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RC5 Background Information



        Here is a short section from the informational RFC on
the RC5 cipher.  It seems to answer many of the questions we have
seen posted to the list.
                --Bob


1. Executive Summary

This document defines three 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, includes padding in the CBC 
mode to allow it to process inputs of any byte length.

The RC5 cipher was invented by Professor Ronald L. Rivest of the 
Massachusetts Institute of Technology in 1994.  It is a very fast and 
simple algorithm that is parameterized by the block size, the number of 
rounds, and key length.  These parameters can be adjusted to meet different 
goals for security, performance, and exportability.

A patent application has been filed for the RC5 cipher.  The terms RC5, 
RC5-CBC, RC5-CBC-Pad and assorted variations on these are trademarks.



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