XOR Cipher

XOR Cipher

XOR (EOR in ARM64) has a special property that makes it perfect for simple encryption: applying XOR twice with the same key recovers the original value.

A ^ K = E     (encrypt)
E ^ K = A     (decrypt)

This works because XOR is its own inverse: (A ^ K) ^ K = A.

How It Works

Given a plaintext message and a key byte, XOR each byte of the message with the key:

LDRB W1, [X0], #1    // Load plaintext byte
EOR W1, W1, W2       // XOR with key
STRB W1, [X3], #1    // Store encrypted byte

Example

Encrypting 'H' (0x48) with key 0x2A:

0x48 = 01001000
0x2A = 00101010
XOR  = 01100010 = 0x62

Decrypting 0x62 with the same key:

0x62 = 01100010
0x2A = 00101010
XOR  = 01001000 = 0x48 = 'H'

Decrypting a Message

To decrypt, iterate over the encrypted bytes, XOR each one with the key, and store the result:

LDR X0, =encrypted
LDR X3, =output
MOV W2, #0x2A        // key

decrypt_loop:
    LDRB W1, [X0], #1
    CBZ W1, done
    EOR W1, W1, W2
    STRB W1, [X3], #1
    B decrypt_loop

Your Task

The .data section contains an encrypted message (5 bytes + null). Each byte has been XOR'd with the key 0x2A (42). Decrypt it to reveal the hidden message and print it followed by a newline.

Hint: The decrypted message is ARM64.