C++ XOR encryption - decryption issue - c++

Question

I followed a tutorial on stephan-brumme website for XOR encryption (unfortunately I cannot include URL because I do not have enough reputation). What I want to do is following: read the content of example.txt file and decrypt the text that it includes. For example, this is the content of example.txt:
\xe7\xfb\xe0\xe0\xe7
This, when decrypted using password "password" should return "hello". This is the code I got:
#include <string>
#include <iostream>
#include <fstream>
using namespace std;
std::string decode(const std::string& input)
{
const size_t passwordLength = 9;
static const char password[passwordLength] = "password";
std::string result = input;
for (size_t i = 0; i < input.length(); i++)
result[i] ^= ~password[i % passwordLength];
return result;
}
int main(int argc, char* argv[])
{
string line;
ifstream myfile ("example.txt");
if (myfile.is_open())
{
while ( getline (myfile,line) )
{
cout << decode(line);
}
myfile.close();
}
return 0;
}
And this is the result of running the application:
click for image
As you can see, the decryption was not successful. Now, if I make it so it doesn't read the .txt, but directly decrypts the text, like this:
cout << decode("\xe7\xfb\xe0\xe0\xe7");
It works perfectly:
click for image
What am I doing wrong here?
Many thanks in advance! :)

Answer 1

Character XOR by same character is zero, so the result may include zero. std::string doesn't like that because zero terminates the string.
You also can use std::vector<char> instead of std::string for the actual encoding/decoding. You would have to change the decode function to handle vector<char>
And read/write the file in binary.
Edit: Using std::string only, and std::string decode(const std::string& input)
int main()
{
std::string line = "hello";
{
line = decode(line);
std::ofstream myfile("example.txt", std::ios::binary);
myfile.write(line.data(), line.size());
//Edit 2 *************
//std::cout << std::hex;
//for (char c : line)
// std::cout << "\\x" << (0xff & c);
//*************
//This will make sure width is always 2
//For example, it will print "\x01\x02" instead of "\x1\x2"
std::cout << std::hex << std::setfill('0');
for (char c : line)
std::cout << "\\x" << std::setw(2) << (0xff & c);
std::cout << std::endl;
}
{
std::ifstream myfile("example.txt", std::ios::binary | std::ios::ate);
int filesize = (int)myfile.tellg();
line.resize(filesize);
myfile.seekg(0);
myfile.read(&line[0], filesize);
line = decode(line);
std::cout << line << std::endl;
}
return 0;
}

Answer 2

I bet example.txt contains the characters '\', 'x', 'e', '7' etc. You have to read those, process all the backslash escapes, and then feed it to decode.
\xe7 is a common way of representing a single character with hex value E7. (Which is quite likely to be the single character 'ç' depending on your character set). If you want to store (encrypted) readable text, I suggest dropping the \x, and having the file contain lines like "e7fbe0e0e7". Then
- read each line into a string.
- Convert each pair of characters from a hex number into an integer, and store the result in a char.
- Store that char in the string.
- Then xor decrypt the string.
Alternatively, ensure the file contains the actual binary characters you need it to.
Also beware that you are XOR-ing with the terminating nul byte of the password. Did you mean to do that?