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.

Types of symmetric-key algorithms

Symmetric-key algorithms can be divided into stream ciphers and block ciphers. Stream ciphers encrypt the bits of the message one at a time, and block ciphers take a number of bits 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 128-bit blocks.

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

Symmetric vs. asymmetric algorithms

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:

Speed

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.

Key management

Main article: Key management

One disadvantage of symmetric-key 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. 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.

Hybrid cryptosystem

Main article: hybrid cryptosystem

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 symmetric-keys at the start of a session. Once a symmetric key is known to all parties of the session, faster symmetric-key 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 and GPG, etc.

Cryptographic primitives based on symmetric ciphers

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.

Construction of symmetric ciphers

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.

Security of symmetric ciphers

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

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