A DES key is ________ bits long.

• A. 40
• B. 56
• C. 100
• D. 128

Answer: (B)

The important idea here is that the length of a DES key refers to how many bits actually determine the encryption, not just how many bits are fed into the key input. In DES, the key input includes bits used for error checking (parity bits), but these parity bits don’t influence the encryption process. The algorithm uses a fixed amount of secret key material, and that fixed amount is what the security relies on. So, when choosing the correct option, you’re selecting the length that represents the actual secret key material DES uses, not the total size of the key input presented to the system. This is why DES is considered weak by modern standards and why stronger algorithms are recommended today.