View source for RC6

{{Short description|Block cipher}}
{{About||the Swedish locomotive|SJ Rc}}
{{more footnotes needed|date=March 2009}}
{{Infobox block cipher
| name          = RC6
| image         = [[Image:RC6 Cryptography Algorithm.JPG|300px|center]]
| caption       = The [[Feistel cipher|Feistel function]] of the RC6 algorithm.
| designers     = [[Ron Rivest]], [[Matt Robshaw]], [[Ray Sidney]], [[Yiqun Lisa Yin]]
| publish date  = 1998
| derived from  = [[RC5]]
| derived to    = 
| related to    =
| certification = [[AES finalist]]
| key size      = 128, 192, or 256 bits
| block size    = 128 bits
| structure     = [[Feistel cipher|Feistel network]] (Type 2)<ref>{{cite conference |title=On Generalized Feistel Networks |last1=Hoang |first1=Viet Tung |last2=Rogaway |first2=Phillip |date=2010 |publisher=Springer |book-title=LNCS 6223 |pages=613–630 |location=USA |conference=CRYPTO 2010|doi=10.1007/978-3-642-14623-7_33 |doi-access=free }}</ref>
| rounds        = 20
| cryptanalysis =
}}
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) [[AES competition|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]]. According to Ron Rivest, the RC stands for ''Ron's Code'',<ref>{{cite web| url = http://people.csail.mit.edu/rivest/faq.html#Ron| title = Rivest FAQ at csail.mit.edu}}</ref> but the RC documentation says simply RC6.

RC6 proper has a [[block size (cryptography)|block size]] of 128 bits and supports [[key size]]s 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 arithmetic|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.

==Encryption/decryption==
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.

<syntaxhighlight lang="C">
// 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]
</syntaxhighlight>

==Possible use in NSA "implants"==
In August 2016, code reputed to be [[Equation Group]] or [[NSA]] "implants" for various network security devices was disclosed.<ref>{{cite news |title=Confirmed: hacking tool leak came from "omnipotent" NSA-tied group |work=Ars Technica |date=August 16, 2016 |url=https://arstechnica.com/security/2016/08/code-dumped-online-came-from-omnipotent-nsa-tied-hacking-group/ }}</ref>  The accompanying instructions revealed that some of these programs use RC6 for confidentiality of network communications.<ref>{{cite web
| url=https://github.com/nneonneo/eqgrp-free-file/blob/master/Firewall/BLATSTING/BLATSTING_20322/opinstructions/install.txt
| title=These instructions guide the INSTALLATION of BLATSTING using ELIGIBLEBACHELOR via NOPEN tunnel.
| website=[[GitHub]]
| access-date=2016-08-16}}
</ref>

==Licensing==
As RC6 was not selected for the [[Advanced Encryption Standard|AES]], it was not guaranteed that RC6 is royalty-free. {{As of|2017|1}}, a web page on the official web site of the designers of RC6, RSA Laboratories, states the following:<ref>{{cite web
| url=https://www.emc.com/emc-plus/rsa-labs/standards-initiatives/rc5-and-rc6.htm
| title=3.6.4 What are RC5 and RC6?
| publisher=[[RSA Laboratories]]
| archive-url=https://web.archive.org/web/20170706114310/https://www.emc.com/emc-plus/rsa-labs/standards-initiatives/rc5-and-rc6.htm
| archive-date=2017-07-06
| access-date=2015-08-02}}</ref>

:"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 ({{US patent|5724428}} and {{US patent|5835600}}); however, the patents expired between 2015 and 2017.

==References==
{{reflist}}

==Bibliography==
{{refbegin}}
* {{cite web
| url=http://people.csail.mit.edu/rivest/pubs/RRSY98.pdf
| archive-url=https://web.archive.org/web/20181223080309/http://people.csail.mit.edu/rivest/rc6.pdf
| archive-date=2018-12-23
| title=The RC6 Block Cipher
| first1=R.L.
| last1=Pavan
| first2=M.J.B.
| last2=Robshaw
| first3=R.
| last3=Sidney
| first4=Y.L.
| last4=Yin.
| date=1998-08-20
| version=v1.1
| access-date=2015-08-02}}

* {{cite web
| url=http://perso.ens-lyon.fr/jean-luc.beuchat/Publications/fpl2003.pdf
| title=FPGA Implementations of the RC6 Block Cipher
| first=Jean-Luc
| last=Beuchat
| archive-url=https://web.archive.org/web/20060505225326/http://perso.ens-lyon.fr/jean-luc.beuchat/Publications/fpl2003.pdf
| archive-date=2006-05-05
| url-status=dead}}

* {{cite web
| url=https://www.theregister.co.uk/2013/12/31/nsa_weapons_catalogue_promises_pwnage_at_the_speed_of_light/
| title=How the NSA hacks PCs, phones, routers, hard disks 'at speed of light': Spy tech catalog leaks
| first=Iain
| last=Thompson
| date=2013-12-31
| website=[[The Register]]
| access-date=2015-08-02}}
{{refend}}

==External links==
* {{cite web
| url=http://embeddedsw.net/Cipher_Reference_Home.html#RC6
| title=Cryptography - 256 bit Ciphers: Reference source code and submissions to international cryptographic designs contests}}

* {{cite web
| url=http://www.users.zetnet.co.uk/hopwood/crypto/scan/cs.html#RC6
| title=Symmetric Ciphers: RC6
| publisher=Standard Cryptographic Algorithm Naming (SCAN)
| date=2009-04-15}}

* {{cite web
| url=https://www.emc.com/emc-plus/rsa-labs/historical/rc6-block-cipher.htm
| title=RC6® Block Cipher
| archive-url=https://web.archive.org/web/20160304055032/https://www.emc.com/emc-plus/rsa-labs/historical/rc6-block-cipher.htm
| archive-date=2016-03-04
| publisher=[[RSA Laboratories]]}}

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[[Category:Block ciphers]]