A symmetric algorithm makes use of the same key for both encryption and decryption. Obviously it is important that the key be kept a secret. Some example uses of symmetric algorithms are to be found in the construction of block ciphers, stream ciphers, and message authentication codes.
The opposite of the symmetric algorithm is the asymmetric algorithm. There are also keyless algorithms which use no keys at all.
L. R. Knudsen, M. J. B. Robshaw, The Block Cipher Companion. Springer, 2011.
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