octets,
C_MAX is 2^64 + 47 octets.
2.5. AEAD_AES_192_CBC_HMAC_SHA_384
AEAD_AES_192_CBC_HMAC_SHA_384 is based on
AEAD_AES_128_CBC_HMAC_SHA_256, but with the following differences:
AES-192 is used instead of AES-128.
SHA-384 is used in HMAC instead of SHA-256.
ENC_KEY_LEN is 24 octets.
MAC_KEY_LEN is 24 octets.
The length of the input key K is 48 octets.
The HMAC-SHA-384 value is truncated to T_LEN=24 octets instead of
16 octets.
The input length restrictions are as for
AEAD_AES_CBC_128_HMAC_SHA_256.
2.6. AEAD_AES_256_CBC_HMAC_SHA_384
AEAD_AES_256_CBC_HMAC_SHA_384 is based on
AEAD_AES_128_CBC_HMAC_SHA_256, but with the following differences:
AES-256 is used instead of AES-128.
SHA-384 is used in HMAC instead of SHA-256.
ENC_KEY_LEN is 32 octets.
MAC_KEY_LEN is 24 octets.
The length of the input key K is 56 octets.
The HMAC-SHA-384 value is truncated to T_LEN=24 octets instead of
16 octets.
The input length restrictions are as for
AEAD_AES_CBC_128_HMAC_SHA_256.
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2.7. AEAD_AES_256_CBC_HMAC_SHA_512
AEAD_AES_256_CBC_HMAC_SHA_512 is based on
AEAD_AES_128_CBC_HMAC_SHA_256, but with the following differences:
AES-256 is used instead of AES-128.
SHA-512 is used in HMAC instead of SHA-256.
ENC_KEY_LEN is 32 octets.
MAC_KEY_LEN is 32 octets.
The length of the input key K is 64 octets.
The HMAC-SHA-512 value is truncated to T_LEN=32 octets instead of
16 octets.
The input length restrictions are as for
AEAD_AES_CBC_128_HMAC_SHA_256.
2.8. Summary
The parameters of the CBC-HMAC algorithms are summarized in the
following table.
+-------------------------------+-------------+-------------+-------+
| algorithm | ENC_KEY_LEN | MAC_KEY_LEN | T_LEN |
+-------------------------------+-------------+-------------+-------+
| AEAD_AES_128_CBC_HMAC_SHA_256 | 16 | 16 | 16 |
| | | | |
| AEAD_AES_192_CBC_HMAC_SHA_384 | 24 | 24 | 24 |
| | | | |
| AEAD_AES_256_CBC_HMAC_SHA_384 | 32 | 24 | 24 |
| | | | |
| AEAD_AES_256_CBC_HMAC_SHA_512 | 32 | 32 | 32 |
+-------------------------------+-------------+-------------+-------+
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3. Randomness Requirements
Each IV MUST be unpredictable to the adversary. It MAY be chosen
uniformly at random, in which case it SHOULD have min-entropy within
one bit of len(IV). Alternatively, it MAY be generated
pseudorandomly, using any method that provides the same level of
security as the block cipher in use. However, if a pseudorandom
method is used, that method MUST NOT make use of ENC_KEY or MAC_KEY.
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4. Rationale
The CBC-HMAC AEAD algorithms defined in this note are intended to be
useful in the following applications:
systems that have the CBC and HMAC algorithms available, but do
not have dedicated AEAD algorithms such as GCM or CCM [RFC5116],
scenarios in which AEAD is useful, but it is undesirable to have
the application maintain a deterministic nonce; see Section 4 of
[RFC5116] for more background,
new systems, such as JSON Cryptography and W3C Web Crypto, which
can omit unauthenticated symmetric encryption altogether by
providing CBC and HMAC through an AEAD interface.
These algorithms are not intended to replace existing uses of AES-CBC
and HMAC, except in those circumstances where the existing use is not
sufficiently secure or sufficiently general-purpose.
The algorithms in this note truncate the HMAC output to half of the
size of the output of the underlying hash function. This size is the
recommended minimum (see Section 5 of [RFC2104]), and this parameter
choice has withstood the test of time.
The length of the associated data input A is included in the HMAC
input to ensure that the encrypter and the decrypter have the same
understanding of that length. Because of this, an attacker cannot
trick the receiver into interpreting the initial bytes of C as the
final bytes of A, or vice-versa.
The padding method used in this note is based on that of Privacy
Enhanced Mail (PEM) and the Public Key Cryptography Standards (PKCS),
because it is implemented in many environments.
The encrypt-then-MAC method is used because of its better security
properties. It would be possible to define AEAD algorithms based on
the MAC-encode-encrypt (MEE) method that is used by the Transport
Layer Security (TLS) protocol [RFC5246]. That alternative would
provide more code-sharing opportunities for an implementation that is
co-resident with a TLS implementation. It is possible (but tricky)
to implement MEE in a way that provides good security, as was shown
in [PRS11]. But its negatives outweigh its positives; its security
is inadequate for some parameter choices, and it has proven to be
very difficult to implement in a way that resists padding oracle and
related timing attacks [V02] [CHVV03] [M04] [DP10] [AP12]. For
future uses of CBC and HMAC, it is better to use encrypt-then-MAC.
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This note uses HMAC-SHA-2 because it is widely deployed, it is
mandated in newer standards, and because SHA1 is being deprecated.
It has been recently announced that the SHA-3 standard will be based
on KECCAK, but this note does not incorporate that hash function. To
do so would be to speculate on the final form of the SHA-3 standard.
In addition, while the use of KECCAK as a hash function is
straightforward, there are multiple options for its use in
authenticated encryption. The focus of this note is the definition
of AEAD algorithms based on currently used cryptographic mechanisms,
so SHA-3 is out of scope.
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5. Test Cases
5.1. AEAD_AES_128_CBC_HMAC_SHA256
K = 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
MAC_KEY = 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
ENC_KEY = 10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
P = 41 20 63 69 70 68 65 72 20 73 79 73 74 65 6d 20
6d 75 73 74 20 6e 6f 74 20 62 65 20 72 65 71 75
69 72 65 64 20 74 6f 20 62 65 20 73 65 63 72 65
74 2c 20 61 6e 64 20 69 74 20 6d 75 73 74 20 62
65 20 61 62 6c 65 20 74 6f 20 66 61 6c 6c 20 69
6e 74 6f 20 74 68 65 20 68 61 6e 64 73 20 6f 66
20 74 68 65 20 65 6e 65 6d 79 20 77 69 74 68 6f
75 74 20 69 6e 63 6f 6e 76 65 6e 69 65 6e 63 65
IV = 1a f3 8c 2d c2 b9 6f fd d8 66 94 09 23 41 bc 04
A = 54 68 65 20 73 65 63 6f 6e 64 20 70 72 69 6e 63
69 70 6c 65 20 6f 66 20 41 75 67 75 73 74 65 20
4b 65 72 63 6b 68 6f 66 66 73
PS = 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10
AL = 00 00 00 00 00 00 01 50
S = 1a f3 8c 2d c2 b9 6f fd d8 66 94 09 23 41 bc 04
c8 0e df a3 2d df 39 d5 ef 00 c0 b4 68 83 42 79
a2 e4 6a 1b 80 49 f7 92 f7 6b fe 54 b9 03 a9 c9
a9 4a c9 b4 7a d2 65 5c 5f 10 f9 ae f7 14 27 e2
fc 6f 9b 3f 39 9a 22 14 89 f1 63 62 c7 03 23 36
09 d4 5a c6 98 64 e3 32 1c f8 29 35 ac 40 96 c8
6e 13 33 14 c5 40 19 e8 ca 79 80 df a4 b9 cf 1b
38 4c 48 6f 3a 54 c5 10 78 15 8e e5 d7 9d e5 9f
bd 34 d8 48 b3 d6 95 50 a6 76 46 34 44 27 ad e5
4b 88 51 ff b5 98 f7 f8 00 74 b9 47 3c 82 e2 db
T = 65 2c 3f a3 6b 0a 7c 5b 32 19 fa b3 a3 0b c1 c4
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C = 1a f3 8c 2d c2 b9 6f fd d8 66 94 09 23 41 bc 04
c8 0e df a3 2d df 39 d5 ef 00 c0 b4 68 83 42 79
a2 e4 6a 1b 80 49 f7 92 f7 6b fe 54 b9 03 a9 c9
a9 4a c9 b4 7a d2 65 5c 5f 10 f9 ae f7 14 27 e2
fc 6f 9b 3f 39 9a 22 14 89 f1 63 62 c7 03 23 36
09 d4 5a c6 98 64 e3 32 1c f8 29 35 ac 40 96 c8
6e 13 33 14 c5 40 19 e8 ca 79 80 df a4 b9 cf 1b
38 4c 48 6f 3a 54 c5 10 78 15 8e e5 d7 9d e5 9f
bd 34 d8 48 b3 d6 95 50 a6 76 46 34 44 27 ad e5
4b 88 51 ff b5 98 f7 f8 00 74 b9 47 3c 82 e2 db
65 2c 3f a3 6b 0a 7c 5b 32 19 fa b3 a3 0b c1 c4
5.2. AEAD_AES_192_CBC_HMAC_SHA384
K = 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
20 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f
MAC_KEY = 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
10 11 12 13 14 15 16 17
ENC_KEY = 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23 24 25 26 27
28 29 2a 2b 2c 2d 2e 2f
P = 41 20 63 69 70 68 65 72 20 73 79 73 74 65 6d 20
6d 75 73 74 20 6e 6f 74 20 62 65 20 72 65 71 75
69 72 65 64 20 74 6f 20 62 65 20 73 65 63 72 65
74 2c 20 61 6e 64 20 69 74 20 6d 75 73 74 20 62
65 20 61 62 6c 65 20 74 6f 20 66 61 6c 6c 20 69
6e 74 6f 20 74 68 65 20 68 61 6e 64 73 20 6f 66
20 74 68 65 20 65 6e 65 6d 79 20 77 69 74 68 6f
75 74 20 69 6e 63 6f 6e 76 65 6e 69 65 6e 63 65
IV = 1a f3 8c 2d c2 b9 6f fd d8 66 94 09 23 41 bc 04
A = 54 68 65 20 73 65 63 6f 6e 64 20 70 72 69 6e 63
69 70 6c 65 20 6f 66 20 41 75 67 75 73 74 65 20
4b 65 72 63 6b 68 6f 66 66 73
PS = 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10
AL = 00 00 00 00 00 00 01 50
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S = 1a f3 8c 2d c2 b9 6f fd d8 66 94 09 23 41 bc 04
ea 65 da 6b 59 e6 1e db 41 9b e6 2d 19 71 2a e5
d3 03 ee b5 00 52 d0 df d6 69 7f 77 22 4c 8e db
00 0d 27 9b dc 14 c1 07 26 54 bd 30 94 42 30 c6
57 be d4 ca 0c 9f 4a 84 66 f2 2b 22 6d 17 46 21
4b f8 cf c2 40 0a dd 9f 51 26 e4 79 66 3f c9 0b
3b ed 78 7a 2f 0f fc bf 39 04 be 2a 64 1d 5c 21
05 bf e5 91 ba e2 3b 1d 74 49 e5 32 ee f6 0a 9a
c8 bb 6c 6b 01 d3 5d 49 78 7b cd 57 ef 48 49 27
f2 80 ad c9 1a c0 c4 e7 9c 7b 11 ef c6 00 54 e3
T = 84 90 ac 0e 58 94 9b fe 51 87 5d 73 3f 93 ac 20
75 16 80 39 cc c7 33 d7
C = 1a f3 8c 2d c2 b9 6f fd d8 66 94 09 23 41 bc 04
ea 65 da 6b 59 e6 1e db 41 9b e6 2d 19 71 2a e5
d3 03 ee b5 00 52 d0 df d6 69 7f 77 22 4c 8e db
00 0d 27 9b dc 14 c1 07 26 54 bd 30 94 42 30 c6
57 be d4 ca 0c 9f 4a 84 66 f2 2b 22 6d 17 46 21
4b f8 cf c2 40 0a dd 9f 51 26 e4 79 66 3f c9 0b
3b ed 78 7a 2f 0f fc bf 39 04 be 2a 64 1d 5c 21
05 bf e5 91 ba e2 3b 1d 74 49 e5 32 ee f6 0a 9a
c8 bb 6c 6b 01 d3 5d 49 78 7b cd 57 ef 48 49 27
f2 80 ad c9 1a c0 c4 e7 9c 7b 11 ef c6 00 54 e3
84 90 ac 0e 58 94 9b fe 51 87 5d 73 3f 93 ac 20
75 16 80 39 cc c7 33 d7
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5.3. AEAD_AES_256_CBC_HMAC_SHA384
K = 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
20 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f
30 31 32 33 34 35 36 37
MAC_KEY = 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
10 11 12 13 14 15 16 17
ENC_KEY = 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23 24 25 26 27
28 29 2a 2b 2c 2d 2e 2f 30 31 32 33 34 35 36 37
P = 41 20 63 69 70 68 65 72 20 73 79 73 74 65 6d 20
6d 75 73 74 20 6e 6f 74 20 62 65 20 72 65 71 75
69 72 65 64 20 74 6f 20 62 65 20 73 65 63 72 65
74 2c 20 61 6e 64 20 69 74 20 6d 75 73 74 20 62
65 20 61 62 6c 65 20 74 6f 20 66 61 6c 6c 20 69
6e 74 6f 20 74 68 65 20 68 61 6e 64 73 20 6f 66
20 74 68 65 20 65 6e 65 6d 79 20 77 69 74 68 6f
75 74 20 69 6e 63 6f 6e 76 65 6e 69 65 6e 63 65
IV = 1a f3 8c 2d c2 b9 6f fd d8 66 94 09 23 41 bc 04
A = 54 68 65 20 73 65 63 6f 6e 64 20 70 72 69 6e 63
69 70 6c 65 20 6f 66 20 41 75 67 75 73 74 65 20
4b 65 72 63 6b 68 6f 66 66 73
PS = 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10
AL = 00 00 00 00 00 00 01 50
S = 1a f3 8c 2d c2 b9 6f fd d8 66 94 09 23 41 bc 04
89 31 29 b0 f4 ee 9e b1 8d 75 ed a6 f2 aa a9 f3
60 7c 98 c4 ba 04 44 d3 41 62 17 0d 89 61 88 4e
58 f2 7d 4a 35 a5 e3 e3 23 4a a9 94 04 f3 27 f5
c2 d7 8e 98 6e 57 49 85 8b 88 bc dd c2 ba 05 21
8f 19 51 12 d6 ad 48 fa 3b 1e 89 aa 7f 20 d5 96
68 2f 10 b3 64 8d 3b b0 c9 83 c3 18 5f 59 e3 6d
28 f6 47 c1 c1 39 88 de 8e a0 d8 21 19 8c 15 09
77 e2 8c a7 68 08 0b c7 8c 35 fa ed 69 d8 c0 b7
d9 f5 06 23 21 98 a4 89 a1 a6 ae 03 a3 19 fb 30
T = dd 13 1d 05 ab 34 67 dd 05 6f 8e 88 2b ad 70 63
7f 1e 9a 54 1d 9c 23 e7
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C = 1a f3 8c 2d c2 b9 6f fd d8 66 94 09 23 41 bc 04
89 31 29 b0 f4 ee 9e b1 8d 75 ed a6 f2 aa a9 f3
60 7c 98 c4 ba 04 44 d3 41 62 17 0d 89 61 88 4e
58 f2 7d 4a 35 a5 e3 e3 23 4a a9 94 04 f3 27 f5
c2 d7 8e 98 6e 57 49 85 8b 88 bc dd c2 ba 05 21
8f 19 51 12 d6 ad 48 fa 3b 1e 89 aa 7f 20 d5 96
68 2f 10 b3 64 8d 3b b0 c9 83 c3 18 5f 59 e3 6d
28 f6 47 c1 c1 39 88 de 8e a0 d8 21 19 8c 15 09
77 e2 8c a7 68 08 0b c7 8c 35 fa ed 69 d8 c0 b7
d9 f5 06 23 21 98 a4 89 a1 a6 ae 03 a3 19 fb 30
dd 13 1d 05 ab 34 67 dd 05 6f 8e 88 2b ad 70 63
7f 1e 9a 54 1d 9c 23 e7
5.4. AEAD_AES_256_CBC_HMAC_SHA512
K = 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
20 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f
30 31 32 33 34 35 36 37 38 39 3a 3b 3c 3d 3e 3f
MAC_KEY = 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
ENC_KEY = 20 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f
30 31 32 33 34 35 36 37 38 39 3a 3b 3c 3d 3e 3f
P = 41 20 63 69 70 68 65 72 20 73 79 73 74 65 6d 20
6d 75 73 74 20 6e 6f 74 20 62 65 20 72 65 71 75
69 72 65 64 20 74 6f 20 62 65 20 73 65 63 72 65
74 2c 20 61 6e 64 20 69 74 20 6d 75 73 74 20 62
65 20 61 62 6c 65 20 74 6f 20 66 61 6c 6c 20 69
6e 74 6f 20 74 68 65 20 68 61 6e 64 73 20 6f 66
20 74 68 65 20 65 6e 65 6d 79 20 77 69 74 68 6f
75 74 20 69 6e 63 6f 6e 76 65 6e 69 65 6e 63 65
IV = 1a f3 8c 2d c2 b9 6f fd d8 66 94 09 23 41 bc 04
A = 54 68 65 20 73 65 63 6f 6e 64 20 70 72 69 6e 63
69 70 6c 65 20 6f 66 20 41 75 67 75 73 74 65 20
4b 65 72 63 6b 68 6f 66 66 73
PS = 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10
AL = 00 00 00 00 00 00 01 50
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S = 1a f3 8c 2d c2 b9 6f fd d8 66 94 09 23 41 bc 04
4a ff aa ad b7 8c 31 c5 da 4b 1b 59 0d 10 ff bd
3d d8 d5 d3 02 42 35 26 91 2d a0 37 ec bc c7 bd
82 2c 30 1d d6 7c 37 3b cc b5 84 ad 3e 92 79 c2
e6 d1 2a 13 74 b7 7f 07 75 53 df 82 94 10 44 6b
36 eb d9 70 66 29 6a e6 42 7e a7 5c 2e 08 46 a1
1a 09 cc f5 37 0d c8 0b fe cb ad 28 c7 3f 09 b3
a3 b7 5e 66 2a 25 94 41 0a e4 96 b2 e2 e6 60 9e
31 e6 e0 2c c8 37 f0 53 d2 1f 37 ff 4f 51 95 0b
be 26 38 d0 9d d7 a4 93 09 30 80 6d 07 03 b1 f6
T = 4d d3 b4 c0 88 a7 f4 5c 21 68 39 64 5b 20 12 bf
2e 62 69 a8 c5 6a 81 6d bc 1b 26 77 61 95 5b c5
C = 1a f3 8c 2d c2 b9 6f fd d8 66 94 09 23 41 bc 04
4a ff aa ad b7 8c 31 c5 da 4b 1b 59 0d 10 ff bd
3d d8 d5 d3 02 42 35 26 91 2d a0 37 ec bc c7 bd
82 2c 30 1d d6 7c 37 3b cc b5 84 ad 3e 92 79 c2
e6 d1 2a 13 74 b7 7f 07 75 53 df 82 94 10 44 6b
36 eb d9 70 66 29 6a e6 42 7e a7 5c 2e 08 46 a1
1a 09 cc f5 37 0d c8 0b fe cb ad 28 c7 3f 09 b3
a3 b7 5e 66 2a 25 94 41 0a e4 96 b2 e2 e6 60 9e
31 e6 e0 2c c8 37 f0 53 d2 1f 37 ff 4f 51 95 0b
be 26 38 d0 9d d7 a4 93 09 30 80 6d 07 03 b1 f6
4d d3 b4 c0 88 a7 f4 5c 21 68 39 64 5b 20 12 bf
2e 62 69 a8 c5 6a 81 6d bc 1b 26 77 61 95 5b c5
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6. Security Considerations
The algorithms defined in this document use the generic composition
of CBC encryption with HMAC authentication, with the encrypt-then-MAC
method defined in Section 4.3 of [BN00]. This method has sound and
well-understood security properties; for details, please see that
reference. Note that HMAC is a good pseudorandom function and is
"strongly unforgeable", and thus meets all of the security goals of
that reference.
Implementations of the encryption and decryption algorithms should
avoid side channels that would leak information about the secret key.
To avoid timing channels, the processing time should be independent
of the secret key. The Encrypt-then-MAC construction used in this
note has some inherent strength against timing attacks because,
during the decryption operation, the authentication check is computed
before the plaintext padding is processed. However, the security of
the algorithm still relies on the absence of timing channels in both
CBC and HMAC. Additionally, comparison between the authentication
tag T and the HMAC output should be done using a constant-time
operation.
During the decryption process, the inputs A and C are mapped into the
input of the HMAC algorithm. It is essential for security that each
possible input to the MAC algorithm corresponds unambiguously to
exactly one pair (A, C) of possible inputs. The fact that this
property holds can be verified as follows. The HMAC input is X = A
|| C || len(A). Let (A,C) and (A',C') denote two distinct input
pairs, in which either 1) A != A' and C = C', 2) C != C and A = A',
or 3) both inequalities hold. We also let X' = A' || C' || len(A').
In cases 1 and 2, X != X' follows immediately. In case 3, if len(A)
!= len(A'), then X != X' directly. If len(A) = len(A'), then X != X
follows from the fact that the initial len(A) bits of X and X' must
be distinct.
There are security benefits to providing both confidentiality and
authentication in a single atomic operation, as done in this note.
This tight binding prevents subtle attacks such as the padding oracle
attack.
As with any block cipher mode of operation, the security of AES-CBC
degrades as the amount of data that is process increases. Each fixed
key value SHOULD NOT be used to protect more than 2^64 bytes of data.
This limit ensures that the AES-CBC algorithm will stay under the
birthday bound, i.e. because of the limit, it is unlikely that there
will be two AES plaintext inputs that are equal. (If this event
occurs, information about the colliding plaintexts is leaked, so it
is desirable to bound the amount of plaintext processed in order to
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make it unlikely.)
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7. Acknowledgements
Thanks are due to Matt Miller for his constructive feedback, Kelly
Burgin, Michael Peck, and Mike Jones for their suggestions and help,
and Jim Schaad, Rob Napier, James Manger, and David Jacobson for
their excellent review and suggestions.
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8. References
8.1. Normative References
[FIPS186-2]
"FIPS 180-2: Secure Hash Standard,", Federal Information
Processing Standard
(FIPS) http://www.itl.nist.gov/fipspubs/fip180-1.htm.
[FIPS197] "FIPS 197: Advanced Encryption Standard (AES)", Federal
Information Processing Standard
(FIPS) http://www.itl.nist.gov/fipspubs/fip197.htm.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4231] Nystrom, M., "Identifiers and Test Vectors for HMAC-SHA-
224, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512",
RFC 4231, December 2005.
[RFC4868] Kelly, S. and S. Frankel, "Using HMAC-SHA-256, HMAC-SHA-
384, and HMAC-SHA-512 with IPsec", RFC 4868, May 2007.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, January 2008.
8.2. Informative References
[AP12] Paterson, K. and N. AlFardan, "Plaintext-Recovery Attacks
Against Datagram TLS", Network and Distributed System
Security Symposium (NDSS)
2012 http://www.isg.rhul.ac.uk/~kp/dtls.pdf, 2012.
[B96] Bellovin, S., "Problem areas for the IP security
protocols", Proceedings of the Sixth Usenix Unix Security
Symposium https://www.cs.columbia.edu/~smb/papers/
badesp.pdf, 1996.
[BN00] "Authenticated encryption: Relations among notions and
analysis of the generic composition paradigm", Proceedings
of ASIACRYPT 2000, Springer-Verlag, LNCS 1976, pp. 531-
545 http://www-cse.ucsd.edu/users/mihir/papers/oem.html.
[CHVV03] Vaudenay, S., Canvel, B., Hiltgen, A., and M. Vuagnoux,
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"Password Interception in a SSL/TLS Channel", CRYPT0
2003 http://lasecwww.epfl.ch/pub/lasec/doc/CHVV03.ps,
2003.
[DP07] Paterson, K. and J. Degabriele, "Attacking the IPsec
Standards in Encryption-only Configurations", IEEE
Symposium on Privacy and
Security http://eprint.iacr.org/2007/125.pdf, 2007.
[DP10] Paterson, K. and J. Degabriele, "On the (in)security of
IPsec in MAC-then-encrypt configurations.", ACM Conference
on Computer and Communications Security (ACM CCS)
2010 http://www.isg.rhul.ac.uk/~kp/CCSIPsecfinal.pdf,
2010.
[M04] Moeller, B., "Security of CBC Ciphersuites in SSL/TLS:
Problems and Countermeasures", Web
Page http://www.openssl.org/~bodo/tls-cbc.txt, 2004.
[PRS11] Paterson, K., Ristenpart, T., and T. Shrimpton, "Tag Size
Does Matter: Attacks and Proofs for the TLS Record
Protocol", IEEE Symposium on Security and Privacy
2012 http://www.isg.rhul.ac.uk/~kp/mee-comp.pdf,
January 2012.
[R02] "Authenticated encryption with Associated-Data",
Proceedings of the 2002 ACM Conference on Computer and
Communication Security (CCS'02), pp. 98-107, ACM Press,
2002. http://www.cs.ucdavis.edu/~rogaway/papers/ad.pdf.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[SP800-38]
Dworkin, M., "NIST Special Publication 800-38:
Recommendation for Block Cipher Modes of Operation", U.S.
National Institue of Standards and Technology http://
csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf.
[V02] Vaudenay, S., "Security Flaws Induced by CBC Padding -
Applications to SSL, IPSEC, WTLS ....", EUROCRYPT 2002 htt
p://lasecwww.epfl.ch/php_code/publications/
search.php?ref=Vau02a, 2002.
[YHR04] Yu, T., Hartman, S., and K. Raeburn, "The Perils of
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Unauthenticated Encryption: Kerberos Version 4", Network
and Distributed Security Symposium (NDSS)
2004 http://web.mit.edu/tlyu/papers/krb4peril-ndss04.pdf,
2004.
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Appendix A. CBC Encryption and Decryption
The Cipher Block Chaining (CBC) mode of operation is defined in
Section 6.2 of [SP800-38]. This section recalls how that mode works,
for the convenience of the reader. The following notation is used:
K denotes the key of the underlying block cipher,
The function CIPHER(K, P) denotes the encryption of the block P
with the block cipher, where P contains exactly b bits,
The function CIPHER-INV(K, C) denotes the decryption of the block
C with the block cipher, where C contains exactly b bits; this is
the inverse operation of CIPHER(), and CIPHER-INV(K, CIPHER(K, P))
= P for all P and all K,
P_1, P_2, ... , P_n denotes the sequence of plaintext blocks,
where each block contains exactly b bits,
C_0, C_1, C_2, ... , C_n denotes the sequence of ciphertext
blocks, where each block contains exactly b bits,
P_i and C_i denote the ith blocks of the plaintext, and
IV denotes the initialization vector, which contains exactly b
bits.
The CBC encryption operation (denoted as CBC-ENC) takes as input a
sequence of n plaintext blocks and produces a sequence of n + 1
ciphertext blocks as follows:
IV = random
C_i = / IV if i=0,
\ CIPHER(K, P_i XOR C_{i-1}) if i=1, 2, ... , n.
CBC-ENC(K, P_1 || P_2 || ... || P_n) returns the value C_0 || C_1 ||
C_2 || ... || C_n. Note that the returned value is one block longer
than the input value, and that C_0 = IV.
The IV MUST be generated using a uniformly random process, or a
pseudorandom process with a cryptographic strength equivalent to that
of the underlying block cipher; see [RFC4086] for background on
random sources. It MUST NOT be predictable to an attacker; in
particular, it MUST NOT be set to the value of any previous
ciphertext blocks.
The CBC decryption operation (denoted as CBC-DEC) takes as input a
sequence of m ciphertext blocks and produces a sequence of m-1
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plaintext blocks as follows:
P_i = CIPHER-INV(K, C_i) XOR C_{i-1} for i=1, 2, ... , n.
The initial block C_0 corresponds to the initialization vector.
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Appendix B. Alternative Interface for Legacy Encoding
In some scenarios, cryptographic data such as the ciphertext,
initialization vector, and message authentication tag are encoded
separately. To allow for the use of the algorithms defined in this
document in such scenarios, this appendix describes an interface in
which those data elements are discrete. New implementations SHOULD
NOT use this interface, because it is incompatible with other
authenticated encryption methods and is more complex; however, it MAY
be useful in scenarios in which the separate encoding is already in
use.
The alternative interface is as follows. The inputs to the
encryption operation the same as those defined in Section 2.1 (the
secret key K, the plaintext P, the associated data A). The outputs
of the encryption operation are:
the initialization vector IV as defined in Appendix A,
the ciphertext C, as defined in Appendix A, and
the message authentication tag T, as defined in Section 2.1.
The inputs to the decryption operation (in addition to K and A) are:
the initialization vector IV as defined in Appendix A,
the ciphertext C as defined in Appendix A, excluding the initial
block C_0 (which is equal to the IV), and
the message authentication tag T, as defined in Section 2.1.
The output of the decryption operation are the same as that defined
in Section 2.2 (either a plaintext value P or a special symbol FAIL
that indicates that the inputs are not authentic).
All processing other than the encoding and decoding of IV, C, and T
is done as defined above. In particular, the IV is an output of the
encryption operation, rather than an input.
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Authors' Addresses
David McGrew
Cisco Systems
13600 Dulles Technology Drive
Herndon, VA 20171
US
Email: mcgrew@cisco.com
URI: http://www.mindspring.com/~dmcgrew/dam.htm
John Foley
Cisco Systems
7025-2 Kit Creek Road
Research Triangle Park, NC 14987
US
Email: foleyj@cisco.com
Kenny Paterson
Royal Holloway, University of London
TW20 0EX
Egham, Surrey TW20 0EX
UK
Phone: +44 1784 414393
Email: Kenny.Paterson@rhul.ac.uk
URI: http://www.isg.rhul.ac.uk/~kp/
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