RC2

The mix-up transformation of RC2; A mixing round consists of applying the "mix-up" transformation four times.

In cryptography, RC2 is a symmetric-key block cipher. Designed by Ronald Rivest in 1987. "RC" stands for "Rivest Cipher", or alternatively, "Ron's Code".

RC2 is a 64-bit block cipher with a variable key size and using 18 rounds.

Rounds are arranged as a source-heavy feistel network, with 16 rounds of one type called "mixing rounds" interleaved by two rounds of another type called "mashing rounds".

The 18 rounds are performed using the following interleaved sequence:

1. perform 5 mixing rounds.
2. perform 1 mashing round.
3. perform 6 mixing rounds.
4. perform 1 mashing round.
5. perform 5 mixing rounds.

RC2 uses key-expansion algorithm by which an expanded key consisting of 64 (16-bit words) is produces depending in a complicated way on every bit of the supplied "variable-length" input key. A mixing round consists of four applications of the "mix-up" transformation, as shown in the diagram. A round is "mashed" by adding to it one of the 16-bit words of the expanded key (RFC 2268).

RC2 is susceptible to a related-key attack using 2³⁴ chosen-plaintext attacks (Kelsey et al., 1997).

The development of RC2 was sponsored by Lotus, who were seeking a custom cipher be exported as part of their Lotus Notes software, after evaluation by the NSA. The NSA suggested a couple of changes, which Ronald 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 under US export regulations for cryptography. Now all 40-bit encryption algorithms are obsolete because they are dangerously susceptible to brute force attacks.

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

References

• Steven Levy, Crypto: How the Code Rebels Beat the Government — Saving Privacy in the Digital Age, ISBN 0-14-024432-8, 2001.
• 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

Other websites

• RFC 2268 - A Description of the RC2(r) Encryption Algorithm
• RSA FAQ: What is RC2? Archived 2006-12-30 at the Wayback Machine
• sci.crypt posting revealing the RC2 algorithm