ChaCha20-Poly1305

ChaCha20-Poly1305 is an authenticated encryption with associated data (AEAD) algorithm, that combines the ChaCha20 stream cipher with the Poly1305 message authentication code.^[1] It has fast software performance, and without hardware acceleration, is usually faster than AES-GCM.^[1]: §B

The two building blocks of the construction, the algorithms Poly1305 and ChaCha20, were both independently designed, in 2005 and 2008, by Daniel J. Bernstein.^[2][3]

In March 2013, a proposal was made to the IETF TLS working group to include Salsa20, a winner of the eSTREAM competition^[4] to replace the aging RC4-based ciphersuites. A discussion followed in the IETF TLS mailing list with various enhancement suggestions, including using Chacha20 instead of Salsa20 and using a universal hashing based MAC for performance. The outcome of this process was the adoption of Adam Langley's proposal for a variant of the original ChaCha20 algorithm (using 32-bit counter and 96-bit nonce) and a variant of the original Poly1305 (authenticating 2 strings) being combined in an IETF draft^[5][6] to be used in TLS and DTLS,^[7] and chosen, for security and performance reasons, as a newly supported cipher.^[8] Shortly after IETF's adoption for TLS, ChaCha20, Poly1305 and the combined AEAD mode are added to OpenSSH via thechacha20-poly1305@openssh.com authenticated encryption cipher^[9][10] but kept the original 64-bit counter and 64-bit nonce for the ChaCha20 algorithm.

In 2015, the AEAD algorithm was standardized in RFC 7539^[11] and in RFC 7634^[12] to be used in IPsec. The same year, it was integrated by Cloudflare as an alternative ciphersuite.^[13]

In 2016 RFC 7905^[14] describes how to use it in the TLS 1.2 and DTLS 1.2 protocols.

In June 2018, RFC 7539 was updated and replaced by RFC 8439.^[1]

The ChaCha20-Poly1305 algorithm takes as input a 256-bit key and a 96-bit nonce to encrypt a plaintext,^[1] with a ciphertext expansion of 128-bit (the tag size). In the ChaCha20-Poly1305 construction, ChaCha20 is used in counter mode to derive a key stream that is XORed with the plaintext. The ciphertext and the associated data is then authenticated using a variant of Poly1305 that first encodes the two strings into one. The way that a cipher and a one time authenticator are combined is precisely identical to AES-GCM construction in how the first block is used to seed the authenticator and how the ciphertext is then authenticated with a 16-byte tag.

The main external difference with ChaCha20 is its 64 byte (512 bit) block size, in comparison to 16 bytes (128 bit) with both AES-128 and AES-256. The larger block size enables higher performance on modern CPUs and allows for larger streams before the 32 bit counter overflows.

The XChaCha20-Poly1305 construction is an extended 192-bit nonce variant of the ChaCha20-Poly1305 construction, using XChaCha20 instead of ChaCha20. When choosing nonces at random, the XChaCha20-Poly1305 construction allows for better security than the original construction. The draft attempt to standardize the construction expired in July 2020.^[15]

Salsa20-Poly1305 and XSalsa20-Poly1305 are variants of the ChaCha20-Poly1305 and XChaCha20-Poly1305 algorithms, using Salsa20 and XSalsa20 in place of ChaCha20 and XChaCha20. They are implemented in NaCl^[16] and libsodium^[17] but not standardized. The variants using ChaCha are preferred in practice as they provide better diffusion per round than Salsa.^[2]

ChaCha20 can be replaced with its reduced-round variants ChaCha12 and ChaCha8, yielding ChaCha12-Poly1305 and ChaCha8-Poly1305. The same modification can be applied to XChaCha20-Poly1305. These are implemented by the RustCrypto team and not standardized.^[18]

ChaCha20-Poly1305 is used in IPsec,^[1] SSH,^[19] TLS 1.2, DTLS 1.2, TLS 1.3,^[14][19] WireGuard,^[20] S/MIME 4.0,^[21] OTRv4^[22] and multiple other protocols and implemented in OpenSSL and libsodium. Additionally, the algorithm is used in the backup software Borg^[23] in order to provide standard data encryption and in the copy-on-write filesystem Bcachefs for the purpose of optional whole filesystem encryption.^[24]

ChaCha20-Poly1305 usually offers better performance than the more prevalent AES-GCM algorithm, except on systems where the CPU(s) have the AES-NI instruction set extension^[1]. As a result, ChaCha20-Poly1305 is sometimes preferred over AES-GCM due to its similar levels of security and in certain use cases involving mobile devices, which mostly use ARM-based CPUs. Because ChaCha20-Poly1305 has less overhead than AES-GCM, ChaCha20-Poly1305 on mobile devices may consume less power than AES-GCM.

The ChaCha20-Poly1305 construction is generally secure in the standard model and the ideal permutation model, for the single- and multi-user setting.^[25] However, similarly to GCM, the security relies on choosing a unique nonce for every message encrypted. Compared to AES-GCM, implementations of ChaCha20-Poly1305 are less vulnerable to timing attacks.

To be noted, when the SSH protocol uses ChaCha20-Poly1305 as underlying primitive, it is vulnerable to the Terrapin attack.

• Authenticated encryption
• Galois/Counter Mode
• Salsa20
• Poly1305

• Josefsson, Simon (2013-03-17). "Salsa20 stream cipher in TLS". mailarchive.ietf.org. IETF. Retrieved 2024-07-31. FYI, we have published -00 of a draft that describes how the Salsa20 stream cipher