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In cryptography, RC6 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 was also submitted to the NESSIE and CRYPTREC projects. It is 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, 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 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.

As RC6 has not been selected for the AES, it is not guaranteed that RC6 is royalty-free. As of January 2007, 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 (U.S. Patents 5,724,428, and 5,835,600).

• R.L. Rivest, M.J.B. Robshaw, R.Sidney, and Y.L. Yin. The RC6 Block Cipher. v1.1, August 1998.
• J. Beuchat FPGA Implementations of the RC6 Block Cipher.

• SCAN's entry on RC6
• RSA Security's RC6 page