The Chacha20 stream cipher. The string argument is the key, and must be of length 16 or 32. The optional iv argument is the initialization vector (also known as the nonce). If present, it must be 8 bytes long. If absent, it is taken to be eight zero bytes. The optional ctr argument is the initial value of the internal counter. If absent, it is taken to be 0. This stream cipher works by xor-ing the input with the output of a key-dependent pseudo random number generator. Thus, decryption is the same function as encryption.
transform src spos dst dpos len encrypts or decrypts len characters, read from byte array src starting at position spos. The resulting len characters are stored in byte array dst starting at position dpos.