ChaCha is a stream cipher designed by Daniel J. Bernstein as a refinement of the Salsa20 cipher. It is engineered for high performance in software, simplicity, and strong resistance to cryptanalytic attacks. The most common implementation, ChaCha20, is frequently combined with the Poly1305 message authenticator to form an Authenticated Encryption with Associated Data (AEAD) construction. This combination, known as ChaCha20-Poly1305, provides confidentiality, integrity, and authenticity and is standardized for use in protocols like TLS 1.3 and SSH.

A. Counter (CTR): This is a mode of operation that turns a block cipher into a stream cipher; it is not a specific cipher derived from Salsa20.

B. Cipher block chaining (CBC): This is a block cipher mode of operation that provides confidentiality but is not a stream cipher and is unrelated to Salsa20.

D. Electronic codebook (ECB): This is the simplest and a highly insecure block cipher mode of operation, not a stream cipher or a variant of Salsa20.

1. Internet Engineering Task Force (IETF). (2018). RFC 8439: ChaCha20 and Poly1305 for IETF Protocols.

Section 1, Introduction: "ChaCha20 is a stream cipher designed by D. J. Bernstein... ChaCha20 is a refinement of the Salsa20 algorithm... This document also specifies the use of ChaCha20 in an AEAD construction, in combination with the Poly1305 authenticator."

2. Bernstein, D. J. (2008). ChaCha, a variant of Salsa20. Workshop record of SASC 2008: The State of the Art of Stream Ciphers.

Page 3, Section 1, Introduction: "This paper introduces ChaCha, a new family of stream ciphers. The ChaCha family is a variant of the Salsa20 family."

3. University of California, Berkeley. (2020). CS 161: Computer Security, Lecture 8: Symmetric Key Cryptography.

Slide 45, "Modern Symmetric Ciphers": Discusses ChaCha20-Poly1305 as a modern, fast, and secure AEAD (Authenticated Encryption with Associated Data) scheme, highlighting its use in TLS 1.3.

4. National Institute of Standards and Technology (NIST). (2001). NIST Special Publication 800-38A: Recommendation for Block Cipher Modes of Operation.

Sections 6.2 (CBC), 6.1 (ECB), and 6.5 (CTR): These sections define CBC, ECB, and CTR as modes of operation for block ciphers, distinguishing them from standalone stream ciphers like ChaCha.