The ChaCha20 stream cipher. Generates a keystream and then XOR's the supplied input with it; use it both to encrypt plaintext and decrypt ciphertext.

Per RFC8439, the key must be exactly 256 bits, and the nonce exactly 96 bits.

>>> let key = "don't tell anyone my secret key!"
>>> let non = "or my nonce!"
>>> let cip = cipher key 1 non "but you can share the plaintext"
>>> cip
"\192*c\248A\204\211n\130y8\197\146k\245\178Y\197=\180_\223\138\146:^\206\&0\v[\201"
>>> cipher key 1 non cip
"but you can share the plaintext"

The ChaCha20 block function. Useful for generating a keystream.

Per RFC8439, the key must be exactly 256 bits, and the nonce exactly 96 bits.