Lab 2: Substitution Cipher - Memecahkan dengan Frekuensi Huruf

cd ~ && mkdir crypto-lab2 && cd crypto-lab2

python3 << 'PYEOF'
import string, random
from collections import Counter

# Buat substitusi acak
alphabet = list(string.ascii_uppercase)
shuffled = alphabet.copy()
random.seed(42)  # reproducible
random.shuffle(shuffled)
subst = dict(zip(alphabet, shuffled))
reverse = {v: k for k, v in subst.items()}

def encrypt(text):
    return ''.join(subst.get(c, c) for c in text.upper())

def decrypt(text, mapping):
    return ''.join(mapping.get(c, c) for c in text)

# Sample text
plaintext = "THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG THIS IS A SECRET MESSAGE"
ciphertext = encrypt(plaintext)

print(f"Plain:  {plaintext}")
print(f"Cipher: {ciphertext}")
print()

# Frequency analysis
freq = Counter(c for c in ciphertext if c in string.ascii_uppercase)
print("=== Frekuensi Huruf (sorted) ===")
for char, count in freq.most_common():
    print(f"  {char}: {count}")
print()

# Bahasa Inggris: E > T > A > O > I > N > S > H > R > D > L > C > U > M
# Tebak: huruf paling sering = E
english_freq = 'ETAOINSHRDLCUMWFGYPBVKJXQZ'
cipher_freq = ''.join(freq.keys())

# Buat mapping tebakan
guess = {}
for i, c in enumerate(cipher_freq):
    if i < len(english_freq):
        guess[c] = english_freq[i]

print("=== Tebakan Awal (E=T, T=A, etc) ===")
partial = decrypt(ciphertext, guess)
print(partial)
print("\n(Catatan: perlu penyesuaian manual untuk hasil sempurna)")
PYEOF