One round (two half-rounds) of the RC5 block cipher
|Key sizes||0 to 2040 bits (128 suggested)|
|Block sizes||32, 64 or 128 bits (64 suggested)|
|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 byte in the key.
RSA Security, which has a patent on the algorithm, 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 December 2014, 3.412% of the keyspace has been searched. At the current rate, it will take approximately 218 years to test every possible remaining key, and thus guarantee completion of the project. The task has inspired many new and novel developments in the field of cluster computing.
- Biryukov A. and Kushilevitz E. (1998). Improved Cryptanalysis of RC5. EUROCRYPT 1998.
- Rivest, R. L. (1994). "Proceedings of the Second International Workshop on Fast Software Encryption (FSE) 1994e" (pdf). pp. 86–96.
- Rivest, R. L, "Block Encryption Algorithm With Data Dependent Rotation", U.S. Patent 5,724,428, issued on 3 March 1998.
- Rivest's paper describing the cipher
- SCAN's entry for the cipher
- RSA Laboratories FAQ — What are RC5 and RC6?
- Helger Lipmaa's links on RC5