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   The message digest produced as output is A, B, C, D. That is, we
   begin with the low-order byte of A, and end with the high-order byte
   of D.

   This completes the description of MD5. A reference implementation in
   C is given in the appendix.

4. Summary

   The MD5 message-digest algorithm is simple to implement, and provides
   a "fingerprint" or message digest of a message of arbitrary length.
   It is conjectured that the difficulty of coming up with two messages
   having the same message digest is on the order of 2^64 operations,
   and that the difficulty of coming up with any message having a given
   message digest is on the order of 2^128 operations. The MD5 algorithm
   has been carefully scrutinized for weaknesses. It is, however, a
   relatively new algorithm and further security analysis is of course
   justified, as is the case with any new proposal of this sort.

5. Differences Between MD4 and MD5

     The following are the differences between MD4 and MD5:

       1.   A fourth round has been added.

       2.   Each step now has a unique additive constant.

       3.   The function g in round 2 was changed from (XY v XZ v YZ) to
       (XZ v Y not(Z)) to make g less symmetric.

       4.   Each step now adds in the result of the previous step.  This
       promotes a faster "avalanche effect".

       5.   The order in which input words are accessed in rounds 2 and
       3 is changed, to make these patterns less like each other.

       6.   The shift amounts in each round have been approximately
       optimized, to yield a faster "avalanche effect." The shifts in
       different rounds are distinct.










Rivest                                                          [Page 6]

RFC 1321              MD5 Message-Digest Algorithm            April 1992


References

   [1] Rivest, R., "The MD4 Message Digest Algorithm", RFC 1320, MIT and
       RSA Data Security, Inc., April 1992.

   [2] Rivest, R., "The MD4 message digest algorithm", in A.J.  Menezes
       and S.A. Vanstone, editors, Advances in Cryptology - CRYPTO '90
       Proceedings, pages 303-311, Springer-Verlag, 1991.

   [3] CCITT Recommendation X.509 (1988), "The Directory -
       Authentication Framework."

APPENDIX A - Reference Implementation

   This appendix contains the following files taken from RSAREF: A
   Cryptographic Toolkit for Privacy-Enhanced Mail:

     global.h -- global header file

     md5.h -- header file for MD5

     md5c.c -- source code for MD5

   For more information on RSAREF, send email to <rsaref@rsa.com>.

   The appendix also includes the following file:

     mddriver.c -- test driver for MD2, MD4 and MD5

   The driver compiles for MD5 by default but can compile for MD2 or MD4
   if the symbol MD is defined on the C compiler command line as 2 or 4.

   The implementation is portable and should work on many different
   plaforms. However, it is not difficult to optimize the implementation
   on particular platforms, an exercise left to the reader. For example,
   on "little-endian" platforms where the lowest-addressed byte in a 32-
   bit word is the least significant and there are no alignment
   restrictions, the call to Decode in MD5Transform can be replaced with
   a typecast.

A.1 global.h

/* GLOBAL.H - RSAREF types and constants
 */

/* PROTOTYPES should be set to one if and only if the compiler supports
  function argument prototyping.
The following makes PROTOTYPES default to 0 if it has not already



Rivest                                                          [Page 7]

RFC 1321              MD5 Message-Digest Algorithm            April 1992


  been defined with C compiler flags.
 */
#ifndef PROTOTYPES
#define PROTOTYPES 0
#endif

/* POINTER defines a generic pointer type */
typedef unsigned char *POINTER;

/* UINT2 defines a two byte word */
typedef unsigned short int UINT2;