Network Working Group M. Oehler (NSA)
R. Glenn (NIST)
Internet Draft May 1, 1996
HMAC-MD5 IP Authentication with Replay Prevention
<draft-ietf-ipsec-ah-hmac-md5-00.txt>
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Abstract
This document describes a keyed-MD5 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 Authentication Header (AH) [RFC-1826] provides integrity and
authentication for IP datagrams. The transform specified in this
document uses a keyed-MD5 mechanism [HMAC-MD5]. The mechanism uses
the (key-less) MD5 hash function [RFC-1321] which produces a message
authentication code. 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. 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].
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]
There is no mandated key size for the HMAC-MD5 transform.
Implementations must support a key length of any size, except zero.
It is advised that keys be chosen as the length of the hash output,
or 128-bits for MD5. For other key lengths, the following concerns
must be considered.
A key length of zero is prohibited and implementations should provide
an alert, since the authentication data would be identical to that of
MD5, solely. Less than 16 bytes is strongly discouraged as it would
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decrease the security strength of the function. Keys longer than 16
bytes are acceptable, but the extra length would not significantly
increase the function strength. A longer key may be advisable if the
randomness of the key is suspect. MD5 operates on 64-byte blocks.
Keys longer than 64 bytes are first hashed using MD5. The resulting
hash is then used to calculate the authentication data.
1.2 Data Size
MD5 produces a 128-bit value which is used as the authentication
data. It is naturally 64 bit aligned and thus, does not need any
padding for machines with native double words.
2. Packet Format
+---------------+---------------+---------------+---------------+
| Next Header | Length | RESERVED |
+---------------+---------------+---------------+---------------+
| SPI |
+---------------+---------------+---------------+---------------+
+ Replay Prevention (optional) |
+---------------+---------------+---------------+---------------+
| |
+ 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 32 bit value used to guarantee that
each packet exchanged between two parties is different. This field
is similar to the one specified in [ESP-DES-MD5]. The SPI is used to
determine whether or not the field is included in the packet (i.e. if
it is not included, the header will have the SPI directly followed by
the Authentication Data). Without this field it is possible to
attack a system by retransmitting packets.
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. The
implementation may accept of out of order packets. The number of
packets to accept out of order is an implementation detail. If a "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.
[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 authentication data is the output of the authentication algorithm
(MD5). This value 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 [RFC-1826].
The Replay Prevention field if present, is included in the
calculation.
The definition and reference implementation of MD5 appears in [RFC-
1321]. Let 'text' denote the data to which HMAC-MD5 is to be applied
and K be the message authentication secret key shared by the parties.
We define two fixed and different strings ipad and opad as follows
(the 'i' and 'o' are mnemonics for inner and outer):
ipad = the byte 0x36 repeated 64 times
opad = the byte 0x5C repeated 64 times.
To compute HMAC-MD5 over the data `text' we perform
MD5(K XOR opad, MD5(K XOR ipad, text))
Namely,
(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) append the data stream 'text' to the 64 byte string resulting
from step (2)
(4) apply MD5 to the stream generated in step (3)
(5) XOR (bitwise exclusive-OR) the 64 byte string computed in
step (1) with opad
(6) append the MD5 result from step (4) to the 64 byte string
resulting from step (5)
(7) apply MD5 to the stream generated in step (6) and output
the result
This computation is described in more detail, along with example
code and performance improvements, in [HMAC-MD5].
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3. Security Considerations
The security of this transform depends heavily on the strength of MD5
and the associated secret key. [HMAC-MD5] contains a detailed
discussion on the strengths and weaknesses of MD5.
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.
References
[RFC-1825] R. Atkinson, "Security Architecture for the Internet
Protocol", RFC-1852, Naval Research Laboratory, July 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-1321] R. Rivest, "The MD5 Message-Digest Algorithm",
RFC-1321, April 1992.
[HMAC-MD5] H. Krawczyk, M. Bellare, R. Canetti, "HMAC-MD5: Keyed-MD5
for Message Authentication", Internet Draft, March, 1996.
[ESP-DES-MD5] J. Hughes, "Combined DES-CBC, MD5, and Replay Prevention
Security Transform", Internet Draft, April, 1996.
Contacts
Michael J. Oehler
National Security Agency
Atn: R23, INFOSEC Research and Development
9800 Savage Road
Fort Meade, MD 20755
mjo@tycho.ncsc.mil
Robert Glenn
NIST
Building 820, Room 455
Gaithersburg, MD 20899
rob.glenn@nist.gov
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