related topics
{math, number, function}
{system, computer, user}
{game, team, player}
{work, book, publish}
{rate, high, increase}

In cryptography, RC5 is a block cipher notable for its simplicity. Designed by Ronald Rivest in 1994[2], 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 byte in the key.


12-round RC5 (with 64-bit blocks) is susceptible to a differential attack using 244 chosen plaintexts.[1] 18–20 rounds are suggested as sufficient protection.

RSA Security, which has a patent on the algorithm,[3] offered a series of US$10,000 prizes for breaking ciphertexts encrypted with RC5, but these contests have been discontinued as of May 2007. A number of these challenge problems have been tackled using distributed computing, organised by Distributed.net. Distributed.net has brute-forced RC5 messages encrypted with 56-bit and 64-bit keys, and is working on cracking a 72-bit key; as of September 2010, 1.043% of the keyspace has been searched. At the current rate, it will take approximately 453 years to test every possible remaining key, and thus guarantee completion of the project.[4]

Full article ▸

related documents
Data element
List of Fourier-related transforms
Z notation
Online algorithm
Euler's theorem
Product of group subsets
Surjective function
The Third Manifesto
Linearity of integration
De Bruijn-Newman constant
Hurwitz polynomial
Group object
Hilbert's Nullstellensatz
Constant folding
Discrete mathematics
Conjugate closure
Dense set
Context-free language
Sigmoid function
Derivative of a constant
Direct sum of groups
Essential singularity
Location parameter
Entropy encoding
Landau's function