This document defines an Experimental PRotocol for the Internet community. This does not specify an Internet standard of any kind. Discussion and suggestions for improvement are requested. Distribution of this memo is unlimited.
Abstract
This document describes the use of keyed SHA 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 (SHA) [FIPS-180-1].
It should be noted that this document specifies a newer version of the SHA than that described in [FIPS-180], which was flawed.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], which defines the overall security plan for IP, and provides important background for this specification.
1.1. Keys
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 up to 160 bits MUST be supported by the implementation, although any particular key may be shorter. Longer keys are encouraged.
1.2. Data Size
SHA's 160-bit output is naturally 32-bit aligned. However, many implementations require 64-bit alignment of the following headers, in which case an additional 32 bits of padding is added, either before or after the SHA output.
The size and position of this padding are negotiated as part of the key management. Padding bits are filled with unspecified implementation dependent (random) values, which are ignored on receipt.
1.3. Performance
Preliminary results indicate that SHA is 62% as fast as md5, and 80% as fast as DES hashing. That is,
SHA < DES < MD5
Nota Bene: Suggestions are sought on alternative authentication algorithms that have significantly faster throughput, are not patent- encumbered, and still retain adequate cryptographic strength.
2. Calculation
The 160-bit digest is calculated as described in [FIPS-180-1]. At the time of writing, a portable C language implementation of SHA is available via FTP from ftp://rand.org/pub/jim/sha.tar.gz.
The form of the authenticated message is
key, keyfill, datagram, key, SHAfill
First, the variable length secret authentication key is filled to the next 512-bit boundary, using the same pad with length technique defined for SHA.
Then, the filled key is concatenated with (immediately followed by) the invariant fields of the entire IP datagram (variant fields are zeroed), 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 SHA itself. The 160-bit SHA 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. The padding technique includes a length which protects arbitrary length keys. Filling to the SHA 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 minimal protection for very long and very short datagrams, and marginal 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.
Security Considerations
Users need to understand that the quality of the security provided by this specification depends completely on the strength of the SHA hash function, the correctness of that algorithm's implementation, the security of the key management mechanism and its implementation, the strength of the key [CN94], 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 SHA algorithm. That is, there are no known attacks on SHA or any of its components that are better than brute force, and the 160-bit hash output by SHA 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 SHA 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.
Comments should be submitted to the ipsec@ans.net mailing list.
References
[CN94] John M. Carroll & Sri Nudiati, "On Weak Keys and Weak Data: Foiling the Two Nemeses", Cryptologia, Vol. 18 No. 23 pp. 253-280, July 1994.
[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).
[RFC-1320] Ronald Rivest, "The MD4 Message-Digest Algorithm", RFC-1320, April 1992.
[RFC-1700] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1700, USC/Information Sciences Institute, October 1994.
[RFC-1825] Atkinson, R., "Security Architecture for the Internet Protocol", RFC-1825, Naval Research Laboratory, July 1995.
[RFC-1826] Atkinson, R., "IP Authentication Header", RFC-1826, Naval Research Laboratory, July 1995.
Author's Address
Questions about this memo can also be directed to:
Perry Metzger Piermont Information Systems Inc. 160 Cabrini Blvd., Suite #2 New York, NY 10033
perry@piermont.com
William Allen Simpson Daydreamer Computer Systems Consulting Services 1384 Fontaine Madison Heights, Michigan 48071
