C++ XOR failing when reaching null byte? - c++

Question

Here is the code I have right now:
#include <iostream>
#include <string>
#include <sstream>
std::string string_to_hex(const std::string& input)
{
static const char* const lut = "0123456789ABCDEF";
size_t len = input.length();
std::string output;
output.reserve(2 * len);
for (size_t i = 0; i < len; ++i)
{
const unsigned char c = input[i];
output.push_back(lut[c >> 4]);
output.push_back(lut[c & 15]);
}
return output;
}
std::string encrypt(std::string msg, std::string key)
{
// Make sure the key is at least as long as the message
std::string tmp(key);
while (key.size() < msg.size())
key += tmp;
// And now for the encryption part
for (std::string::size_type i = 0; i < msg.size(); ++i)
msg[i] ^= key[i];
return msg;
}
std::string decrypt(std::string msg, std::string key)
{
return encrypt(msg, key); // lol
}
int main()
{
std::cout << string_to_hex(encrypt("Hello World!", "monkey")) << std::endl;
std::cout << decrypt("\x25\x0A\x02\x07\x0A\x59\x3A\x00\x1C\x07\x01\x58", "monkey") << std::endl;
std::cout << string_to_hex(encrypt("Hello. This is a test of encrypting strings in C++.", "monkey")) << std::endl;
std::cout << decrypt("\x25\x0A\x02\x07\x0A\x57\x4D\x3B\x06\x02\x16\x59\x04\x1C\x4E\x0A\x45\x0D\x08\x1C\x1A\x4B\x0A\x1F\x4D\x0A\x00\x08\x17\x00\x1D\x1B\x07\x05\x02\x59\x1E\x1B\x1C\x02\x0B\x1E\x1E\x4F\x07\x05\x45\x3A\x46\x44\x40", "monkey") << std::endl;
}
The output is the following:
250A02070A593A001C070158
Hello W
250A02070A574D3B06021659041C4E0A450D081C1A4B0A1F4D0A000817001D1B070502591E1B1C020B1E1E4F0705453A464440
Hello. This is a test of e
The decryption seems to stop when reaching a \x00. Does anyone have any ideas on how to fix or get around that?
Thanks!

Answer 1

The std::string constructor that takes in a char* assumes that the input is a null-terminated string, so even though your string literal has lots of data in it past the null byte, when you pass it into your function the std::string constructor will stop reading as soon as it hits that null byte.
You have a couple of options to fix this. As one option, the std::string type has a two-argument constructor where you can give a pointer to the first element in the string and the past-the-end byte of the string. The std::string will then initialize itself to the text in that range, ignoring intermediary null terminators.
char s1[] = "\x25\x0A\x02\x07\x0A\x59\x3A\x00\x1C\x07\x01\x58";
char s2[] = "\x25\x0A\x02\x07\x0A\x57\x4D\x3B\x06\x02\x16\x59\x04\x1C\x4E\x0A\x45\x0D\x08\x1C\x1A\x4B\x0A\x1F\x4D\x0A\x00\x08\x17\x00\x1D\x1B\x07\x05\x02\x59\x1E\x1B\x1C\x02\x0B\x1E\x1E\x4F\x07\x05\x45\x3A\x46\x44\x40";
std::cout << string_to_hex(encrypt("Hello World!", "monkey")) << std::endl;
std::cout << decrypt(std::string(std::begin(s1), std::end(s1)-1), "monkey") << std::endl;
std::cout << string_to_hex(encrypt("Hello. This is a test of encrypting strings in C++.", "monkey")) << std::endl;
std::cout << decrypt(std::string(std::begin(s2), std::end(s2)-1), "monkey") << std::endl;
Demo.