decrypt.py

import base64

from cryptography.hazmat.primitives.asymmetric import padding
from cryptography.hazmat.primitives import hashes
from cryptography.hazmat.primitives import serialization
from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes

IVLength = 16
AESKeyLength = 32


def decrypt_rsa(private_key, message):
    index = message.index(b":")
    enc_aes_key = message[:index]
    encrypted_message = message[index + 1:]

    aes_key = private_key.decrypt(
        base64.b64decode(enc_aes_key),
        padding=padding.OAEP(
            mgf=padding.MGF1(algorithm=hashes.SHA1()),
            algorithm=hashes.SHA1(),
            label=None
        ),
    )

    return decrypt_aes(aes_key, encrypted_message)


def decrypt_aes(aes_key, encrypted_message):
    iv, ciphertext = encrypted_message.split(b":")

    cipher = Cipher(
        algorithms.AES(bytes.fromhex(aes_key.decode())),
        modes.CBC(bytes.fromhex(iv.decode()))
    )
    decryptor = cipher.decryptor()
    decrypted_data = decryptor.update(bytes.fromhex(ciphertext.decode())) + decryptor.finalize()

    return decrypted_data


def main():
    encrypted_message = b"message here"

    with open("key.pem", "rb") as private_key:
        loaded_key = serialization.load_pem_private_key(private_key.read(), password=None)
        decrypted_data = decrypt_rsa(loaded_key, encrypted_message)
        print(decrypted_data.decode())


if __name__ == '__main__':
    main()