RC2 - Definition and Overview

RC2
The MIX transformation of RC2; four of these comprise a MIXING round.
General
Designer(s)	Ron Rivest
First published	source code leaked 1996 (designed 1987)
Derived from	-
Cipher(s) based on this design	-
Algorithm detail
Block size(s)	64 bits
Key size(s)	8–128 bits, in steps of eight bits; default 64 bits
Structure	Source-heavy Feistel network
Number of rounds	16 of type MIXING; 2 of type MASHING
Best cryptanalysis
A related-key attack is possible requiring 234 chosen plaintexts (Kelsey et al, 1997).

In cryptography, RC2 is a block cipher designed by Ron Rivest in 1987. "RC" stands for "Ron's Code" or "Rivest Cipher"; other ciphers designed by Rivest include RC4, RC5 and RC6.

The development of RC2 was sponsored by Lotus, who were seeking a custom cipher that, after evaluation by the NSA, could be exported as part of their Lotus Notes software. The NSA suggested a couple of changes, which Rivest incorporated. After further negotiations, the cipher was approved for export in 1989. Along with RC4, RC2 with a 40-bit key size was treated favourably under US export regulations for cryptography.

Initially, the details of the algorithm were kept secret — proprietary to RSA Security — but on 29th January, 1996, source code for RC2 was anonymously posted to the Internet on the Usenet forum, sci.crypt. A similar disclosure had occurred earlier with RC4. It is unclear whether the poster had access to the specifications or whether it had been reverse engineered.

RC2 is a 64-bit block cipher with a variable size key. Its 18 rounds are arranged as a source-heavy Feistel network, with 16 rounds of one type (MIXING) punctuated by two rounds of another type (MASHING). A MIXING round consists of four applications of the MIX transformation, as shown in the following diagram:

RC2 is vulnerable to a related-key attack using 2³⁴ chosen plaintexts (Kelsey et al, 1997).

Sources

• Steven Levy, Crypto: How the Code Rebels Beat the Government — Saving Privacy in the Digital Age, ISBN 0140244328, 2001.

References

• Lars R. Knudsen, Vincent Rijmen, Ronald L. Rivest, Matthew J. B. Robshaw: On the Design and Security of RC2. Fast Software Encryption 1998: 206–221
• John Kelsey, Bruce Schneier, David Wagner: Related-key cryptanalysis of 3-WAY, Biham-DES, CAST, DES-X, NewDES, RC2, and TEA. ICICS 1997: 233–246

External links

• RFC 2268: A Description of the RC2(r) Encryption Algorithm (http://www.faqs.org/rfcs/rfc2268.html)
• RSA FAQ: What is RC2? (http://www.rsasecurity.com/rsalabs/node.asp?id=2249)
• sci.crypt posting revealing the RC2 algorithm (http://groups.google.com/groups?selm=4ehmfs%246nq%40utopia.hacktic.nl)