CHACHA20POLY1305

ChaCha20-Poly1305 is an authenticated encryption with associated data (AEAD) scheme that combines the ChaCha20 stream cipher with the Poly1305 message authentication code. It provides both confidentiality for the plaintext and integrity and authenticity for the ciphertext and any associated data. The construction uses a 256-bit key and a 12-byte (96-bit) nonce. A Poly1305 key is first derived from ChaCha20 by encrypting a 32-byte block with counter set to 0; the actual encryption uses ChaCha20 with the same key and nonce and a counter starting at 1. The Poly1305 tag authenticates the associated data, the ciphertext, and their respective lengths, producing a 16-byte authentication tag appended to the ciphertext.

ChaCha20-Poly1305 was designed by Daniel J. Bernstein and colleagues and was standardized for IETF protocols in RFC 7539, with continued guidance in RFC 8439. It is favored for high performance on platforms lacking hardware-accelerated AES and for constant-time operation to resist timing side-channel attacks. It has seen widespread adoption in TLS as a drop-in replacement for AES-GCM in suitable environments, and it is also used in other protocols such as SSH and various secure messaging systems. A well-known variant is XChaCha20-Poly1305, which uses an extended nonce to allow larger numbers of messages per key.

Security and usage notes: each message must use a unique nonce with the same key; nonce reuse can completely compromise security. Implementations should perform constant-time authentication checks and avoid revealing partial results. Proper nonce management and adherence to the standard are essential for preserving the scheme’s security guarantees.