Editing RC5 (section)

==Cryptanalysis==
Twelve-round RC5 (with 64-bit blocks) is susceptible to a [[differential cryptanalysis|differential attack]] using 2<sup>44</sup> chosen plaintexts.<ref name="Biryukov">{{cite conference |last1=Biryukov |first1=Alex |authorlink1=Alex Biryukov  |last2=Kushilevitz |first2=Eyal |date=31 May 1998 |title=Improved Cryptanalysis of RC5 |conference=EUROCRYPT 1998 |doi=10.1007/BFb0054119 |doi-access=free |url=https://link.springer.com/content/pdf/10.1007/BFb0054119.pdf}}</ref> 18&ndash;20 rounds are suggested as sufficient protection.

A number of these challenge problems have been tackled using [[distributed computing]], organised by [[Distributed.net]]. Distributed.net has [[Brute force attack|brute-forced]] RC5 messages encrypted with 56-bit and 64-bit keys and has been working on cracking a 72-bit key since November 3, 2002.<ref name="distributed.net: Project RC5">{{cite web |title=distributed.net: Project RC5 |url=http://www.distributed.net/RC5 |website=www.distributed.net |access-date=14 December 2019}}</ref> As of July 26, 2023, 10.409% of the keyspace has been searched and based on the rate recorded that day, it would take a little more than 59 years to complete 100% of the keyspace.<ref>{{Cite web|url=https://stats.distributed.net/projects.php?project_id=8|title=stats.distributed.net - RC5-72 Overall Project Stats|website=stats.distributed.net}}</ref> The task has inspired many new and novel developments in the field of cluster computing.<ref>{{cite press release|url=https://www.umassd.edu/news/2014/1-in-the-world-in-code-cracking-challenge-list.html |title=PlayStation 3 supercomputer places UMass Dartmouth #1 in the world in code cracking challenge list |publisher=[[University of Massachusetts Dartmouth]] |date=24 September 2014 |access-date=2024-01-24 |url-status=live |archive-url=https://web.archive.org/web/20220629064757/https://www.umassd.edu/news/2014/1-in-the-world-in-code-cracking-challenge-list.html |archive-date=2022-06-29 }}</ref>

[[RSA Security]], which had a (now expired) patent on the algorithm,<ref>Rivest, R. L, "Block Encryption Algorithm With Data Dependent Rotation", {{US patent|5724428}}, issued on 3 March 1998, expired 1 November 2015.</ref> offered a series of US$10,000 prizes for breaking [[ciphertext]]s encrypted with RC5, but these contests were discontinued as of May 2007.<ref name="distributed.net: Project RC5"/> As a result, distributed.net decided to fund the monetary prize. The individual who discovers the winning key will receive US$1,000, their team (if applicable) will receive US$1,000, and the [[Free Software Foundation]] will receive US$2,000.<ref>{{cite web |title=distributed.net: staff blogs – 2008 – September – 08 |url=http://blogs.distributed.net/2008/09/08/02/09/bovine/ |access-date=15 December 2019}}</ref>