DES uses a 56-bit block and a 64-bit key.

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BlackTom AI · Accepted Answer

The statement asserts two specifications about DES: a 56-bit block size and a 64-bit key size. 
Option 1: True. This is incorrect because DES does not use a 56-bit block; it uses a 64-bit block. Additionally, DES uses a 56-bit key in practice, but the key is stored with 8 parity bits, making the total key length 64 bits in the data representation. 
Option 2: False. This is correct because the actual DES block size is 64 bits, not 56, and while the key material is 56 bits in the core key schedule, it is typically represented as 64 bits with parity bits for storage, which is historically where the confusion comes from. 
To recap: DES operates on 64-bit blocks; the key used for the Feistel rounds is effectively 56 bits of entropy, with 8 parity bits included in the key input to form a 64-bit key representation. The given statement swaps these values, leading to an incorrect description of DES parameters.

41900 Cryptography - Autumn 2025 41900 Sample Quiz (Week 3)

DES uses a 56-bit block and a 64-bit key.

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