Editing Block cipher (section)

===RC5===
[[File:RC5 InfoBox Diagram.svg|thumb|160px|right|One round (two half-rounds) of the RC5 block cipher]]
{{Main|RC5}}
RC5 is a block cipher designed by [[Ron Rivest|Ronald Rivest]] in 1994 which, unlike many other ciphers, has a variable block size (32, 64, or 128 bits), key size (0 to 2040 bits), and a number of rounds (0 to 255). The original suggested choice of parameters was 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 arithmetic|modular]] additions and XORs. The general structure of the algorithm is a [[Feistel cipher|Feistel]]-like a 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 (mathematical constant)|e]] and the [[golden ratio]] as sources of "[[nothing up my sleeve number]]s". The tantalizing simplicity of the algorithm together with the novelty of the data-dependent rotations has made RC5 an attractive object of study for cryptanalysts.

12-round RC5 (with 64-bit blocks) is susceptible to a [[differential cryptanalysis|differential attack]] using 2<sup>44</sup> chosen plaintexts.<ref name="Biryukov">Biryukov A. and Kushilevitz E. (1998). Improved Cryptanalysis of RC5. EUROCRYPT 1998.</ref> 18–20 rounds are suggested as sufficient protection.