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Symmetric-key algorithms are a class of algorithms for cryptography that use trivially related, often identical, cryptographic keys for both decryption and encryption.

The encryption key is trivially related to the decryption key, in that they may be identical or there is a simple transform to go between the two keys. The keys, in practice, represent a shared secret between two or more parties that can be used to maintain a private information link.

Other terms for symmetric-key encryption are secret-key, single-key, shared-key, one-key and eventually private-key encryption. Use of the latter term does conflict with the term private key in public-key cryptography.

Some examples of popular and well-respected symmetric algorithms include Twofish, Serpent, AES (aka Rijndael), Blowfish, CAST5, RC4, TDES, and IDEA.

Unlike symmetric algorithms, asymmetric key algorithms use a different key for encryption than for decryption. I.e., a user knowing the encryption key of an asymmetric algorithm can encrypt messages, but cannot derive the decryption key and cannot decrypt messages encrypted with that key. A short comparison of these two types of algorithms is given below:

Symmetric-key algorithms are generally much less computationally intensive than asymmetric key algorithms. In practice, asymmetric key algorithms are typically hundreds to thousands times slower than symmetric key algorithms.

- Main article: Key management

- Main article: hybrid cryptosystem

Systems that use such a hybrid approach include SSL, PGP and GPG, etc.

Symmetric ciphers are often used to achieve other cryptographic primitives than just encryption.

Encrypting a message does not guarantee that this message is not changed while encrypted. Hence often a message authentication code is added to a ciphertext to ensure that changes to the ciphertext will be noted by the receiver. Message authentication codes can be constructed from symmetric ciphers (e.g. CBC-MAC). However, these messages authentication codes cannot be used for non-repudiation purposes.

Another application is to build hash functions from block ciphers. See one-way compression function for descriptions of several such methods.

- Main article: Feistel cipher

Many modern block ciphers are based on a construction proposed by Horst Feistel. Feistel's construction allows to build invertible functions from other functions that are itself not invertible.

Symmetric ciphers have historically been susceptible to known-plaintext attacks, chosen plaintext attacks, differential cryptanalysis and linear cryptanalysis. Careful construction of the functions for each round can greatly reduce the chances of a success

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Last updated on Tuesday October 07, 2008 at 10:28:59 PDT (GMT -0700)

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This article is licensed under the GNU Free Documentation License.

Last updated on Tuesday October 07, 2008 at 10:28:59 PDT (GMT -0700)

View this article at Wikipedia.org - Edit this article at Wikipedia.org - Donate to the Wikimedia Foundation

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