RC6

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.

For the Swedish locomotive, see SJ Rc.

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)[1]
Rounds	20

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.

Key expansion

Same key expansion as in RC5: https://en.m.wikipedia.org/w/index.php?title=RC5&wprov=rarw1

Encryption/decryption

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.

// 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]

Possible use in NSA "implants"

In August 2016, code reputed to be Equation Group or NSA "implants" for various network security devices was disclosed.^[2] The accompanying instructions revealed that some of these programs use RC6 for confidentiality of network communications.^[3]

Licensing

As RC6 was not 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:^[4]

"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 have required licensing and royalty payments for any products using the RC6 algorithm. RC6 was a patented encryption algorithm (U.S. patent 5,724,428 and U.S. patent 5,835,600); however, the patents expired between 2015 and 2017.