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

Limitations

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.

In spite of these limits, applications must enforce a limit on the maximum size of a ciphertext to decrypt. Very large messages should be split in multiple chunks instead of being encrypted as a single ciphertext:

• This keeps memory usage in control,

• A corrupted chunk can be immediately detected before the whole ciphertext is received,

• Large messages provide more wiggle room for attacks.

Applications are also encouraged to limit the number of attempts an adversary can make, for example by closing a session after a large number of decryption failures.

Assuming a 2^-32 attack success probability, the following tables summarize how many messages should be encrypted with a single key before switching to a new key, as well as how many brute force decryption attempts an attacker should be allowed to make to prevent forgery.

Note that the latter is not a practical concern due to application limits, noisiness, storage and bandwidth requirements. The maximum number of encryptions is the most important criteria for selecting a secure primitive.

• For 16 KB long messages:

• For 1 MB long messages:

• For 1 GB long messages:

• For 64 GB long messages:

Nonces

Hk represents a keyed hash function that is safe against length-extension attacks, such as BLAKE2 or the HMAC construction.

TL;DR: which one should I use?

XChaCha20-Poly1305-IETF is the safest choice.

Other choices are only present for interoperability with other libraries that don't implement XChaCha20-Poly1305-IETF yet.

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.

It will soon become an IETF standard.

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.

Robustness

Ciphertexts are expected to be decrypted and verified using the same key as the key initially used for encryption.

Trying to decrypt a ciphertext with a different key may still allow an authentication tag verification to pass. Doing so would produce a different message, and would not compromise the security of the original message.

But this may be an issue if an attacker has the ability to force a recipient to use a different key.

If that turns out to be a concern, this can be solved in different ways:

• In an interactive protocol, if the set of valid keys is known by the party decrypting the ciphertext:

  • By incorporating an application-defined key identifier in the nonce

  • By including an application-defined key identifier in the additional data

A key identifier can be anything allowing the application to map that identifier to an actual secret key. It doesn't have to be secret. It can be a user id. But using a hash of the secret key is not recommended.

• In non-interactive protocol, or if the set of valid keys is not known in advance (ex: password-based encryption):

  • By prepending H(key || nonce || ciphertext_tag) to the ciphertext, and verifying this prior to decryption. This can be done with crypto_generichash() or crypto_auth() and crypto_auth_verify().

References

• ​Limits on Authenticated Encryption Use in TLS (Atul Luykx, Kenneth G. Paterson).