Which of the following is NOT an effective key length in 3DES?

• A. 40 bits
• B. 110 bits
• C. Both A and B
• D. Neither A nor B

Answer: (D)

In 3DES, the number of bits in the key isn’t the sole measure of security—the practical strength against attacks defines the effective key length. Even though 3DES can use up to 168 bits of key material, the best-known attacks on triple DES reduce the practical security to about 112 bits. That means the real protection you get from a 3DES key is roughly 112 bits, not the full 168.

A 40-bit key is far too small to be effective today. With only 2^40 possibilities, an attacker can brute-force it quickly with modern hardware, so it doesn’t provide meaningful security.

The number 110 bits isn’t the established standard for 3DES’s effective security. The commonly cited practical strength is about 112 bits, so 110 bits isn’t recognized as an effective length.

So, neither 40 bits nor 110 bits constitutes an effective key length for 3DES.