Triple DES is used in its two-key variant, with K1 = K3. What is the effective key length?

Answer

112 bits

Options - A. 112 bits - B. 168 bits - C. 56 bits - D. 128 bits Why - A. Correct — two independent 56-bit keys give 2 × 56 = 112 bits of effective key. - B. 168 bits is the THREE-key variant, 3 × 56. Attaching it to the two-key variant is the trap this question exists for. - C. 56 bits is single DES. Two keys are in play here, not one. - D. 128 bits is an AES key size. 3DES key lengths are always multiples of 56.

SB5e ch2 §2.1; ch20 §20.1–20.3