Network Working Group S. Chang (NIST)
R. Glenn (NIST)
March 20, 1997
Internet Draft
HMAC-SHA-1-96 IP Authentication with Replay Prevention
<draft-ietf-ipsec-ah-hmac-sha-1-96-00.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 [RFC-2104]. A replay prevention
field is also specified.
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Contents
1. Introduction...................................................3
1.1 Terminology.................................................3
1.2 Keys........................................................4
1.3 Data Size...................................................4
2 Packet Format..................................................5
2.1 Replay Prevention...........................................5
2.2 Authentication Data Calculation.............................6
3. Security Considerations........................................7
Acknowledgments....................................................7
References.........................................................8
Authors' Addresses.................................................8
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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 [RFC-2104]. 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 specified 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: Keyed Hashing for
Message Authentication" [RFC-2104].
All implementations that claim conformance or compliance with the IP
Authentication Header specification [RFC-1826] SHOULD implement this
HMAC-SHA-1-96 transform.
1.1 Terminology
In this document, the words that are used to define the
significance of each particular requirement are usually capitalized.
These words are:
- MUST
This word or the adjective "REQUIRED" means that the item is an
absolute requirement of the specification.
- SHOULD
This word or the adjective "RECOMMENDED" means that there might
exist valid reasons in particular circumstances to ignore this item,
but the full implications should be understood and the case carefully
weighed before taking a different course.
- MAY
This word or the adjective "OPTIONAL" means that this item is truly
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optional. One vendor might choose to include the item because a
particular marketplace requires it or because it enhances the product,
for example; another vendor may omit the same item.
For the purpose of this specification, the terms conformance and
compliance are synonymous.
1.2 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. [RFC-2104]
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
be the length of the hash algorithm 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.
1.3 Data Size
HMAC-SHA-1 generates a message digest of 160 bits. HMAC-SHA-1-96 uses
the first or left most 96 bits as the Authentication Data. This
procedure is known as truncation. In the case of this transform,
truncation is used to help maintain 64-bit packet header alignment,
eliminate unnecessary overhead, and potentially provided stronger
authentication. [RFC-2104] provides more information on the
advantages and disadvantages of truncation.
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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. The Length field will
always be set to 4 (128 bits) for HMAC-SHA-1-96.
2.1 Replay Prevention
The Replay Prevention field is a 32-bit value used to guarantee that
each packet exchanged between two parties is different. Each IPsec
Security Association specifies whether Replay Prevention is used for
that Security Association. The Replay Prevention field is always
included in the calculation of the Authentication Data. If Replay
Prevention is NOT in use, the 32-bit value is set to 0, included in
the calculation of the Authentication Data, and ignored upon receipt
with regard to checking for replay. This field is used to help
prevent attacks in which a message is stored and re-used later,
replacing or repeating the original. Replay Prevention SHOULD be
implemented.
Replay Prevention SHOULD be implemented. If Replay Prevention is not
implemented, the 32-bit field remains are part of the AH and is
treated as if Replay Prevention is NOT in use (i.e. the 32-bit value
is set to 0, included in the calculation of the Authentication Data,
and ignored upon receipt with regard to checking for replay.
The 32-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 2^32
packets using a single key.
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Upon receipt, the replay value is assured to be increasing. An
implementation MAY accept out of order packets. If an "out of order
window" is supported, an implementation MUST guarantee that any and
all packets accepted out of order have not arrived before. That is,
an implementation MUST accept any packet, at most, once. The size of
the window is a negotiated value specified by the IPsec Security
Association.
[ESP-DES-MD5] provides more information on negotiated windows sizes,
example code that implements a 32 packet replay window, and a test
routine to show how it could be implemented.
When the destination address is a multicast address and more than one
sender is sharing the same IPsec Security Association to that
multicast destination address, then Replay Prevention SHOULD NOT be
enabled. When Replay Prevention is desired for a multicast session
having multiple senders to the same multicast destination address,
each sender SHOULD have its own IPsec Security Association.
2.2 Authentication Data Calculation
The Authentication Data is the output of the SHA authentication
algorithm as 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, used or not, is included in the calculation.
To compute HMAC-SHA-1 over the data 'text', the following is
calculated:
SHA (K XOR opad, SHA (K XOR ipad, text))
The result of which is truncated to 96 bits (retaining the left most
bits) to produce HMAC-SHA-1-96.
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. The two strings are:
ipad = the byte 0x36 repeated 64 times,
opad = the byte 0x5C repeated 64 times.
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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 16 bytes it will be appended with 48 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) use the left most 96 bits of the result obtained in (7) as the final
result
A similar computation is described in more detail, along with example
code and performance improvements, in [RFC-2104]. Implementers
should consult [RFC-2104] for more information on the HMAC 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. [RFC-2104]
contains a detailed discussion on the strengths and weaknesses of
HMAC algorithms. [HMAC-TESTS] contains test vectors and example code
to assist in verifying the correctness of HMAC-SHA-1 code.
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 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.
[RFC-2104] H. Krawczyk, M. Bellare, R. Canetti, "HMAC: Keyed Hashing
for Message Authentication", RFC-2104, February, 1997.
[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)
[ESP-DES-MD5] J. Hughes, "Combined DES-CBC, MD5, and Replay Prevention
Security Transform", Internet Draft, September 1996.
[HMAC-TESTS] P. Cheng, R. Glenn, "Test Cases for HMAC-MD5 and HMAC-SHA-1",
Internet Draft, March 1997.
Authors' Addresses
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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