Function transcrypt 

pub fn transcrypt(
    key: &EncryptionKey,
    init_ctr: u32,
    input: &[u8],
    output: &mut [u8],
) -> Result<(), EncryptionError>

XOR input with the Keccak-256 CTR keystream, writing to output.

The keystream is generated by derive_segment_key for each 32-byte block, starting at init_ctr and incrementing by 1 per block.

output must be at least as long as input. Bytes in output beyond input.len() are left untouched.