Which statement about DES keys is true?

• A. They are 40 bits
• B. They are 56 bits
• C. They are 100 bits
• D. They are 128 bits

Answer: (B)

DES uses a 64-bit key input, but 8 bits are parity bits and not used in the encryption process. That leaves 56 bits of actual key material, so the effective key length is 56 bits. This is why DES keys are described as 56-bit: the key material that actually governs the DES rounds is 56 bits long. Historically, this relatively small key size made brute-force attacks feasible, which led to moves toward stronger options like triple DES and, later, AES.