Which legacy symmetric block cipher uses 56-bit keys?

• A. AES
• B. DES
• C. ECB Mode
• D. Padding

Answer: (B)

Key length is the focus here. DES, the Data Encryption Standard, is a legacy symmetric block cipher that uses a 56-bit key. In practice, the key is often shown as 64 bits with every eighth bit used as parity, so only 56 bits contribute to the actual secret. DES operates on 64-bit blocks through 16 rounds in a Feistel structure, with subkeys derived from that 56-bit key. Because the key space is only 2^56, it can be brute-forced more easily than modern ciphers, which is why DES is considered insecure today. To bolster security, people used Triple-DES (3DES), which effectively increases the key length, but DES itself remains a legacy algorithm. Modern standards use AES, which supports much larger keys (128, 192, or 256 bits). The other options aren’t a cipher or relate to modes/mitigation rather than the actual key size of DES.