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Authenticated Encryption with Additional Data

This operation:

  • Encrypts a message with a key and a nonce to keep it confidential
  • Computes an authentication tag. This tag is used to make sure that the message, as well as optional, non-confidential (non-encrypted) data, haven't been tampered with.

A typical use case for additional data is to authenticate protocol-specific metadata about the message, such as its length and encoding.

Supported constructions

libsodium supports two popular constructions: AES256-GCM and ChaCha20-Poly1305 (original version and IETF version), as well as a variant of the later with an extended nonce: XChaCha20-Poly1305.

The "combined mode" API of each construction appends the authentication tag to the ciphertext. The "detached mode" API stores the authentication tag in a separate location.

Availability and interoperability

Construction Key size Nonce size Block size MAC size Availability
AES256-GCM 256 bits 96 bits 128 bits 128 bits libsodium >= 1.0.4 but requires hardware support. IETF standard; also implemented in many other libraries.
ChaCha20-Poly1305 256 bits 64 bits 512 bits 128 bits libsodium >= 0.6.0. Also implemented in {Libre,Open,Boring}SSL.
ChaCha20-Poly1305-IETF 256 bits 96 bits 512 bits 128 bits libsodium >= 1.0.4. IETF standard; also implemented in Ring, {Libre,Open,Boring}SSL and other libraries.
XChaCha20-Poly1305-IETF 256 bits 192 bits 512 bits 128 bits libsodium >= 1.0.12.

Limitations

Construction Max bytes for a single (key,nonce) Max bytes for a single key
AES256-GCM ~ 64 GB ~ 350 GB (for ~16 KB long messages)
ChaCha20-Poly1305 No practical limits (~ 2^64 bytes) Up to 2^64* messages, no practical total size limits
ChaCha20-Poly1305-IETF 256 GB Up to 2^64* messages, no practical total size limits
XChaCha20-Poly1305-IETF No practical limits (~ 2^64 bytes) Up to 2^64* messages, no practical total size limits

These figures assume an untruncated (128-bit) authentication tag.

* Although periodic rekeying remains highly recommended, online protocols leveraging additional data to discard old messages don't have practical limitations on the total number of messages.

Nonces

Construction Safe options to choose a nonce
AES256-GCM Counter, permutation
ChaCha20-Poly1305 Counter, permutation
ChaCha20-Poly1305-IETF Counter, permutation
XChaCha20-Poly1305-IETF Counter, permutation, random, Hk(random ‖ m)

TL;DR: which one should I use?

XChaCha20-Poly1305-IETF is the safest choice.

AES256-GCM

The current implementation of this construction is hardware-accelerated and requires the Intel SSSE3 extensions, as well as the aesni and pclmul instructions.

Intel Westmere processors (introduced in 2010) and newer meet the requirements.

There are no plans to support non hardware-accelerated implementations of AES-GCM, as correctly mitigating side-channels in a software implementation comes with major speed tradeoffs, that defeat the whole point of AES-GCM over ChaCha20-Poly1305.

ChaCha20-Poly1305

While AES is very fast on dedicated hardware, its performance on platforms that lack such hardware is considerably lower. Another problem is that many software AES implementations are vulnerable to cache-collision timing attacks.

ChaCha20 is considerably faster than AES in software-only implementations, making it around three times as fast on platforms that lack specialized AES hardware. ChaCha20 is also not sensitive to timing attacks.

Poly1305 is a high-speed message authentication code.

The combination of the ChaCha20 stream cipher with the Poly1305 authenticator was proposed in January 2014 as an alternative to the Salsa20-Poly1305 construction. ChaCha20-Poly1305 was implemented in major operating systems, web browsers and crypto libraries shortly after. It eventually became an official IETF standard in May 2015.

The ChaCha20-Poly1305 implementation in libsodium is portable across all supported architectures.

XChaCha20-Poly1305

XChaCha20-Poly1305 applies the construction described in Daniel Bernstein's Extending the Salsa20 nonce paper to the ChaCha20 cipher in order to extend the nonce size to 192-bit.

This extended nonce size allows random nonces to be safely used, and also facilitates the construction of misuse-resistant schemes.

The XChaCha20-Poly1305 implementation in libsodium is portable across all supported architectures.

The main limitation of XChaCha20-Poly1305 is that it is not widely implemented in other libraries yet. This construction also requires at least libsodium 1.0.12.

Additional data

These functions accept an optional, arbitrary long "additional data" parameter. These data are not present in the ciphertext, but are mixed in the computation of the authentication tag.

A typical use for these data is to authenticate version numbers, timestamps or monotonically increasing counters in order to discard previous messages and prevent replay attacks.

References