Which of the following is one of the effective key lengths in 3DES?

• A. 56 bits
• B. 100 bits
• C. 112 bits
• D. None of the above

Answer: (C)

Three DES keys in 3DES give a nominal 168-bit key, but practical security is reduced by the meet-in-the-middle attack, which makes the effective work factor about 2^112. So the effective key length for 3DES is around 112 bits. That’s why 112 bits is the correct effective length. A 56-bit figure would only reflect a single DES key, not the combined 3DES strength. A 100-bit figure isn’t a standard effective length for 3DES, and “none of the above” would be incorrect because 112 bits is a valid effective length.