Sekur Julius

SOLUTION

Reading source code, we can see:

• The function takes a message (msg) and a shift value (shift).
• If the character is a space, it appends ‘0’ to the ciphertext.
• If the character is not an uppercase letter (A-Z), it appends the character as is.
• If the character is an uppercase letter, it calculates the new character by shifting it within the alphabet using the Caesar cipher logic.

Now we write decrypt code, reverse the formula

import os
def julius_decrypt(ct, shift):
    msg = ''
    for c in ct:
        if c == '0':
            msg += ' '
        elif not ord('A') <= ord(c) <= ord('Z'):
            msg += c
        else:
            o = ord(c) - 65
            msg += chr (65 + (0 - shift) % 26)
    return msg

def decrypt(ct, key):
    for shift in reversed (key):
        ct = julius_decrypt(ct, shift)
    return ct

# Read the encrypted message from output.txt
with open('output.txt', 'r') as f:
    encrypted_msg = f.read()

# Generate the same secure key (this is just for demonstration; in practice, you should store the key securely)
secure_key = os.urandom(1337)

# Convert the secure key to shift values (0-25)
# Here we take the first 26 bytes and map them to shifts
key_shifts = [b % 26 for b in secure_key[:26]]

decrypted_msg = decrypt(encrypted_msg, key_shifts)

print(decrypted_msg)

We got the message!