Lab 4: ⊗ XOR Cipher - Fondasi Kriptografi Modern

cd ~ && mkdir crypto-lab4 && cd crypto-lab4

python3 << 'PYEOF'
import os

# 1. Single-byte XOR
def xor_single(data, key):
    return bytes(b ^ key for b in data)

plaintext = b"HELLO XOR CIPHER"
key = 0x42

cipher = xor_single(plaintext, key)
decrypted = xor_single(cipher, key)

print(f"Plaintext:  {plaintext}")
print(f"Key:        0x{key:02x}")
print(f"Cipher (hex): {cipher.hex()}")
print(f"Decrypted:    {decrypted}")
print()

# 2. Multi-byte XOR (repeating key)
def xor_repeating(data, key):
    return bytes(data[i] ^ key[i % len(key)] for i in range(len(data)))

text = b"This is a secret message with XOR encryption!"
key = b"KEY"

cipher = xor_repeating(text, key)
decrypted = xor_repeating(cipher, key)

print(f"Text:    {text}")
print(f"Key:     {key}")
print(f"Cipher:  {cipher.hex()}")
print(f"Decrypt: {decrypted}")
print()

# 3. XOR untuk encrypt file
with open('/tmp/plain.txt', 'w') as f:
    f.write("Data rahasia: password=admin123")

with open('/tmp/plain.txt', 'rb') as f:
    data = f.read()

# Encrypt
key_bytes = b'MY_SECRET_KEY'
encrypted = xor_repeating(data, key_bytes)

with open('/tmp/encrypted.bin', 'wb') as f:
    f.write(encrypted)

# Decrypt
decrypted = xor_repeating(encrypted, key_bytes)
print(f"File decrypt: {decrypted.decode()}")
PYEOF