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Subject: CIKS-1, Spectr-H64, Cipher security summary, Advanced Encryption Standard, RC4
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One round (two half-rounds) of the RC5 block cipher
Designers Ron Rivest
First published 1994
Successors RC6, Akelarre
Cipher detail
Key sizes 0 to 2040 bits (128 suggested)
Block sizes 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]

In cryptography, RC5 is a symmetric-key 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. has brute-forced RC5 messages encrypted with 56-bit and 64-bit keys, and is working on cracking a 72-bit key; as of February 2014, 3.112% of the keyspace has been searched. At the current rate, it will take approximately 287 years to test every possible remaining key, and thus guarantee completion of the project.[4] The task has inspired many new and novel developments in the field of cluster computing.[5]

See also


  1. ^ a b Biryukov A. and Kushilevitz E. (1998). Improved Cryptanalysis of RC5. EUROCRYPT 1998.
  2. ^ Rivest, R. L. (1994). "Proceedings of the Second International Workshop on Fast Software Encryption (FSE) 1994e" (pdf). pp. 86–96. 
  3. ^ Rivest, R. L, "Block Encryption Algorithm With Data Dependent Rotation", U.S. Patent 5,724,428, issued on 3 March 1998.
  4. ^ [1]
  5. ^ [2]

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
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