In cryptography, a block cipher is a symmetric key cipher operating on fixed-length groups of bits, called blocks, with an unvarying transformation. A block cipher encryption algorithm might take a 128-bit block of plaintext as input, and output a corresponding 128-bit block of ciphertext...
In cryptography, RC5 is a block cipher notable for its simplicity. Designed by Ronald Rivest in 1994, RC stands for "Rivest Cipher", or alternatively, "Ron's Code"...
Ronald Linn Rivest is a cryptographer. He is the Andrew and Erna Viterbi Professor of Computer Science at MIT's Department of Electrical Engineering and Computer Science and a member of MIT's Computer Science and Artificial Intelligence Laboratory...
Matthew John Barton "Matt" Robshaw is a cryptographer. Formerly a lecturer at Royal Holloway, University of London, Robshaw currently belongs to the cryptography research group at France Telecom's Orange Labs. He also coordinates the Symmetric Techniques Virtual Lab for ECRYPT...
Advanced Encryption Standard is a specification for the encryption of electronic data. It has been adopted by the U.S. government and is now used worldwide. It supersedes DES...
(AES) competition. The algorithm was one of the five finalists, and was also submitted to the NESSIE
NESSIE
NESSIE was a European research project funded from 2000–2003 to identify secure cryptographic primitives. The project was comparable to the NIST AES process and the Japanese Government-sponsored CRYPTREC project, but with notable differences from both...
CRYPTREC is the Cryptography Research and Evaluation Committees set up by the Japanese Government to evaluate and recommend cryptographic techniques for government and industrial use...
projects. It is a proprietary algorithm, patented by RSA Security
RSA Security
RSA, the security division of EMC Corporation, is headquartered in Bedford, Massachusetts, United States, and maintains offices in Australia, Ireland, Israel, the United Kingdom, Singapore, India, China, Hong Kong and Japan....
In modern cryptography, symmetric key ciphers are generally divided into stream ciphers and block ciphers. Block ciphers operate on a fixed length string of bits. The length of this bit string is the block size...
In cryptography, key size or key length is the size measured in bits of the key used in a cryptographic algorithm . An algorithm's key length is distinct from its cryptographic security, which is a logarithmic measure of the fastest known computational attack on the algorithm, also measured in bits...
s of 128, 192 and 256 bits, but, like RC5, it can be parameterised to support a wide variety of word-lengths, key sizes and number of rounds. RC6 is very similar to RC5 in structure, using data-dependent rotations, modular
Modular arithmetic
In mathematics, modular arithmetic is a system of arithmetic for integers, where numbers "wrap around" after they reach a certain value—the modulus....
addition and XOR operations; in fact, RC6 could be viewed as interweaving two parallel RC5 encryption processes. However, RC6 does use an extra multiplication operation not present in RC5 in order to make the rotation dependent on every bit in a word, and not just the least significant few bits.
Encryption/Decryption
// Encryption/Decryption with RC6-w/r/b
//
// Input: Plaintext stored in four w-bit input registers A, B, C & D
// r is the number of rounds
// w-bit round keys S[0, ... , 2r + 3]
//
// Output: Ciphertext stored in A, B, C, D
//
// Encryption Procedure:
B = B + S[0]
D = D + S[1]
for i = 1 to r do
{
t = (B*(2B + 1)) <<< lg w
u = (D*(2D + 1)) <<< lg w
A = ((A ⊕ t) <<< u) + S[2i]
C = ((C ⊕ u) <<< t) + S[2i + 1]
(A, B, C, D) = (B, C, D, A)
}
A = A + S[2r + 2]
C = C + S[2r + 3]
// Decryption Procedure:
C = C - S[2r + 3]
A = A - S[2r + 2]
for i = r downto 1 do
{
(A, B, C, D) = (D, A, B, C)
u = (D*(2D + 1)) <<< lg w
t = (B*(2B + 1)) <<< lg w
C = ((C - S[2i + 1]) >>> t) ⊕ u
A = ((A - S[2i]) >>> u) ⊕ t
}
D = D - S[1]
B = B - S[0]
Advanced Encryption Standard is a specification for the encryption of electronic data. It has been adopted by the U.S. government and is now used worldwide. It supersedes DES...
, it is not guaranteed that RC6 is royalty-free. , a web page on the official web site of the designers of RC6, RSA Laboratories, states the following:
"We emphasize that if RC6 is selected for the AES, RSA Security will not require any licensing or royalty payments for products using the algorithm".
The emphasis on the word "if" suggests that RSA Security Inc. may now require licensing and royalty payments for any products using the RC6 algorithm. RC6 is a patented encryption algorithm ( and ).
