Expert 2: Differential Cryptanalysis - Menganalisis Perbedaan untuk Menemukan Kunci

cd ~ && mkdir crypto-exp2 && cd crypto-exp2

python3 << 'PYEOF'
# === SIMPLIFIED BLOCK CIPHER ===
# Untuk demonstrasi, kita buat cipher sederhana dengan S-box

SBOX = [0xE, 0x4, 0xD, 0x1, 0x2, 0xF, 0xB, 0x8,
        0x3, 0xA, 0x6, 0xC, 0x5, 0x9, 0x0, 0x7]

def encrypt_block(plain, key):
    """Simple 4-bit block cipher with S-box substitution + XOR key"""
    return SBOX[plain ^ key]

# === DIFFERENTIAL ANALYSIS ===
print("=== Differential Analysis ==*")
print()

# Pilih input difference (ΔP)
delta_p = 1  # bedakan bit terakhir

# Cari output difference untuk setiap key
print(f"Input difference ΔP = {delta_p}")
print()
print(f"{'Key':<8} {'P1':<8} {'P2':<8} {'C1':<8} {'C2':<8} {'ΔC':<8} {'Consistent?'}")
print("-"*60)

for key in range(16):
    p1 = 0x00
    p2 = p1 ^ delta_p
    c1 = encrypt_block(p1, key)
    c2 = encrypt_block(p2, key)
    delta_c = c1 ^ c2
    print(f"{key:<8} {p1:02x}<8 {p2:02x}<8 {c1:02x}<8 {c2:02x}<8 {delta_c:02x}<8")

print()
print("📌 Analisis:")
print("  - Untuk ΔP=1, hanya key tertentu yang menghasilkan ΔC tertentu")
print("  - Dengan beberapa pasangan (P1,C1), (P2,C2), kita bisa")
print("    mempersempit kemungkinan key")
print("  - Untuk S-box 4-bit: 8 pasangan cukup untuk menentukan key!")
print()

# === ATTACK: recover key with differential pairs ===
print("=== Key Recovery ==*")
known_pairs = [(0x00, encrypt_block(0x00, 0x07)), 
               (0x01, encrypt_block(0x01, 0x07))]

candidate_keys = []
for key in range(16):
    match = True
    for p, c in known_pairs:
        if encrypt_block(p, key) != c:
            match = False
            break
    if match:
        candidate_keys.append(key)

print(f"From 2 plaintext-ciphertext pairs:")
print(f"  Candidate keys: {candidate_keys}")
print(f"  (Differential analysis bisa persempit dari 16 ke ~2-4)")
PYEOF