What is the effective key size of DES?

The Data Encryption Standard (DES) algorithm specifies a key of 64 bits in length. However, within this 64-bit block, every eighth bit (bits 8, 16, 24, 32, 40, 48, 56, and 64) is designated as a parity bit for error detection. These parity bits are discarded before the key-scheduling process begins. Consequently, only 56 of the 64 bits are actually used to generate the subkeys for the encryption rounds. This makes the effective key size 56 bits, which defines the algorithm's cryptographic strength against brute-force attacks.

B. 64 bits: This is the nominal key size, including the 8 parity bits, not the effective key size used in the cryptographic operations.

C. 128 bits: This is a common key size for modern symmetric algorithms like the Advanced Encryption Standard (AES), not for the legacy DES algorithm.

D. 1024 bits: This key length is characteristic of asymmetric cryptographic algorithms, such as RSA, not symmetric block ciphers like DES.

1. National Institute of Standards and Technology (NIST). (1999). FIPS PUB 46-3

Data Encryption Standard (DES). U.S. Department of Commerce. In Section 3

"THE ALGORITHM

" it states

"The 64 bits of the key are denoted by K1

K2

...

K64. The bits K8

K16

...

K64 are for error detection... The 56 bits used in the algorithm are selected from the 64-bit key." (Page 4).

2. Boneh

D. (n.d.). CS255 Introduction to Cryptography

Lecture 5: DES. Stanford University. The lecture notes state

"DES uses a 64-bit key

but 8 of these bits are parity bits. So the effective key length is 56 bits." (Slide 10

"DES: The Data Encryption Standard").

3. Katz

J.

& Lindell

Y. (2014). Introduction to Modern Cryptography (2nd ed.). CRC Press. In Chapter 6

"The Data Encryption Standard (DES)

" Section 6.2

"A High-Level Description of DES

" it is explained that the initial 64-bit key is subjected to a permutation (PC-1) that discards the parity bits

resulting in a 56-bit key for the key-scheduling algorithm. (Page 178).