Which legacy cipher uses a 56-bit key and is insecure for modern use?

• A. One-Time Pad
• B. DES
• C. AES
• D. CBC Mode

Answer: (B)

A 56-bit key is too small for modern security, which is why DES is considered insecure today. The strength of a cipher against brute-force attacks largely comes from its key space size. With a 56-bit key, there are 2^56 possible keys to try. That search space is small enough for attackers with today’s hardware to exhaust in a practical amount of time, making it feasible to recover the plaintext by trial and error. DES uses exactly that 56-bit key, so it’s labeled a legacy cipher and is no longer considered secure for modern use. The other options don’t fit: the One-Time Pad is theoretically unbreakable if the key is truly random and as long as the message, not a fixed 56-bit scheme; AES uses much larger keys (128/192/256 bits) and is the standard today; CBC Mode is a mode of operation, not a standalone cipher, and it would be used with a modern cipher rather than being defined by a 56-bit key.