A simple adaptive cipher based on the logical XOR operation.

How it works

Encoding

The XOR cipher will encrypt a message by using the Boolean XOR (exclusive or) operation. A XOR B returns 1 if and only if A and B are different.

To encrypt a message with a given key, we first convert the string and the key into their ASCII equivalents, and we then XOR every character with the key. For example, if we want to encrypt XOR with 134 as a key, we would do:

    X        O        R
01011000 01001111 01010010    # String in ASCII
10000110 10000110 10000110    # Repeating key 134
--------------------------
11011110 11001001 11010100    # XOR result
    Þ        É        Ô       # Result converted back into plain text

When implemented in python, we get char ^ key.

As you might have noticed, in this cipher, a given character is always replaced by the same character. This makes frequency analysis easier. To avoid that, we can use a non-repeating key eg. 29, 62, 134, where we loop through the keys to encode each character.

 X  O  R
88  79  82
29  62 134
----------
69 113 212
 E  q   Ô

Decoding

The XOR operation has a couple of properties, including:

A XOR A = 0
A XOR 0 = A
(A XOR B) XOR C = A XOR (B XOR C)

So, from that, we can conclude that:

B XOR A XOR A = B XOR 0 = B

Therefore, to decrypt a message, we need to re-XOR it with the same key, aka. re-encode it.

    Þ        É        Ô       # Encoded string
11011110 11001001 11010100    # String in  ASCII
10000110 10000110 10000110    # Repeating key 134
--------------------------
01011000 01001111 01010010    # XOR result
    X        O        R       # Decoded string

Cons

• Frequency analysis can be used to crack the code if the key is a single byte.

Implementations

Source: CrypTools Cipher Repository