| RC5 | |
 One round (two half-rounds) of the RC5 block cipher
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| General | |
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| Designer(s): | Ron Rivest |
| First published: | 1994 |
| Successor(s): | RC6, Akelarre |
| Cipher detail | |
| Key size(s): | 0 to 2040 bits (128 suggested) |
| Block size(s): | 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.[1] | |
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.
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Description
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.
Cryptanalysis
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, who have 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- and 64-bit keys, and is, as of December 3rd, 2002, working on cracking a 72-bit key. At the current rate, it will take approximately 1,000 years to test every possible key, and thus guarantee completion of the project. [1]
See also
References
- ^ a b Biryukov A. and Kushilevitz E. (1998). Improved Cryptanalysis of RC5. EUROCRYPT 1998.
- ^ Rivest, R. L. (1994). "The RC5 Encryption Algorithm" (pdf). Proceedings of the Second International Workshop on Fast Software Encryption (FSE) 1994e: 86–96.
- ^ Rivest, R. L, "Block Encryption Algorithm With Data Dependent Rotation", U.S. Patent 5,724,428, issued on 3 March 1998.
External links
- 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
- RSA's patent via Google.
