Generate XChaCha20 stream for the given 32-byte key and 12-byte nonce (last 8 bytes of 24 byte nonce prefixed with 4 zero bytes) and write it into dst and return it.
Nonces MUST be generated using an CSPRNG to generate a sufficiently random nonce such that a collision is highly unlikely to occur.
stream is like streamXOR with all-zero src.
Encrypt src with XChaCha20 stream generated for the given 32-byte key and 8-byte (as in original implementation) or 12-byte (as in RFC7539) nonce and write the result into dst and return it.
dst and src may be the same, but otherwise must not overlap.
Nonce length is set in such a way that given it's generated via a CSPRNG then there's little concern of collision for roughly 2^96 messages while reusing a secret key and not encountering nonce reuse vulnerabilities.
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HChaCha is a one-way function used in XChaCha to extend nonce.
It takes 32-byte key and 16-byte src and writes 32-byte result into dst and returns it.