In cryptography, the Cellular Message Encryption Algorithm (CMEA) is a block cipher that was used for increasing the security of mobile phones in USA. CMEA is among four cryptographic primitives pointed out in a Telecommunications Industry Association (TIA) standard, and the control channel is encrypted using this, rather than the voice data.
In 1997, a group of cryptographers made attacks on the cipher giving evidence that it had plethora of weaknesses that give it a trivial effective strength of a 24-bit to 32-bit cipher. NSA had been accused of pressurizing the original designers into crippling CMEA; however, the NSA has denied any role in the design or selection of the algorithm.
CMEA is described in U.S. Patent 5,159,634. It is byte-oriented, with varying block size: 2 to 6 bytes. The key size is 64 bits. For a modern cipher, they both are a lot smaller than usual. The algorithm is made up of only 3 passes over the data: a non-linear left-to-right diffusion operation, an un-keyed linear mixing, and another non-linear diffusion that is in fact the inverse of the first.
The non-linear operations use a keyed lookup table called the T-box, which makes use of an un-keyed lookup table called the “CaveTable”. The algorithm is self-inverse; re-encrypting the ciphertext with the same key and decrypting it is equivalent to one another.
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