Symmetrickey 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 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.
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.
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Symmetrickey algorithms can be divided into stream ciphers and block ciphers. Stream ciphers encrypt the bytes of the message one at a time, and block ciphers take a number of bytes and encrypt them as a single unit. Blocks of 64 bits have been commonly used; the Advanced Encryption Standard algorithm approved by NIST in December 2001 uses 128bit blocks.
Some examples of popular and wellrespected symmetric algorithms include Twofish, Serpent, AES (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:
Symmetrickey 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.
One disadvantage of symmetrickey algorithms is the requirement of a shared secret key, with one copy at each end. In order to ensure secure communications between everyone in a population of n people a total of n(n − 1)/2 keys are needed, which is the total number of possible communication channels.^{[1]} To limit the impact of a potential discovery by a cryptographic adversary, they should be changed regularly and kept secure during distribution and in service. The process of selecting, distributing and storing keys is known as key management, and is difficult to achieve reliably and securely.
In modern cryptosystems designs, both asymmetric (public key) and symmetric algorithms are used to take advantage of the virtues of both. Asymmetric algorithms are used to distribute symmetrickeys at the start of a session. Once a symmetric key is known to all parties of the session, faster symmetrickey algorithms using that key can be used to encrypt the remainder of the session. This simplifies the key distribution problem, because asymmetric keys only have to be distributed authentically, whereas symmetric keys need to be distributed in an authentic and confidential manner.
Systems that use such a hybrid approach include SSL, PGP, 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. CBCMAC). However, these messages authentication codes cannot be used for nonrepudiation purposes.
Another application is to build hash functions from block ciphers. See oneway compression function for descriptions of several such methods.
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 themselves not invertible.
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.
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.
