What does Triple DES use to produce a ciphertext block?

• A. A 64-bit plaintext block and a 128-bit key
• B. A 64-bit plaintext block and a 56-bit key
• C. A 128-bit plaintext block and a 64-bit key
• D. A 32-bit plaintext block and a 256-bit key

Answer: (B)

Triple DES (3DES) encrypts data by applying the Data Encryption Standard (DES) algorithm three times to each data block. It is important to understand the specifications of DES to grasp how Triple DES modifies them.

In Triple DES, a 64-bit plaintext block is used, which aligns with the original DES specification that processes data in blocks of this size. The key size for 3DES is a bit more complex; although the original DES utilized a 56-bit key, Triple DES effectively employs three separate keys, which can lead to an effective key length of up to 168 bits when all three are used. However, when referring to the fundamental structure of DES, the critical aspect is that it starts with a 64-bit plaintext block and a 56-bit key per encryption cycle.

Thus, the accurate choice reflects that Triple DES uses a 64-bit plaintext block alongside a 56-bit key for its encryption process, adhering to the principles established by DES while enhancing security through multiple encryption iterations.