What purpose does an initialization vector (IV) serve for encryption?

An Initialization Vector (IV) is a random or pseudo-random number used with a secret key for encryption. Its primary purpose is to ensure that when the same plaintext is encrypted multiple times with the same key, the resulting ciphertext is unique for each encryption. This prevents attackers from inferring relationships between messages by observing patterns in the ciphertext. The IV does not need to be secret but must be unique for each encryption session using the same key.

A. Key and algorithm negotiation is a function of protocols like TLS or IKE, not the IV itself.
C. Converting passphrases to cryptographic keys is the function of a Key Derivation Function (KDF), such as PBKDF2.
D. This describes a key exchange mechanism (like Diffie-Hellman) or key encapsulation, not the function of an IV.

1. National Institute of Standards and Technology (NIST). (2001). Special Publication 800-38A: Recommendation for Block Cipher Modes of Operation. Section 6, "Confidentiality Modes," and Appendix B, "The Generation of Initialization Vectors," explain that the IV's purpose is to randomize the encryption to produce distinct ciphertexts from the same plaintext.

2. Dworkin, M. (2016). NIST Special Publication 800-38D: Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC. Section 8.2, "IV Uniqueness Requirement," states, "The probability that the authenticated encryption function ever will be invoked with the same IV and the same key on two (or more) distinct sets of plaintext data shall be negligible."