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A key should therefore be large enough that a brute force attack (possible against any encryption algorithm) is infeasible -- i.e, would take too long to execute. Shannon's work on information theory showed that to achieve perfect secrecy, it is necessary for the key length to be at least as large as that of the message to be transmitted. In light of this, and the practical difficulty of managing such long keys, modern cryptographic practice has discarded the notion of perfect secrecy as a requirement for encryption, and instead focuses on computational security. Under this definition, the computational requirements of breaking an encrypted text must be infeasible for an attacker.
Even if a cipher is unbreakable by exploiting structural weaknesses in the algorithm, it is possible to run through the entire space of keys in what is known as a brute force attack. Since longer keys require more work to brute force search, a long enough key will require more work than is feasible. Thus, length of the key is important in resisting this type of attack.
A key of length n (bits) means that there are 2n possible keys. This number grows extremely rapidly as n increases. However, any comfort is seriously reduced because of Moore's lawMoore's law is an empirical observation stating, in effect, that at our rate of technological development and advances in the semiconductor industry, the complexity of integrated circuits doubles every 18 months. See exponential growth. It is attributed t, which suggests that computing power, including that available to an attacker, doubles roughly every 18 months.
Because of this, US Government export policy has long restricted the 'strength' of cryptography which can be sent out of the country. The metric used is the length of key. For many years the limit was 40 bits40-bit encryption is a key size for symmetric encryption representing a low-level of security where the key is forty bits in length (five bytes). Forty bits can represent a total of 240 possible keys. Although this is a large number (about a trillion, and. Needless to say, a key length of 40 bits offers little protection today even against a casual attacker with very limited computing resources. The restrictions have not been removed (it is still illegal to export some cryptographic products), but the limit was effectively raised to 128-bit key lengths in 1999For the album by Prince, see 1999 (album 1999 is a common year starting on Friday (see link for calendar), and was designated the International Year of Older Persons by the UN. Events Kosovo War Former child star Gary Coleman files for bankruptcy Y2K prep/ 2000.
When the Data Encryption Standard cipher was released in 1977, a key length of 56 bits was thought to be sufficient (though there was speculation at the time that the NSA has deliberately reduced the key size from the original value of 112 bits, in IBM's Lucifer cipher, or 64 bits, in one of the versions of what was adopted as DES) so as to limit the 'strength' of encryption available to non-US users. The NSA has major computing resources and a large budget; some thought that 56 bits was NSA-breakable in the late '70s. However, by the late 90s, it became clear that DES could be cracked in a few days' time-frame with custom-built hardware such as could be purchased by a large corporation. The book Cracking DES (O'Reilly and Associates) tells of the successful attempt to break 56-bit DES by a brute force attack mounted by a cyber civil rights group with limited resources. 56 bits is now considered insufficient length for symmetric algorithm keys, and may have been for some time. More technically and financially capable organizations were surely able to do the same long before the effort described in the book. Distributed.net and its volunteers broke a 64-bit RC5 key in several years, using hundreds of thousands of (mostly home) computers.
The NSA's Skipjack algorithm used in its Fortezza program employs 80 bit keys.
DES has been replaced in many applications by triple DES or 3DES, which has 112-bit keys.
The Advanced Encryption Standard published in 2001 uses a key size of (at minimum) 128 bits. It also can use keys up to 256 bits (a specification requirement for submissions to the AES contest). 128 bits is currently thought, by many observers, to be sufficient for the foreseeable future for symmetric algorithms of AES's quality. The U.S. Government requires 192 or 256-bit AES keys for TOP SECRET data.
As of 2003, the U.S. National Institute for Standards and Technology, NIST, is proposing that 80-bit keys be phased out by 2015.
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