(What describes how Counter (CTR) mode encryption functions?)

Counter (CTR) mode is a block cipher mode of operation that effectively transforms a block cipher into a stream cipher. It does not encrypt the plaintext directly. Instead, it generates a keystream by encrypting successive values of a "counter." This counter is typically initialized with a nonce (a number used once) to ensure each stream is unique. The resulting encrypted counter blocks (the keystream) are then XORed with the plaintext blocks to produce the ciphertext. This process allows for parallel encryption and decryption.

B. This description is more aligned with Cipher Feedback (CFB) mode, which can be self-synchronizing. CTR mode is not self-synchronizing.

C. This describes Electronic Codebook (ECB) mode, where each block is encrypted independently without any chaining or feedback mechanism.

D. This describes Cipher Block Chaining (CBC) mode, where the ciphertext from the previous block is used to encrypt the next plaintext block.

1. National Institute of Standards and Technology (NIST). (2001). SP 800-38A: Recommendation for Block Cipher Modes of Operation. Section 6.5, "The Counter (CTR) Mode," page 13.

2. Boneh, D., & Shoup, V. (2017). A Graduate Course in Applied Cryptography. Version 0.5. Chapter 4, "Symmetric Encryption," Section 4.5, "Modes of Operation," page 101. (Available at https://crypto.stanford.edu/~dabo/cryptobook/)