How many bits is the effective length of the key of the Data Encryption Standard algorithm?

A.
168

B.
128

C.
56

D.
64

Explanation:
Data Encryption Standard (DES) has had a long and rich history within the computer community. NIST invited
vendors to submit data encryption algorithms to be used as a cryptographic standard. IBM had already been
developing encryption algorithms to protect financial transactions. In 1974, IBM’s 128-bit algorithm, named
Lucifer, was submitted and accepted. The NSA modified this algorithm to use a key size of 64 bits (with 8 bits
used for parity, resulting in an effective key length of 56 bits) instead of the original 128 bits, and named it the
Data Encryption Algorithm (DEA).
NOTE DEA is the algorithm that fulfills DES, which is really just a standard. So DES is the standard and DEA is
the algorithm, but in the industry we usually just refer to it as DES. The CISSP exam may refer to the algorithm
by either name, so remember both.
Incorrect Answers:
A: The Data Encryption Standard algorithm has an effective key length of 56 bits, not 168 bits.
B: The Data Encryption Standard algorithm has an effective key length of 56 bits, not 128 bits.
D: The Data Encryption Standard algorithm has an effective key length of 56 bits, not 64 bits.

Harris, Shon, All In One CISSP Exam Guide, 6th Edition, McGraw-Hill, 2013, p. 800