Cellular Message Encryption Algorithm

Infobox block cipher
name = CMEA


caption =
designers = James A. Reeds III
publish date = 1991
derived from =
derived to =
key size = 64 bits
block size = 16-64 bits
structure =
rounds = 3
cryptanalysis = 338 chosen plaintexts break all block sizes, 40-80 known plaintexts break 24-bit blocks, and 4 known plaintexts break 16-bit blocksIn 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 Telecommunications 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.

CMEA is described in US patent|5159634. It is byte-oriented, with variable block size, typically 2 to 6 bytes. The key size is only 64 bits. Both of these are unusually small for a modern cipher. The algorithm consists of only 3 passes over the data: a non-linear left-to-right diffusion operation, an unkeyed 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 uses an unkeyed lookup table called the "CaveTable". The algorithm is self-inverse; re-encrypting the ciphertext with the same key is equivalent to decrypting it.

CMEA is severely insecure. There is a chosen-plaintext attack, effective for all block sizes, using 338 chosen plaintexts. For 3-byte blocks (typically used to encrypt each dialled digit), there is a known-plaintext attack using 40 to 80 known plaintexts. For 2-byte blocks, 4 known plaintexts suffice.

ee also

* A5/1, The european equivalent
* ORYX
* CAVE

References

* cite conference
author = David Wagner, Bruce Schneier, John Kelsey
title = Cryptanalysis of the Cellular Message Encryption Algorithm
booktitle = Advances in Cryptology - CRYPTO '97, 17th Annual International Cryptology Conference
pages = pp.526–537
date = August 1997
location = Santa Barbara, California
url = http://www.schneier.com/paper-cmea.html
format = PDF/PostScript
accessdate = 2007-02-07

External links

* [http://www.schneier.com/cmea.html The attack on CMEA]
* [http://www.snapshield.com/www_problems/United_States/New_Digital.htm Press release and the NSA response]