RFC 2841 (rfc2841) - Page 2 of 9

IP Authentication using Keyed SHA1 with Interleaved Padding (IP-MAC)

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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