40-bit encryption refers to a 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, nearly two hundred times the world's human population), it is possible to break this degree of encryption using a very limited amount of computing power (as of 2015, of any modern personal computer) in a brute-force attack, i.e., trying out each possible key in turn.
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. 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, and would be able to break 40-bit DES encryption in about two seconds.
40-bit encryption was common in software released before 1999, especially those based on the RC2 and RC4 algorithms, 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." 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.
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. 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.
As a general rule, modern symmetric encryption algorithms such as AES use key lengths of 128, 192 and 256 bits.
- As of 2015, a 40-bit keyspace can be exhaustively searched in under a minute using a COTS PC of moderate specification: in 1998, the EFF's Deep Crack could test 5x1010 (56-bit, 16 orders of magnitude larger keyspace than 40-bit) DES keys per second, leading to a brute-force key recovery in 56 hours. See http://pnylab.com/pny/papers/sk/sk/index.html
- Schneier 1996, p. 154.
- Schneier 1996, p. 153.
- Grimmett 2001.
- Schneier 1996, p. 615.
- "Frequently Asked Questions (FAQ) About the Electronic Frontier Foundation's "DES Cracker" Machine". Electronic Frontier Foundation. 16 July 1998. Retrieved 23 Mar 2012.
- Grimmett, Jeanne J. (2001). Encryption Export Controls (pdf) (Report). Congressional Research Service Report RL30273.
- Schneier, Bruce (1996). Applied Cryptography (Second Edition ed.). John Wiley & Sons. ISBN 0-471-11709-9.