RFC 2040 (rfc2040) - Page 1 of 29

The RC5, RC5-CBC, RC5-CBC-Pad, and RC5-CTS Algorithms

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Network Working Group                                         R. Baldwin
Request for Comments: 2040                       RSA Data Security, Inc.
Category: Informational                                        R. Rivest
                                     MIT Laboratory for Computer Science
                                             and RSA Data Security, Inc.
                                                            October 1996


         The RC5, RC5-CBC, RC5-CBC-Pad, and RC5-CTS Algorithms

Status of this Memo

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

Acknowledgments

   We would like to thank Steve Dusse, Victor Chang, Tim Mathews, Brett
   Howard, and Burt Kaliski for helpful suggestions.

Table of Contents

     1.        Executive Summary .......................  1
     2.        Overview ................................  2
     3.        Terminology and Notation ................  3
     4.        Description of RC5 Keys .................  4
     5.        Description of RC5 Key Expansion ........  6
     6.        Description of RC5 Block Cipher ......... 10
     7.        Description of RC5-CBC and RC5-CBC-Pad .. 12
     8.        Description of RC5-CTS .................. 18
     9.        Test Program and Vectors ................ 19
     10.       Security Considerations ................. 26
     11.       ASN.1 Identifiers ....................... 28
     References ........................................ 28
     Authors' Addresses ................................ 29

1.  Executive Summary

   This document defines four ciphers with enough detail to ensure
   interoperability between different implementations.  The first cipher
   is the raw RC5 block cipher.  The RC5 cipher takes a fixed size input
   block and produces a fixed sized output block using a transformation
   that depends on a key.  The second cipher, RC5-CBC, is the Cipher
   Block Chaining (CBC) mode for RC5.  It can process messages whose
   length is a multiple of the RC5 block size.  The third cipher, RC5-
   CBC-Pad, handles plaintext of any length, though the ciphertext will
   be longer than the plaintext by at most the size of a single RC5



Baldwin & Rivest             Informational
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