cryptography Cryptography, or cryptology (from grc, , translit=kryptós "hidden, secret"; and ''graphein'', "to write", or ''-logia'', "study", respectively), is the practice and study of techniques for secure communication in the presence of adve ...

, the Cellular Message Encryption Algorithm (CMEA) is a

block cipher In cryptography, a block cipher is a deterministic algorithm operating on fixed-length groups of bits, called ''blocks''. Block ciphers are specified cryptographic primitive, elementary components in the design of many cryptographic protocols and ...

which was used for securing

mobile phone A mobile phone, cellular phone, cell phone, cellphone, handphone, hand phone or pocket phone, sometimes shortened to simply mobile, cell, or just phone, is a portable telephone that can make and receive telephone call, calls over a radio freq ...

s in the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., federal district, five ma ...

. CMEA is one of four cryptographic primitives specified in a

Telecommunications Industry Association The Telecommunications Industry Association (TIA) is accredited by the American National Standards Institute (ANSI) to develop voluntary, consensus-based industry standards for a wide variety of Information and Communication Technologies (ICT) p ...

(TIA) standard, and is designed to

encrypt In cryptography, encryption is the process of encoding information. This process converts the original representation of the information, known as plaintext, into an alternative form known as ciphertext. Ideally, only authorized parties can decip ...

the control channel, rather than the voice data. In 1997, a group of cryptographers published attacks on the

cipher In cryptography, a cipher (or cypher) is an algorithm for performing encryption or decryption—a series of well-defined steps that can be followed as a procedure. An alternative, less common term is ''encipherment''. To encipher or encode i ...

showing it had several weaknesses which give it a trivial effective strength of a 24-bit to 32-bit cipher. Some accusations were made that the

NSA The National Security Agency (NSA) is a national-level intelligence agency of the United States Department of Defense, under the authority of the Director of National Intelligence (DNI). The NSA is responsible for global monitoring, collectio ...

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 . It is

byte-oriented Byte-oriented framing protocol is "a communications protocol in which full bytes are used as control codes. Also known as character-oriented protocol." For example UART communication is byte-oriented. The term "character-oriented" is deprecated, ...

, with variable block size, typically 2 to 6 bytes. The

key size In cryptography, key size, key length, or key space refer to the number of bits in a key used by a cryptographic algorithm (such as a cipher). Key length defines the upper-bound on an algorithm's security (i.e. a logarithmic measure of the fastes ...

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 In computer science, a lookup table (LUT) is an array that replaces runtime computation with a simpler array indexing operation. The process is termed as "direct addressing" and LUTs differ from hash tables in a way that, to retrieve a value v w ...

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 A chosen-plaintext attack (CPA) is an attack model for cryptanalysis which presumes that the attacker can obtain the ciphertexts for arbitrary plaintexts.Ross Anderson, ''Security Engineering: A Guide to Building Dependable Distributed Systems''. ...

, 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 The known-plaintext attack (KPA) is an attack model for cryptanalysis where the attacker has access to both the plaintext (called a crib), and its encrypted version ( ciphertext). These can be used to reveal further secret information such as s ...

using 40 to 80 known plaintexts. For 2-byte blocks, 4 known plaintexts suffice. The "improved" CMEA, CMEA-I, is not much better: chosen-plaintext attack of it requires less than 850 plaintexts in its adaptive version.

References

External links

The attack on CMEA

{{Cryptography navbox , block Broken block ciphers

See also

References

External links