In
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 adver ...
, CDMF (Commercial Data Masking Facility) is an algorithm developed at
IBM in 1992 to reduce the
security strength of the
56-bit DES
Des is a masculine given name, mostly a short form (hypocorism) of Desmond. People named Des include:
People
* Des Buckingham, English football manager
* Des Corcoran, (1928–2004), Australian politician
* Des Dillon (disambiguation), sever ...
cipher to that of
40-bit encryption
40-bit encryption refers to a (now broken) key size of forty bits, or five bytes, for symmetric encryption; this represents a relatively low level of security. A forty bit length corresponds to a total of 240 possible keys. Although this is a larg ...
, at the time a requirement of U.S. restrictions on
export of cryptography. Rather than a separate cipher from DES, CDMF constitutes a
key generation algorithm, called ''key shortening''. It is one of the cryptographic algorithms supported by
S-HTTP.
Algorithm
Like DES, CDMF accepts a 64-bit input
key
Key or The Key may refer to:
Common meanings
* Key (cryptography), a piece of information that controls the operation of a cryptography algorithm
* Key (lock), device used to control access to places or facilities restricted by a lock
* Key (map ...
, but not all bits are used.
The algorithm consists of the following steps:
#Clear bits 8, 16, 24, 32, 40, 48, 56, 64 (ignoring these bits as DES does).
#
XOR
Exclusive or or exclusive disjunction is a logical operation that is true if and only if its arguments differ (one is true, the other is false).
It is symbolized by the prefix operator J and by the infix operators XOR ( or ), EOR, EXOR, , ...
the result with its encryption under DES using the key 0xC408B0540BA1E0AE.
#Clear bits 1, 2, 3, 4, 8, 16, 17, 18, 19, 20, 24, 32, 33, 34, 35, 36, 40, 48, 49, 50, 51, 52, 56, 64.
#Encrypt the result under DES using the key 0xEF2C041CE6382FE6.
The resulting 64-bit data is to be used as a DES key. Due to step 3, a
brute force attack needs to test only 2
40 possible keys.
References
*
* , IBM's patent on CDMF
ISO/IEC9979-0005 Register Entry (PDF) registered October 29, 1994
*
* , defines S-HTTP
Cryptographic algorithms
Data Encryption Standard
Key management
Block ciphers
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