40-bit encryption refers to a (now broken)
key size of forty bits, or five
byte
The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable uni ...
s, for
symmetric encryption
Symmetric-key algorithms are algorithms for cryptography that use the same cryptographic keys for both the encryption of plaintext and the decryption of ciphertext. The keys may be identical, or there may be a simple transformation to go between th ...
; this represents a relatively low
level of security. A forty bit length corresponds to a total of 2
40 possible keys. Although this is a large number in human terms (about a
trillion
''Trillion'' is a number with two distinct definitions:
*1,000,000,000,000, i.e. one million million, or (ten to the twelfth power), as defined on the short scale. This is now the meaning in both American and British English.
* 1,000,000,000,00 ...
), 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. 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
The Data Encryption Standard (DES ) is a symmetric-key algorithm for the encryption of digital data. Although its short key length of 56 bits makes it too insecure for modern applications, it has been highly influential in the advancement of cry ...
(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
In cryptography, RC4 (Rivest Cipher 4, also known as ARC4 or ARCFOUR, meaning Alleged RC4, see below) is a stream cipher. While it is remarkable for its simplicity and speed in software, multiple vulnerabilities have been discovered in RC4, ren ...
algorithms which had special "7-day" export review policies, 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 browser
A web browser is application software for accessing websites. When a user requests a web page from a particular website, the browser retrieves its files from a web server and then displays the page on the user's screen. Browsers are used o ...
s were designed to have an effective key size of 40 bits when using
Secure Sockets Layer to protect
e-commerce
E-commerce (electronic commerce) is the activity of electronically buying or selling of products on online services or over the Internet. E-commerce draws on technologies such as mobile commerce, electronic funds transfer, supply chain manag ...
. 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.
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
*
56-bit encryption
*
Content Scramble System
Footnotes
References
*
*
*
{{Refend
Symmetric-key cryptography
History of cryptography
Encryption debate