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+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.
+
+Table of Contents
+
+   1.   Introduction ............................................. 2
+   1.1. Keys ..................................................... 2
+   1.2. Data Size ................................................ 2
+   1.3. Performance .............................................. 3
+   2.   Calculation .............................................. 3
+   A.   Changes .................................................. 5
+   Security Considerations ....................................... 6
+   Acknowledgements .............................................. 6
+   References .................................................... 7
+   Contacts ...................................................... 8
+   Editor's Note ................................................. 8
+   Full Copyright Statement ...................................... 9
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+Metzger & Simpson               Historic                        [Page 1]
+
+RFC 2841                     AH SHA1 IP-MAC                November 2000
+
+
+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-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.
+
+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 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.
+
+   4) the SHA1 output is variably bit-positioned within 192-bits (24
+      octets).
+
+
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+
+Metzger & Simpson               Historic                        [Page 2]
+
+RFC 2841                     AH SHA1 IP-MAC                November 2000
+
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+   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
+   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.
+
+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
+   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.
+
+
+
+Metzger & Simpson               Historic                        [Page 3]
+
+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.
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+Metzger & Simpson               Historic                        [Page 4]
+
+RFC 2841                     AH SHA1 IP-MAC                November 2000
+
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+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.
+
+   Minor editorial changes.
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+Metzger & Simpson               Historic                        [Page 5]
+
+RFC 2841                     AH SHA1 IP-MAC                November 2000
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+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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+Metzger & Simpson               Historic                        [Page 6]
+
+RFC 2841                     AH SHA1 IP-MAC                November 2000
+
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+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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+Metzger & Simpson               Historic                        [Page 7]
+
+RFC 2841                     AH SHA1 IP-MAC                November 2000
+
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+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
+
+   EMail: wsimpson@UMich.edu
+          wsimpson@GreenDragon.com (preferred)
+
+Editor's Note
+
+   This paper was originally submitted May 1, 1996.
+
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+Metzger & Simpson               Historic                        [Page 8]
+
+RFC 2841                     AH SHA1 IP-MAC                November 2000
+
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+Full Copyright Statement
+
+   Copyright (C) The Internet Society (2000).  All Rights Reserved.
+
+   This document and translations of it may be copied and furnished to
+   others, and derivative works that comment on or otherwise explain it
+   or assist in its implementation may be prepared, copied, published
+   and distributed, in whole or in part, without restriction of any
+   kind, provided that the above copyright notice and this paragraph are
+   included on all such copies and derivative works.  However, this
+   document itself may not be modified in any way, such as by removing
+   the copyright notice or references to the Internet Society or other
+   Internet organizations, except as needed for the purpose of
+   developing Internet standards in which case the procedures for
+   copyrights defined in the Internet Standards process must be
+   followed, or as required to translate it into languages other than
+   English.
+
+   The limited permissions granted above are perpetual and will not be
+   revoked by the Internet Society or its successors or assigns.
+
+   This document and the information contained herein is provided on an
+   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
+   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
+   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
+   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
+   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
+
+Acknowledgement
+
+   Funding for the RFC Editor function is currently provided by the
+   Internet Society.
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+Metzger & Simpson               Historic                        [Page 9]
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