XOR Cipher

The XOR Cipher is a symmetric encryption technique that operates at the bit level, combining each bit of the plaintext with a corresponding bit from a secret key using the exclusive OR (XOR) operation. It is simple but effective for situations where the key is as long as the message, forming the basis of the one-time pad. Its strength comes from the reversibility of the XOR operation: applying the same key twice restores the original plaintext.

Each character in the plaintext is converted into its binary representation. Then, each bit is XORed with the corresponding key bit to produce the ciphertext. Decryption applies the same XOR operation with the same key to retrieve the original plaintext.

XOR Cipher: Encoding

To encode a message, each character of the plaintext is XORed with the corresponding character of the key. For example, using the key XMCKL to encode “HELLO” (assuming ASCII encoding):

Plaintext (ASCII binary):
H → 01001000
E → 01000101
L → 01001100
L → 01001100
O → 01001111

Key (ASCII binary):
X → 01011000
M → 01001101
C → 01000011
K → 01001011
L → 01001100

Encoding (XOR each bit):
01001000 XOR 01011000 = 00010000 → P (ASCII 16)
01000101 XOR 01001101 = 00001000 → H (ASCII 8)
01001100 XOR 01000011 = 00001111 → O (ASCII 15)
01001100 XOR 01001011 = 00000111 → G (ASCII 7)
01001111 XOR 01001100 = 00000011 → C (ASCII 3)

Ciphertext: [binary values or mapped ASCII characters]

XOR Cipher: Decoding

To decode, apply XOR again with the same key on the ciphertext:

Ciphertext (binary):
00010000 → P (ASCII 16)
00001000 → H (ASCII 8)
00001111 → O (ASCII 15)
00000111 → G (ASCII 7)
00000011 → C (ASCII 3)

Key (same as above):
X → 01011000
M → 01001101
C → 01000011
K → 01001011
L → 01001100

Decoding (XOR with key):
00010000 XOR 01011000 = 01001000 → H
00001000 XOR 01001101 = 01000101 → E
00001111 XOR 01000011 = 01001100 → L
00000111 XOR 01001011 = 01001100 → L
00000011 XOR 01001100 = 01001111 → O

Plaintext: HELLO

XOR Cipher: Notes

- XOR is reversible: ciphertext XOR key = plaintext. - If the key is random, as long as the message, and never reused, the XOR Cipher becomes a perfect one-time pad and is unbreakable. - If the key is reused or predictable, patterns can be exploited, making the cipher vulnerable. - It is widely used in modern cryptography for stream ciphers, simple obfuscation, and error-checking operations.