The MD2 Message-Digest Algorithm
RFC 1319

Document Type RFC - Historic (April 1992; Errata)
Obsoleted by RFC 6149
Last updated 2013-03-02
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Network Working Group                                         B. Kaliski
Request for Comments: 1319                              RSA Laboratories
Updates: RFC 1115                                             April 1992

                     The MD2 Message-Digest Algorithm

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard.  Distribution of this memo is
   unlimited.

Acknowlegements

   The description of MD2 is based on material prepared by John Linn and
   Ron Rivest.  Their permission to incorporate that material is greatly
   appreciated.

Table of Contents

   1. Executive Summary                                                1
   2. Terminology and Notation                                         2
   3. MD2 Algorithm Description                                        2
   4. Summary                                                          4
   References                                                          5
   APPENDIX A - Reference Implementation                               5
   Security Considerations                                            17
   Author's Address                                                   17

1. Executive Summary

   This document describes the MD2 message-digest algorithm. The
   algorithm takes as input a message of arbitrary length and produces
   as output a 128-bit "fingerprint" or "message digest" of the input.
   It is conjectured that it is computationally infeasible to produce
   two messages having the same message digest, or to produce any
   message having a given prespecified target message digest. The MD2
   algorithm is intended for digital signature applications, where a
   large file must be "compressed" in a secure manner before being
   signed with a private (secret) key under a public-key cryptosystem
   such as RSA.

   License to use MD2 is granted for non-commerical Internet Privacy-
   Enhanced Mail [1-3].

   This document is an update to the August 1989 RFC 1115 [3], which
   also gives a reference implementation of MD2. The main differences

Kaliski                                                         [Page 1]
RFC 1319              MD2 Message-Digest Algorithm            April 1992

   are that a textual description of MD2 is included, and that the
   reference implementation of MD2 is more portable.

   For OSI-based applications, MD2's object identifier is

   md2 OBJECT IDENTIFIER ::=
   iso(1) member-body(2) US(840) rsadsi(113549) digestAlgorithm(2) 2}

   In the X.509 type AlgorithmIdentifier [4], the parameters for MD2
   should have type NULL.

2. Terminology and Notation

   In this document, a "byte" is an eight-bit quantity.

   Let x_i denote "x sub i". If the subscript is an expression, we
   surround it in braces, as in x_{i+1}. Similarly, we use ^ for
   superscripts (exponentiation), so that x^i denotes x to the i-th
   power.

   Let X xor Y denote the bit-wise XOR of X and Y.

3. MD2 Algorithm Description

   We begin by supposing that we have a b-byte message as input, and
   that we wish to find its message digest. Here b is an arbitrary
   nonnegative integer; b may be zero, and it may be arbitrarily large.
   We imagine the bytes of the message written down as follows:

                   m_0 m_1 ... m_{b-1}

   The following five steps are performed to compute the message digest
   of the message.

3.1 Step 1. Append Padding Bytes

   The message is "padded" (extended) so that its length (in bytes) is
   congruent to 0, modulo 16. That is, the message is extended so that
   it is a multiple of 16 bytes long. Padding is always performed, even
   if the length of the message is already congruent to 0, modulo 16.

   Padding is performed as follows: "i" bytes of value "i" are appended
   to the message so that the length in bytes of the padded message
   becomes congruent to 0, modulo 16. At least one byte and at most 16
   16 bytes are appended.

   At this point the resulting message (after padding with bytes) has a
   length that is an exact multiple of 16 bytes. Let M[0 ... N-1] denote

Kaliski                                                         [Page 2]
RFC 1319              MD2 Message-Digest Algorithm            April 1992

   the bytes of the resulting message, where N is a multiple of 16.

3.2 Step 2. Append Checksum

   A 16-byte checksum of the message is appended to the result of the
   previous step.

   This step uses a 256-byte "random" permutation constructed from the
   digits of pi. Let S[i] denote the i-th element of this table. The
   table is given in the appendix.

   Do the following:

      /* Clear checksum. */
      For i = 0 to 15 do:
         Set C[i] to 0.
      end /* of loop on i */

      Set L to 0.

      /* Process each 16-word block. */
      For i = 0 to N/16-1 do

         /* Checksum block i. */
         For j = 0 to 15 do
            Set c to M[i*16+j].
            Set C[j] to S[c xor L].
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