Network Working Group P. Metzger Request for Comments: 2841 Piermont Category: Historic W. Simpson Obsoletes: 1852 DayDreamer November 2000
IP Authentication using Keyed SHA1 with Interleaved Padding (IP-MAC)
Status of this Memo
This memo defines a Historic Document for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This document describes the use of keyed SHA1 with the IP Authentication Header.
The Authentication Header (AH) [RFC-1826] provides integrity and authentication for IP datagrams. This specification describes the AH use of keys with the Secure Hash Algorithm (SHA1) [FIPS-180-1]. This SHA1-IP-MAC algorithm uses a leading and trailing key (a variant of the "envelope method"), with alignment padding between both keys and data.
It should be noted that this document specifies a newer version of SHA than that described in [FIPS-180], which was flawed. The older version is not interoperable with the newer version.
This document assumes that the reader is familiar with the related document "Security Architecture for the Internet Protocol" [RFC- 1825], that defines the overall security plan for IP, and provides important background for this specification.
The secret authentication key shared between the communicating parties SHOULD be a cryptographically strong random number, not a guessable string of any sort.
The shared key is not constrained by this transform to any particular size. Lengths of 160-bits (20 octets) MUST be supported by the implementation, although any particular key may be shorter. Longer keys are encouraged.
SHA1's 160-bit output is naturally 32-bit aligned. However, many implementations require 64-bit alignment of the following headers.
Therefore, several options are available for data alignment (most preferred to least preferred):
1) only the most significant 128-bits (16 octets) of output are used.
2) an additional 32-bits (4 octets) of padding is added before the SHA1 output.
3) an additional 32-bits (4 octets) of padding is added after the SHA1 output.
4) the SHA1 output is variably bit-positioned within 192-bits (24 octets).
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RFC 2841 AH SHA1 IP-MAC November 2000
The size and position of the output are negotiated as part of the key management. Padding bits are filled with unspecified implementation dependent (random) values, which are ignored on receipt.
Discussion:
Although truncation of the output for alignment purposes may appear to reduce the effectiveness of the algorithm, some analysts of attack verification suggest that this may instead improve the overall robustness [PO95a].
Preliminary results indicate that SHA1 is 62% as fast as MD5, and 80% as fast as DES hashing. That is:
SHA1 < DES < MD5
This appears to be a reasonable performance tradeoff, as SHA1 internal chaining is significantly longer than either DES or MD5:
DES < MD5 < SHA1
Nota Bene: Suggestions are sought on alternative authentication algorithms that have significantly faster throughput, are not patent- encumbered, and still retain adequate cryptographic strength.
The 160-bit digest is calculated as described in [FIPS-180-1]. A portable C language implementation of SHA1 is available via FTP from ftp://rand.org/pub/jim/sha.tar.gz.
First, the variable length secret authentication key is filled to the next 512-bit boundary, using the same pad-with-length technique defined for SHA1. The padding technique includes a length that protects arbitrary length keys.
Next, the filled key is concatenated with (immediately followed by) the invariant fields of the entire IP datagram (variant fields are zeroed). This is also filled to the next 512-bit boundary, using the same pad-with-length technique defined for SHA1. The length includes the leading key and data.
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RFC 2841 AH SHA1 IP-MAC November 2000
Then, the filled data is concatenated with (immediately followed by) the original variable length key again. A trailing pad-with-length to the next 512-bit boundary for the entire message is added by SHA1 itself.
Finally, the 160-bit SHA1 digest is calculated, and the result is inserted into the Authentication Data field.
Discussion:
The leading copy of the key is padded in order to facilitate copying of the key at machine boundaries without requiring re- alignment of the following datagram. Filling to the SHA1 block size also allows the key to be prehashed to avoid the physical copy in some implementations.
The trailing copy of the key is not necessary to protect against appending attacks, as the IP datagram already includes a total length field. It reintroduces mixing of the entire key, providing protection for very long and very short datagrams, and robustness against possible attacks on the IP length field itself.
When the implementation adds the keys and padding in place before and after the IP datagram, care must be taken that the keys and/or padding are not sent over the link by the link driver.
Added shortened 128-bit output, and clarify output text.
Added tradeoff text.
Changed padding technique to comply with Crypto '95 recommendations.
Updated references.
Updated contacts.
Minor editorial changes.
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RFC 2841 AH SHA1 IP-MAC November 2000
Security Considerations
Users need to understand that the quality of the security provided by this specification depends completely on the strength of the SHA1 hash function, the correctness of that algorithm's implementation, the security of the key management mechanism and its implementation, the strength of the key, and upon the correctness of the implementations in all of the participating nodes.
The SHA algorithm was originally derived from the MD4 algorithm [RFC-1320]. A flaw was apparently found in the original specification of SHA [FIPS-180], and this document specifies the use of a corrected version [FIPS-180-1].
At the time of writing of this document, there are no known flaws in the SHA1 algorithm. That is, there are no known attacks on SHA1 or any of its components that are better than brute force, and the 160- bit hash size of SHA1 is substantially more resistant to brute force attacks than the 128-bit hash size of MD4 and MD5.
However, as the flaw in the original SHA1 algorithm shows, cryptographers are fallible, and there may be substantial deficiencies yet to be discovered in the algorithm.
Acknowledgements
Some of the text of this specification was derived from work by Randall Atkinson for the SIP, SIPP, and IPv6 Working Groups.
Preliminary performance analysis was provided by Joe Touch.
Padding the leading copy of the key to a hash block boundary for increased performance was originally suggested by William Allen Simpson.
Padding the leading copy of the key to a hash block boundary for increased security was suggested by [KR95]. Including the key length for increased security was suggested by David Wagner.
Padding the datagram to a hash block boundary to avoid (an impractical) key recovery attack was suggested by [PO95b].
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RFC 2841 AH SHA1 IP-MAC November 2000
References
[FIPS-180] "Secure Hash Standard", Computer Systems Laboratory, National Institute of Standards and Technology, U.S. Department Of Commerce, May 1993.
Also known as: 58 Fed Reg 27712 (1993).
[FIPS-180-1] "Secure Hash Standard", National Institute of Standards and Technology, U.S. Department Of Commerce, April 1995.
Also known as: 59 Fed Reg 35317 (1994).
[KR95] Kaliski, B., and Robshaw, M., "Message authentication with MD5", CryptoBytes (RSA Labs Technical Newsletter), vol.1 no.1, Spring 1995.
[PO95a] Preneel, B., and van Oorshot, P., "MDx-MAC and Building Fast MACs from Hash Functions", Advances in Cryptology -- Crypto '95 Proceedings, Santa Barbara, California, August 1995.
[PO95b] Preneel, B., and van Oorshot, P., "On the Security of Two MAC Algorithms", Presented at the Rump Session of Crypto '95, Santa Barbara, California, August 1995.
[RFC 1320] Rivest, R., "The MD4 Message-Digest Algorithm", RFC 1320, April 1992.
[RFC 1700] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC 1700, October 1994.
[RFC 1825] Atkinson, R., "Security Architecture for the Internet Protocol", RFC 1825, July 1995.
[RFC 1826] Atkinson, R., "IP Authentication Header", RFC 1826, July 1995.
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RFC 2841 AH SHA1 IP-MAC November 2000
Contacts
Comments about this document should be discussed on the photuris@adk.gr mailing list.
This document is a submission to the IP Security Working Group of the Internet Engineering Task Force (IETF). The working group can be contacted via the current chairs:
Questions about this document can also be directed to:
Perry Metzger Piermont Information Systems Inc. 160 Cabrini Blvd., Suite #2 New York, NY 10033
EMail: perry@piermont.com
William Allen Simpson DayDreamer Computer Systems Consulting Services 1384 Fontaine Madison Heights, Michigan 48071
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