A symmetric-key algorithm is an algorithm for cryptography that uses the same cryptographic key to encrypt and decrypt the message. (Actually, it is sufficient for it to be easy to compute the decryption key from the encryption key and vice versa.) Other terms for symmetric-key encryption are single-key and private-key encryption. Use of the latter term is discouraged because of conflict with the term private key in public key cryptography.
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.
Symmetric-key algorithms are generally much faster to execute electronically than asymmetric key algorithms. The disadvantage of symmetric-key algorithms is the requirement of a shared secret key. since keys are subject to unauthorized copying (most ominously, by The Opposition), keys should be changed frequently to render any copied keys worthless. The consequent requirement to choose and distribute keys without error and without loss is a difficulty. In practice, it is a very severe problem, and has never been adequately solved. Given the presence of humans in the distribution mechanisms, there is no prospect of a solution.