RC5

In cryptography, RC5 is a simple symmetric-key block cipher. Designed by Ronald Rivest in 1994,[1] RC5 is a parameterized algorithm with a variable block size, a variable key size, and a variable number of rounds. "RC" stands for "Rivest Cipher", or alternatively, "Ron's Code".

In order to provide varieties of security and efficiency levels; RC5 has a variable block size (32, 64 or 128 bits), variable key size (0 to 2040 bits) and variable 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.[1][2]

A key feature of RC5 is the use of data-dependent rotations; one of the goals of RC5 was to study and evaluate operations of block ciphers 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 simplicity of the algorithm together with the novelty of the data-dependent rotations has made RC5 an attractive subject to study by cryptanalysts.

Cryptanalysis

12-round RC5 (with 64-bit blocks) is susceptible to a differential attack using 244 chosen plaintexts.[3] 18–20 rounds are suggested as sufficient protection.

RSA Security, which has a patent on the algorithm,[4] 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 now is working on cracking a 72-bit key. At the current rate (as of November 12, 2008), it will take approximately 1,000 years to test every possible key to complete the project.

References
  1. Rivest, R. L. (1994). "The RC5 Encryption Algorithm" (PDF). Proceedings of the Second International Workshop on Fast Software Encryption (FSE) 1994e. pp. 86–96.
  2. "What are RC5 and RC6". RSA Security. Archived from the original on 2006-12-29. Retrieved 2008-11-12.
  3. Biryukov A. and Kushilevitz E. (1998). Improved Cryptanalysis of RC5. EUROCRYPT 1998.
  4. Rivest, R. L, "Block Encryption Algorithm With Data Dependent Rotation", U.S. Patent 5,724,428, issued on 3 March 1998.

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