A Description of the RC2(r) Encryption Algorithm (RFC2268)
Original Publication Date: 1998-Mar-01
Included in the Prior Art Database: 2019-Feb-15
Internet Society Requests For Comment (RFCs)
This memo describes a conventional (secret-key) block encryption algorithm, called RC2, which may be considered as a proposal for a DES replacement. This memo provides information for the Internet community. It does not specify an Internet standard of any kind.
Network Working Group R. Rivest Request for Comments: 2268 MIT Laboratory for Computer Science Category: Informational and RSA Data Security, Inc. March 1998
A Description of the RC2(r) Encryption Algorithm
Status of this Memo
This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (1998). All Rights Reserved.
This memo is an RSA Laboratories Technical Note. It is meant for informational use by the Internet community.
This memo describes a conventional (secret-key) block encryption algorithm, called RC2, which may be considered as a proposal for a DES replacement. The input and output block sizes are 64 bits each. The key size is variable, from one byte up to 128 bytes, although the current implementation uses eight bytes.
The algorithm is designed to be easy to implement on 16-bit microprocessors. On an IBM AT, the encryption runs about twice as fast as DES (assuming that key expansion has been done).
1.1 Algorithm description
We use the term "word" to denote a 16-bit quantity. The symbol + will denote twos-complement addition. The symbol & will denote the bitwise "and" operation. The term XOR will denote the bitwise "exclusive-or" operation. The symbol ˜ will denote bitwise complement. The symbol ^ will denote the exponentiation operation. The term MOD will denote the modulo operation.
There are three separate algorithms involved:
Key expansion. This takes a (variable-length) input key and produces an expanded key consisting of 64 words K,...,K.
Rivest Informational [Page 1]
RFC 2268 RC2(r) Encryption Algorithm March 1998
Encryption. This takes a 64-bit input quantity stored in words R, ..., R and encrypts it "in place" (the result is left in R, ..., R).
Decryption. The inverse operation to encryption.
2. Key expansion
Since we will be dealing with eight-bit byte operations as well as 16-bit word operations, we will use two alternative notations
for referring to the key buffer:
For word operations, we will refer to the positions of the buffer as K, ..., K; each K[i] is a 16-bit word.
For byte operations, we will refer to the key buffer as L, ..., L; each L[i] is an eight-bit byte.
These are alternative views of the same data buffer. At all times it will be true that
K[i] = L[2*i] + 256*L[2*i+1].
(Note that the low-order byte of each K word is given before the high-order byte.)
We will assume that exactly T bytes of key are supplied, for some T in the range 1 <= T <= 128. (Our current implementation uses T = 8.) However, regardless of T, the algorithm has a maximum effective key length in bits, denoted T1. That is, the search space is 2^(8*T), or 2^T1, whichever is smaller.
The purpose of the key-expansion algorithm is to modify the key buffer so that each bit of the expanded key depends in a complicated way on every bit of the supplied input key...