RC6

RC6

The Feistel function of the RC6 algorithm.
General
Designers	Ron Rivest, Matt Robshaw, Ray Sidney, Yiqun Lisa Yin
First published	1998
Derived from	RC5
Certification	AES finalist
Cipher detail
Key sizes	128, 192, or 256 bits
Block sizes	128 bits
Structure	Feistel network (Type 2)
Rounds	20

In cryptography, RC6 (Rivest cipher 6) is a symmetric key block cipher derived from RC5. It was designed by Ron Rivest, Matt Robshaw, Ray Sidney, and Yiqun Lisa Yin to meet the requirements of the Advanced Encryption Standard (AES) competition. The algorithm was one of the five finalists, and also was submitted to the NESSIE and CRYPTREC projects. It was a proprietary algorithm, patented by RSA Security.

RC6 proper has a block size of 128 bits and supports key sizes of 128, 192, and 256 bits up to 2040-bits, but, like RC5, it may 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 addition, and XOR operations; in fact, RC6 could be viewed as interweaving two parallel RC5 encryption processes, although 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.

Note that the key expansion algorithm is practically identical to that of RC5. The only difference is that for RC6, more words are derived from the user-supplied key.

In August 2016, code reputed to be Equation Group or NSA "implants" for various network security devices was disclosed. The accompanying instructions revealed that some of these programs use RC6 for confidentiality of network communications.

As RC6 has not been selected for the AES, it was not guaranteed that RC6 is royalty-free. As of January 2017^[update], a web page on the official web site of the designers of RC6, RSA Laboratories, states the following: