RC2

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RC2

Image:RC2 InfoBox Diagram.png

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).

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Sources

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

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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

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