Exclusive-OR encryption, while not a public-key system such as RSA, is almost unbreakable through brute force methods. It is susceptible to patterns, but this weakness can be avoided through first compressing the file (so as to remove patterns). Exclusive-or encryption requires that both encryptor and decryptor have access to the encryption key, but the encryption algorithm, while extremely simple, is nearly unbreakable.
Exclusive-OR encrytion works by using the boolean algebra function exclusive-OR (XOR). XOR is a binary operator (meaning that it takes two arguments – similar to the addition sign, for example). By its name, exclusive-OR, it is easy to infer (correctly, no less) that it will return true if one, and only one, of the two operators is true. The truth table is as follows:
A B A XOR B
T T F
T F T
F T T
F F F
The idea behind exclusive-OR encryption is that it is impossible to reverse the operation without knowing the initial value of one of the two arguments. Exclusive-OR encryption works on the principle that if you have the encrypted string and the encryption key you can always decrypt correctly. If you don’t have the key, it is impossible to decrypt it without making entirely random keys and attempting each one of them until the decryption program’s output is something akin to readable text. The longer you make the encryption key, the more difficult it becomes to break it.
The actual way exclusive-OR encryption is used is to take the key and encrypt a file by repeatedly applying the key to successive segments of the file and storing the output. The output will be the equivalent of an entirely random program, as the key is generated randomly. Once a second person has access to the key, that person is able to decrypt the files, but without it, decryption is almost impossible. For every bit added to the length of the key, you double the number of tries it will take to break the encryption through brute force.
C++ uses ^ for bit-level exclusive-OR. To encrypt a single character you can use char x=x^key; if you have a key of one byte. To encrypt a string of characters with a longer key, you can use something akin to the following code:
char string=”Welcome Home”;
for(int x=0; x<12; x++)
The program encrypts each character in the string using the ^ bit operator to exclusive-OR the string value with the key value for each character.