40-bit encryption
{{Short description|Key size for symmetric encryption}}
{{Use mdy dates|date=July 2011}}
40-bit encryption refers to a (now broken) key size of forty bits, or five bytes, for symmetric encryption; this represents a relatively low level of security. A forty bit length corresponds to a total of 240 possible keys. Although this is a large number in human terms (about a trillion), it is possible to break this degree of encryption using a moderate amount of computing power in a brute-force attack, i.e., trying out each possible key in turn.
Description
A typical home computer in 2004 could brute-force a 40-bit key in a little under two weeks, testing a million keys per second; modern computers are able to achieve this much faster. Using free time on a large corporate network or a botnet would reduce the time in proportion to the number of computers available.{{sfn|Schneier|1996|p=154}} With dedicated hardware, a 40-bit key can be broken in seconds. The Electronic Frontier Foundation's Deep Crack, built by a group of enthusiasts for US$250,000 in 1998, could break a 56-bit Data Encryption Standard (DES) key in days,{{sfn|EFF-1998}} and would be able to break 40-bit DES encryption in about two seconds.{{sfn|Schneier|1996|p=153}}
40-bit encryption was common in software released before 1999, especially those based on the RC2 and RC4 algorithms which had special "7-day" export review policies,{{Citation needed|date=November 2014}} when algorithms with larger key lengths could not legally be exported from the United States without a case-by-case license. "In the early 1990s ... As a general policy, the State Department allowed exports of commercial encryption with 40-bit keys, although some software with DES could be exported to U.S.-controlled subsidiaries and financial institutions."{{sfn|Grimmett|2001|p=}}{{sfn|Schneier|1996|p=615}} As a result, the "international" versions of web browsers were designed to have an effective key size of 40 bits when using Secure Sockets Layer to protect e-commerce. Similar limitations were imposed on other software packages, including early versions of Wired Equivalent Privacy. In 1992, IBM designed the CDMF algorithm to reduce the strength of 56-bit DES against brute force attack to 40 bits, in order to create exportable DES implementations.
Obsolescence
All 40-bit and 56-bit encryption algorithms are obsolete, because they are vulnerable to brute force attacks, and therefore cannot be regarded as secure.{{cite web|last1=University of California at Berkeley Public Information Office|title=The only legally exportable cryptography level is totally insecure; UC Berkeley grad student breaks challenge cipher in hours|url=http://www.berkeley.edu/news/media/releases/97legacy/code.html|publisher=The Regents of the University of California|access-date=2015-12-14|date=1997-01-29|quote=This is the final proof of what we've known for years: 40-bit encryption technology is obsolete.}}{{cite web|author1=Fitzmaurice, Ellen|author2=Tamaki, Kevin|title=Decoding the Encryption Debate: U.S. export restrictions and 'key recovery' policies are ineffectual as well as burdensome to business|url=https://www.latimes.com/archives/la-xpm-1997-06-01-me-64597-story.html|website=Los Angeles Times|access-date=2015-12-14|date=1997-06-01|quote=But recent advances in computing technology have rendered 40-bit encryption dangerously weak and export limits commercially obsolete.}} As a result, virtually all Web browsers now use 128-bit keys, which are considered strong. Most Web servers will not communicate with a client unless it has 128-bit encryption capability installed on it.
Public/private key pairs used in asymmetric encryption (public key cryptography), at least those based on prime factorization, must be much longer in order to be secure; see key size for more details.
As a general rule, modern symmetric encryption algorithms such as AES use key lengths of 128, 192 and 256 bits.
See also
Footnotes
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References
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- {{cite web|url=https://w2.eff.org/Privacy/Crypto/Crypto_misc/DESCracker/HTML/19980716_eff_des_faq.html|title=Frequently Asked Questions (FAQ) About the Electronic Frontier Foundation's "DES Cracker" Machine|publisher=Electronic Frontier Foundation|date=16 July 1998|access-date=23 Mar 2012|ref={{SfnRef|EFF-1998}}|archive-url=https://web.archive.org/web/20120918165854/http://w2.eff.org/Privacy/Crypto/Crypto_misc/DESCracker/HTML/19980716_eff_des_faq.html|archive-date=September 18, 2012|url-status=dead|df=mdy-all}}
- {{cite report|url=http://www.au.af.mil/au/awc/awcgate/crs/rl30273.pdf|format=pdf|title=Encryption Export Controls|first=Jeanne J.|last=Grimmett|series=Congressional Research Service Report RL30273|year=2001|access-date=July 26, 2011|archive-date=February 28, 2019|archive-url=https://web.archive.org/web/20190228095041/http://www.au.af.mil/au/awc/awcgate/crs/rl30273.pdf|url-status=dead}}
- {{cite book|last=Schneier|first=Bruce|title=Applied Cryptography|edition=Second|publisher=John Wiley & Sons|year=1996|isbn=0-471-11709-9}}
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Category:Symmetric-key cryptography