In cryptography, the Cellular Message Encryption Algorithm (CMEA) is a block cipher which was used for securing mobile phones in the United States. CMEA is one of four cryptographic primitives specified in a Telecommuncations Industry Association (TIA) standard, and is designed to encrypt the control channel, rather than the voice data. In 1997, a group of cryptographers (Wagner et. al., 1997) published attacks on the cipher showing it had several weaknesses. Some accusations were made that the NSA had pressured the original designers into crippling CMEA, but the NSA has denied any role in the design or selection of the algorithm. The ECMEA and SCEMA ciphers are derived from CMEA.
Ciphers, on the other hand, work at a lower level: the level of individual letters, small groups of letters, or, in modern schemes, individual bits.
Historical pen and paper ciphers used in the past are sometimes known as classical ciphers.
Symmetric key ciphers can be distinguished into two types, depending on whether they work on blocks of symbols of fixed size (block ciphers), or on a continuous stream of symbols (stream ciphers).
In cryptography, a block cipher is a symmetric key cipher which operates on fixed-length groups of bits, termed blocks, with an unvarying transformation.
Block ciphers can be contrasted with stream ciphers; a stream cipher operates on individual digits one at a time, and the transformation varies during the encryption.
The distinction between the two types is not always clear-cut: a block cipher, when used in certain modes of operation, acts effectively as a stream cipher.
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