There are many use-cases where a PHP user is currently using sodium_compat's implementation of this low-level XChaCha20 API. For example, multi-part message processing (in low-memory settings) for a ciphertext that was encrypted with XChaCha20-Poly1305 (rather than the secretstream API).
Adding this function to ext/sodium offers better performance and lowers users' memory usage with the polyfill, and ensures that users coming from other languages that provide libsodium bindings have a more consistent experience with our bindings. This is a win-win.
This patch follows the libsodium precedent of adding functions instead of optional parameters to existing functions. The parameter order is also consistent with the C API.
https://doc.libsodium.org/advanced/stream_ciphers/xchacha20#usage
Closes GH-8276.
Paragon Initiative Enterprises is aware of PHP applications that use sodium_compat's ParagonIE\Sodium\Core\XChaCha20 class directly for stream encryption.
Greater performance and security assurance is offered by exposing libsodium's crypto_stream_xchacha20 API to PHP users.
It's acceptable to only include this change in PHP 8.1+; the offending applications are more than welcome to either install ext/sodium from PECL or upgrade to 8.1 when it comes out later this year.
Ref: https://github.com/jedisct1/libsodium-php/pull/211
