Network Working Group P Metzger
Internet Draft Piermont
W A Simpson
DayDreamer
expires in six months April 1996
IP Authentication using Keyed SHA1 with Data Padding
draft-simpson-ah-sha-kdp-00.txt
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Abstract
This document describes the use of keyed SHA1 with the IP Authentica-
tion Header.
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1. Introduction
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-KDP 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It should be noted that this document specifies a newer version of
the SHA than that described in [FIPS-180], which was flawed.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], that defines the overall security plan for IP, and pro-
vides important background for this specification.
1.1. Keys
The secret authentication key shared between the communicating par-
ties SHOULD be a cryptographically strong random number, not a guess-
able 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.
1.2. Data Size
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.
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4) the SHA1 output is variably bit-positioned within 192-bits (24
octets).
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].
1.3. Performance
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 inter-
nal 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.
2. Calculation
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.
The form of the authenticated message is:
SHA1( key, keyfill, datagram, datafill, key, sha1fill )
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 pro-
tects arbitrary length keys.
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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.
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 copy-
ing 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.
A. Changes
Changes from RFC-1852:
Use of SHA1 term (as always intended).
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.
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Minor editorial changes.
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 implementa-
tions 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 specifica-
tion of SHA [FIPS-180], and this document specifies the use of a cor-
rected 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, cryptogra-
phers 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 Ran-
dall 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 Simp-
son.
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 impracti-
cal) key recovery attack was suggested by [PO95b].
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References
[FIPS-180]
"Secure Hash Standard", Computer Systems Laboratory,
National Institute of Standards and Technology, U.S. Depart-
ment 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]
Ronald Rivest, "The MD4 Message-Digest Algorithm", RFC-1320,
April 1992.
[RFC-1700]
Reynolds, J., and Postel, J., "Assigned Numbers", STD 2, RFC
1700, USC/Information Sciences Institute, October 1994.
[RFC-1825]
Atkinson, R., "Security Architecture for the Internet Proto-
col", RFC-1825, Naval Research Laboratory, July 1995.
[RFC-1826]
Atkinson, R., "IP Authentication Header", RFC-1826, Naval
Research Laboratory, July 1995.
Contacts
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Comments about this document should be discussed on the ipsec-
dev@terisa.com mailing list.
Questions about this document 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
wsimpson@UMich.edu
wsimpson@GreenDragon.com (preferred)
bsimpson@MorningStar.com
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