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[edit]
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[edit]
Examples of popular and wellrespected symmetric algorithms include Twofish, Serpent, AES (Rijndael), Blowfish, CAST5, RC4, 3DES, and IDEA.^{[citation needed]}
Cryptographic primitives based on symmetric ciphers[edit]
Symmetric ciphers are commonly used to achieve other cryptographic primitives than just encryption.^{[citation needed]}
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).^{[citation needed]}
However, symmetric ciphers cannot be used for nonrepudiation purposes. See the ISO 138882 standard.^{[citation needed]}
Another application is to build hash functions from block ciphers. See oneway compression function for descriptions of several such methods.^{[citation needed]}
Construction of symmetric ciphers[edit]
Main article:
Feistel cipher
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.^{[citation needed]}
Security of symmetric ciphers[edit]
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.^{[citation needed]}
Key generation[edit]
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.^{[citation needed]}


Common
algorithms 


Less common
algorithms 


Other
algorithms 


Design 


Attack
(cryptanalysis) 


Standardization 


Utilization 





Widely used ciphers 


eSTREAM Portfolio 


Other ciphers 


Theory 


Attacks 






»»