Symmetrickey algorithms^{[1]} are a class of algorithms for cryptography that use the same cryptographic keys for both encryption of plaintext and decryption of ciphertext. The keys may be identical or there may be a simple transformation 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.^{[2]} This requirement that both parties have access to the secret key is one of the main drawbacks of symmetric key encryption, in comparison to publickey encryption.^{[3]}
Types of symmetrickey algorithms
Symmetrickey encryption can use either stream ciphers or block ciphers.^{[4]}

Stream ciphers encrypt the digits (typically bytes) of a message one at a time.

Block ciphers take a number of bits and encrypt them as a single unit, padding the plaintext so that it is a multiple of the block size. Blocks of 64 bits have been commonly used. The Advanced Encryption Standard (AES) algorithm approved by NIST in December 2001 uses 128bit blocks.
Implementations
Examples of popular symmetric algorithms include Twofish, Serpent, AES (Rijndael), Blowfish, CAST5, RC4, 3DES, Skipjack, Safer+/++ (Bluetooth), and IDEA.
Cryptographic primitives based on symmetric ciphers
Symmetric ciphers are commonly 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. CBCMAC).
However, symmetric ciphers cannot be used for nonrepudiation purposes except by involving additional parties. See the ISO/IEC 138882 standard.
Another application is to build hash functions from block ciphers. See oneway compression function for descriptions of several such methods.
Construction of symmetric ciphers
Many modern block ciphers are based on a construction proposed by Horst Feistel. Feistel's construction makes it possible to build invertible functions from other functions that are themselves not invertible.
Security of symmetric ciphers
Symmetric ciphers have historically been susceptible to knownplaintext attacks, chosen plaintext attacks, differential cryptanalysis and linear cryptanalysis. Careful construction of the functions for each round can greatly reduce the chances of a successful attack.
Key generation
When used with asymmetric ciphers for key transfer, pseudorandom key generators are nearly always used to generate the symmetric cipher session keys. However, lack of randomness in those generators or in their initialization vectors is disastrous and has led to cryptanalytic breaks in the past. Therefore, it is essential that an implementation uses a source of high entropy for its initialization.
Notes

^ Other terms for symmetrickey encryption are secretkey, singlekey, sharedkey, onekey, and privatekey encryption. Use of the last and first terms can create ambiguity with similar terminology used in publickey cryptography. Symmetrickey cryptography is to be contrasted with asymmetrickey cryptography.

^ Delfs, Hans & Knebl, Helmut (2007). "Symmetrickey encryption". Introduction to cryptography: principles and applications. Springer.

^ Mullen, Gary & Mummert, Carl (2007). Finite fields and applications. American Mathematical Society. p. 112.

^ Pelzl & Paar (2010). Understanding Cryptography. Berlin: SpringerVerlag. p. 30.


Common
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Less common
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Other
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Design



Attack
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Standardization



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Widely used ciphers



eSTREAM Portfolio



Other ciphers



Theory



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