selamat_pagi

selamat pagi

If you talk in another language, nobody can understand what you say! Check out this message I sent in Indonesian. To add some extra security, I also applied a monoalphabetic substitution cipher on it!

Problem

• Reverse the monoalphabetic substitution
• The original message is in Indonesian

Solve

I assume the original text consist of the following words:

• lactf (flag format, 100% sure)
• selamat pagi (the challenge title)
• bendera (trasnlation of "flag" in Indonesian)

By comparing with the cipher text, we can find the following substitution:

• lactf -> bkvim
• selamat pagi -> wzbkdki ckse
• bendera -> Lzfqztk

So I write the following program

import string

knowns = {
    # cipher: original
    "bkvim": "lactf",
    "Lzfqztk": "bendera",
    "wzbkdki": "selamat",
    "ckse": "pagi",
}

def monoalphabetic_substitution(ciphertext, substitution_key):
    plaintext = []
    for char in ciphertext:
        if not char.isalpha():
            plaintext.append(char)
            continue

        if char not in substitution_key:
            plaintext.append('*')
            continue

        plaintext.append(substitution_key[char])
    return ''.join(plaintext)

substitution_key = {}
for key,value in knowns.items():
    key = key.lower()
    value = value.lower()
    assert len(key) == len(value)
    for i in range(len(key)):
        a,b = key[i], value[i]
        # check if it already exist
        if a in substitution_key:
            assert substitution_key[a] == b
            continue
        substitution_key.update({a:b, a.upper():b.upper()})

missing_key = set([ c for c in string.ascii_lowercase if c not in substitution_key.keys()])
missing_value = set([ c for c in string.ascii_lowercase if c not in substitution_key.values()])
assert len(missing_key) == len(missing_value)

with open('message.txt', 'r') as file:
    ciphertext = file.read()
decrypted_text = monoalphabetic_substitution(ciphertext, substitution_key)

print("---")
print(ciphertext)
print("---")
print(decrypted_text)
print("---")

Although there are some missing character, we can find them with the help of google translate.

The problem can be solve by putting those character back to the known list, and repeat a few times.

Final known list:

knowns = {
    # cipher: original
    "bkvim": "lactf",
    "Lzfqztk": "bendera",
    "wzbkdki": "selamat",
    "ckse": "pagi",
    "x": "h",
    "a": "y",
    "u": "k",
    "j": "u",
}