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 store protocol-specific metadata about the message, such as its length and encoding.
The chosen construction uses encrypt-then-MAC and decryption will never be performed, even partially, before verification.
Variants
libsodium implements three versions of the ChaCha20-Poly1305 construction:
The original construction can safely encrypt up to 2^64 messages with the same key (even more with most protocols), without any practical limit to the size of a message (up to 2^64 bytes for a 128-bit tag).
The IETF variant. It can safely encrypt a practically unlimited number of messages, but individual messages cannot exceed 64*(2^32)-64 bytes (approximatively 256 GB).
The XChaCha20 variant, introduced in libsodium 1.0.12. It can safely encrypt a practically unlimited number of messages of any sizes, and random nonces are safe to use.
The first two variants are fully interoperable with other crypto libaries. The XChaCha20 variant is currently only implemented in libsodium, but is the recommended option if interoperability is not a concern.
They all share the same security properties when used properly, and are accessible via a similar API.
The crypto_aead_chacha20poly1305_*() set of functions implements the original construction, the crypto_aead_chacha20poly1305_ietf_*() functions implement the IETF version, and the crypto_aead_xchacha20poly1305_ietf_*() functions implement the XChaCha20 variant.
The constants are the same, except for the nonce size.