Network Working Group S. Chang (NIST)
R. Glenn (NIST)
October 30, 1996
Internet Draft
HMAC-SHA IP Authentication with Replay Prevention
<draft-ietf-ipsec-ah-hmac-sha-03.txt>
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Abstract
This document describes a keyed-SHA transform to be used in
conjunction with the IP Authentication Header [RFC-1826]. The
particular transform is based on [HMAC-MD5]. An option is also
specified to guard against replay attacks.
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Contents
1. Introduction...................................................3
1.1 Keys........................................................3
1.2 Data Size...................................................4
2 Packet Format..................................................4
2.1 Replay Prevention...........................................4
2.2 Authentication Data Calculation.............................5
3. Security Considerations........................................6
ACKNOWLEDGMENTS....................................................6
REFERENCES.........................................................6
CONTACTS...........................................................6
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1. Introduction
The IP Authentication Header (AH) provides integrity and
authentication for IP datagrams [RFC-1826]. The transform specified
in this document uses a keyed-SHA mechanism based on [HMAC-MD5]. The
mechanism uses the (key-less) SHA hash function [FIPS-180-1] which
produces a message digest. When combined with an AH Key,
authentication data is produced. This value is placed in the
Authentication Data field of the AH [RFC-1826]. This value is also
the basis for the data integrity service offered by the AH protocol.
To provide protection against replay attacks, a Replay Prevention
field is included as a transform option. This field is used to help
prevent attacks in which a message is stored and re-used later,
replacing or repeating the original. The Security Parameters Index
(SPI) [RFC-1825] is used to determine whether this option is included
in the AH.
Familiarity with the following documents is assumed: "Security
Architecture for the Internet Protocol" [RFC-1825], "IP
Authentication Header" [RFC-1826], and "HMAC-MD5: Keyed-MD5 for
Message Authentication" [HMAC-MD5].
All implementations that claim conformance or compliance with the IP
Authentication Header specification [RFC-1826] SHOULD implement this
HMAC-SHA transform.
1.1 Keys
The AH Key is used as a shared secret between two communicating
parties. The Key is not a cryptographic key as used in a traditional
sense. Instead, the AH key (shared secret) is hashed with the
transmitted data and thus, assures that an intervening party cannot
duplicate the authentication data.
Even though an AH key is not a cryptographic key, the rudimentary
concerns of cryptographic keys still apply. Consider that the
algorithm and most of the data used to produce the output is known.
The strength of the transform lies in the singular mapping of the key
(which needs to be strong) and the IP datagram (which is known) to
the authentication data. Thus, implementations should, and as
frequently as possible, change the AH key. Keys need to be chosen at
random, or generated using a cryptographically strong pseudo-random
generator seeded with a random seed. [HMAC-MD5]
All conforming and compliant implementations MUST support a key
length of 160 bits or less. Implementations SHOULD support longer
key lengths as well. It is advised that the key length be chosen to
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be the length of the hash output, which is 160 bits for SHA. For
other key lengths the following concerns MUST be considered.
A key length of zero is prohibited and implementations MUST prevent
key lengths of zero from being used with this transform, since no
effective authentication could be provided by a zero-length key. SHA
operates on 64-byte blocks. Keys longer than 64-bytes are first
hashed using SHA. The resulting hash is then used to calculate the
authentication data."
1.2 Data Size
SHA generates a message digest of 160 bits. To maintain 64-bit word
alignment, all conforming and compliant implementations MUST include
the ability to pad the message digest to 192 bits as described in
this paragraph. Implementations MAY also include the ability to use
the 160 bit message digest with out the pad when 64-bit alignment is
not required. Padding is added by appending 32 zero bits to SHA
message digest. The length of the Authentication Data, specified in
the Length field of the AH in 32-bit words, should include the
padding bits, if present. Upon receipt, the value of the padded bits
MUST be zero and are otherwise ignored.
2. Packet Format
+---------------+---------------+---------------+---------------+
| Next Header | Length | RESERVED |
+---------------+---------------+---------------+---------------+
| SPI |
+---------------+---------------+---------------+---------------+
| Replay Prevention |
| |
+---------------+---------------+---------------+---------------+
| |
+ Authentication Data |
| |
+---------------+---------------+---------------+---------------+
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
The Next Header, RESERVED, and SPI fields are specified in [RFC-
1826]. The Length field is the length of the Replay Prevention field
and the Authentication Data in 32-bit words.
2.1 Replay Prevention
The Replay Prevention field is a 64-bit value used to guarantee that
each packet exchanged between two parties is different. Each IPsec
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Security Association specifies whether Replay Prevention is used for
that Security Association. If Replay Prevention is NOT in use, then
the Authentication Data field will directly follow the SPI field.
This field is used to help prevent attacks in which a message is
stored and re-used later, replacing or repeating the original.
The 64-bit field is an up counter starting at a value of 1.
The secret shared key must not be used for a period of time that
allows the counter to wrap, that is, to transmit more than 2^64
packets using a single key.
Upon receipt, the replay value is assured to be increasing. The
implementation may accept out of order packets. The number of packets
to accept out of order is an implementation detail. If an "out of
order window" is supported, the implementation shall ensure that any
and all packets accepted out of order are guaranteed not to have
arrived before. That is, the implementation will accept any packet at
most once.
When the destination address is a multicast address, replay
protection is in use, and more than one sender is sharing the same
IPsec Security Association to that multicast destination address,
then Replay Protection SHOULD NOT be enabled. When replay protection
is desired for a multicast session having multiple senders to the
same multicast destination address, each sender SHOULD have its own
IPsec Security Association.
[ESP-DES-MD5] provides example code that implements a 32 packet
replay window and a test routine to show how it works.
2.2 Authentication Data Calculation
The computation of the 160-bit SHA digest is described
in [FIPS-180-1]. The digest is calculated over
the entire IP datagram. Fields within the datagram that are variant
during transit and the authentication data field itself must contain
all zeros prior to the computation [RFC-1826].
The Replay Prevention field, if present, is included in the calculation.
To compute HMAC-SHA over the data 'text', the following is calculated:
SHA (K XOR opad, SHA (K XOR ipad, text))
K denotes the secret key shared by the parties. If K is longer
than 64-bytes it MUST first be hashed using SHA.
In this case, K is the resulting hash. The variables 'ipad', 'opad'
denote fixed strings for inner and outer padding respectively.
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The two strings are:
ipad = the byte 0x36 repeated 64 times,
opad = the byte 0x5C repeated 64 times.
The calculation of the authentication data consists of the following steps:
(1) append zeros to the end of K to create a 64 byte string (e.g., if K is
of length 20 bytes it will be appended with 44 zero bytes 0x00)
(2) XOR (bitwise exclusive-OR) the 64 byte string computed in step (1) with
ipad
(3) concatenate to the 64 byte string resulting from step (2) the data
stream 'text'
(4) apply SHA to the stream generated in step (3)
(5) XOR the 64 byte string computed in step (1) with opad
(6) concatenate to the 64 byte string resulting from step (5) the SHA result
of step (4)
(7) apply SHA to the stream generated in step (6)
(8) The sender then zero pads the resulting hash to a 64-bit boundary
for word alignment. IPv4 implemenations choosing not to pad will not
zero pad the resulting hash. The receiver then compares the generated
160-bit hash to the first 160-bits of authentication data contained in
the AH.
A similar computation is described in more detail, along with example
code and performance improvements, in [HMAC-MD5]. Implementers
should consult [HMAC-MD5] for more information on this technique
for keying a cryptographic hash function.
3. Security Considerations
The security provided by this transform is based on the strength of
SHA, the correctness of the algorithm's implementation, the security
of the key management mechanism and its implementation, the strength
of the associated secret key, and upon the correctness of the
implementations in all of the participating systems.
At this time there are no known cryptographic attacks against SHA
[SCHNEIER]. The 160-bit digest makes SHA more resistant to brute
force attacks than MD4 and MD5 which produce a 128-bit digest.
Acknowledgments
This document is largely based on text written by Hugo Krawczyk. The
format used was derived from work by William Simpson and Perry Metzger.
The text on replay prevention is derived directly from work by Jim
Hughes.
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References
[RFC-1825] R. Atkinson, "Security Architecture for the Internet Protocol",
RFC-1825, August 1995.
[RFC-1826] R. Atkinson, "IP Authentication Header",
RFC-1826, August 1995.
[RFC-1828] P. Metzger, W. A. Simpson, "IP Authentication using Keyed MD5",
RFC-1828, August 1995.
[HMAC-MD5] H. Krawczyk, M. Bellare, R. Canetti, "HMAC-MD5: Keyed-MD5
for Message Authentication", Internet Draft, March, 1996.
[FIPS-180-1] NIST, FIPS PUB 180-1: Secure Hash Standard, April 1995.
[URL] http://csrc.nist.gov/fips/fip180-1.txt (ascii)
[URL] http://csrc.nist.gov/fips/fip180-1.ps (postscript)
[SCHNEIER] B. Schneier, "Applied Cryptography Protocols, Algorithms, and
Source Code in C", John Wiley & Sons, Inc. 1994.
[ESP-DES-MD5] J. Hughes, "Combined DES-CBC, MD5, and Replay Prevention
Security Transform", Internet Draft, April, 1996.
Contacts
Shu-jen Chang
NIST
Building 820, Room 456
Gaithersburg, MD 20899
shu-jen.chang@nist.gov
Robert Glenn
NIST
Building 820, Room 455
Gaithersburg, MD 20899
rob.glenn@nist.gov
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