The ext-sodium API exposes the following functions:
* sodium_crypto_stream(int $len, string $nonce, string $key): string
* sodium_crypto_stream_keygen(): string
* sodium_crypto_stream_xor(string $message, string $nonce, string $key):
string
(And the new XChaCha variant landing in PHP 8.1, of course.)
This is good for, like, 99% of the use cases for the crypto_stream API.
However, there is a missed opportunity: Being able to seek arbitrary
starting points in the XSalsa20 or XChaCha20 keystream when
encrypting/decrypting data.
The underlying libsodium API has a distinct feature for this: Functions
with the _ic suffix
<https://github.com/jedisct1/libsodium/blob/6d566070b48efd2fa099bbe9822914455150aba9/src/libsodium/crypto_stream/salsa20/ref/salsa20_ref.c#L64-L66>
.
Let's add an optional integer argument to the following PHP functions:
- sodium_crypto_stream_xor
- sodium_crypto_stream_xchacha20_xor
The only question I have is: Should this land in 8.1 (it's small and
self-contained) or 8.2?
