Decrypt AES encrypted data using a key in turn encrypted using RSA 256

decrypt_payload.py

from Crypto.Cipher import AES
from Crypto.PublicKey import RSA
from Crypto.Cipher import AES, PKCS1_v1_5
from Crypto.Random import get_random_bytes
import base64
import sys
import zlib


def decrypt(session_key, privkey_fname, payload_fname, out_fname):
    enc_session_key = base64.b64decode(session_key)
    private_key = RSA.import_key(open(privkey_fname).read(), "p")

    # Decrypt the session key with the private RSA key
    sentinel = get_random_bytes(16)
    cipher_rsa = PKCS1_v1_5.new(private_key)
    session_key = cipher_rsa.decrypt(enc_session_key, sentinel)

    if session_key == sentinel:
        raise Exception("unable to decrypt session key")

    # Load the encrypted file into memory
    with open(payload_fname, 'rb') as f:
        nonce = f.read(12)
        ciphertext = f.read()

    tag = ciphertext[len(ciphertext)-16:]
    # Decrypt the data with the AES session key
    cipher_aes = AES.new(session_key, AES.MODE_GCM, nonce)
    data = zlib.decompress(cipher_aes.decrypt_and_verify(
        ciphertext[:len(ciphertext)-16], tag))

    with open(out_fname, 'wb') as f:
        f.write(data)


if __name__ == '__main__':
    session_key = sys.argv[1]
    privkey_fname = sys.argv[2]
    payload_fname = sys.argv[3]
    out_fname = payload_fname + '.dec'
    if len(sys.argv) >= 5:
        out_fname = sys.argv[4]

    decrypt(session_key, privkey_fname, payload_fname, out_fname)