(A security analyst is using 3DES for data encryption. Which 3DES key size is valid?)

Triple DES (3DES) is a symmetric-key block cipher that applies the Data Encryption Standard (DES) algorithm three times to each data block. It has two primary keying options defined by NIST. The most common option, Two-Key 3DES, uses two independent 56-bit keys (K1 and K2), where the process is Encrypt-Decrypt-Encrypt (EDE) with the key sequence K1, K2, K1. This results in an effective key size of 112 bits (56 bits + 56 bits). While a three-key option exists with a 168-bit key, its effective strength is also considered to be 112 bits due to meet-in-the-middle attacks.

A. 128-bit is a standard key size for the Advanced Encryption Standard (AES), not 3DES.

B. 2,048-bit is a common key size for asymmetric algorithms like RSA, not for a symmetric block cipher like 3DES.

C. 56-bit is the key size for the original, single DES algorithm, which 3DES was designed to replace due to its vulnerability to brute-force attacks.

1. National Institute of Standards and Technology (NIST). (2017). Special Publication 800-67 Revision 2: Recommendation for the Triple Data Encryption Algorithm (TDEA) Block Cipher. Section 3, "TDEA Keying Options," p. 4. Retrieved from https://doi.org/10.6028/NIST.SP.800-67r2

2. Katz, J., & Lindell, Y. (2014). Introduction to Modern Cryptography (2nd ed.). CRC Press. Chapter 6, "Constructions of Pseudorandom Permutations," Section 6.2.4, "Stronger Block Ciphers from Weaker Ones," pp. 193-195.

3. Rivest, R. L. (1998). Lecture 4: The Data Encryption Standard (DES). MIT 6.857 Computer and Network Security. Retrieved from https://ocw.mit.edu/courses/6-857-computer-and-network-security-spring-2017/resources/lecture-4-the-data-encryption-standard-des/ (Discusses DES key size and mentions Triple DES).