RC5

RC5

One round (two half-rounds) of the RC5 block cipher
General
Designers	Ron Rivest
First published	1994
Successors	RC6, Akelarre
Cipher detail
Key sizes	0 to 2040 bits (128 suggested)
Block sizes	32, 64 or 128 bits (64 suggested)
Structure	Feistel-like network
Rounds	1-255 (12 suggested originally)
Best public cryptanalysis
12-round RC5 (with 64-bit blocks) is susceptible to a differential attack using 244 chosen plaintexts.

In cryptography, RC5 is a symmetric-key block cipher notable for its simplicity. Designed by Ronald Rivest in 1994,RC stands for "Rivest Cipher", or alternatively, "Ron's Code" (compare RC2 and RC4). The Advanced Encryption Standard (AES) candidate RC6 was based on RC5.

Unlike many schemes, RC5 has a variable block size (32, 64 or 128 bits), key size (0 to 2040 bits) and number of rounds (0 to 255). The original suggested choice of parameters were a block size of 64 bits, a 128-bit key and 12 rounds.

A key feature of RC5 is the use of data-dependent rotations; one of the goals of RC5 was to prompt the study and evaluation of such operations as a cryptographic primitive. RC5 also consists of a number of modular additions and eXclusive OR (XOR)s. The general structure of the algorithm is a Feistel-like network. The encryption and decryption routines can be specified in a few lines of code. The key schedule, however, is more complex, expanding the key using an essentially one-way function with the binary expansions of both e and the golden ratio as sources of "nothing up my sleeve numbers". The tantalising simplicity of the algorithm together with the novelty of the data-dependent rotations has made RC5 an attractive object of study for cryptanalysts. The RC5 is basically denoted as RC5-w/r/b where w=word size in bits, r=number of rounds, b=number of 8-bit bytes in the key.

RC5 encryption and decryption both expand the random key into 2(r+1) words that will be used sequentially (and only once each) during the encryption and decryption processes. All of the below comes from Rivest's revised paper on RC5.