Encrypt/decrypt data using the Salsa20 function.

This function is symmetric, so it can be used for both encryption and decryption.

Encryption key.

Build a key out of bytes. The key must be either 16 or 32 bytes long.

Nonce. A random unique code that should be unique for each message.

Build a nonce out of bytes. The nonce must be either 8 bytes long.