"if(num_cypher_bytes<100000)// or num_bytes" If num_bytes << num_cypher_bytes (and even more if we know the key length) then the encryption leaks information.

"if(num_cypher_bytes<100000)// or num_bytes" If num_bytes << num_cypher_bytes (and even more if we know the key length) then the encryption leaks information. https://en.wikipedia.org/wiki/XOR_cipher https://en.wikipedia.org/wiki/XOR_cipher Otherwise this is a stream cipher and it is really secure but it's inconvenient :) But it is a good starting point for anybody who's interested in crypto.