DES uses a key of what size?

DES uses a key of what size?

• A. 64-bit
• B. 56-bit
• C. 40-bit
• D. 128-bit

Answer: (B)

DES uses a 56-bit key. Although the key input to DES is presented as 64 bits, eight of those bits are parity bits and aren’t used in the actual key material. The key schedule drops those parity bits and uses the remaining 56 bits to generate the sixteen 48-bit subkeys used in each round. That means the effective key length—the number of possible secret keys—is 56 bits (2^56 possibilities). The parity bits are there for error checking, not to increase security, so the key strength is tied to 56 bits. The other sizes don’t match DES’s design: a 64-bit interpretation would imply using parity bits as part of the secret, while 40-bit or 128-bit do not reflect DES’s established specification.

DES uses a 56-bit key. Although the key input to DES is presented as 64 bits, eight of those bits are parity bits and aren’t used in the actual key material. The key schedule drops those parity bits and uses the remaining 56 bits to generate the sixteen 48-bit subkeys used in each round. That means the effective key length—the number of possible secret keys—is 56 bits (2^56 possibilities). The parity bits are there for error checking, not to increase security, so the key strength is tied to 56 bits. The other sizes don’t match DES’s design: a 64-bit interpretation would imply using parity bits as part of the secret, while 40-bit or 128-bit do not reflect DES’s established specification.