RFC 1852 (rfc1852) - Page 2 of 6
IP Authentication using Keyed SHA
Alternative Format: Original Text Document
RFC 1852 AH SHA September 1995 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 (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 It should be noted that this document specifies a newer version of the SHA than that described in [FIPS-180], which was flawed.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, Metzger & Simpson Experimental
