How to decrypt OpenSSL AES-encrypted files in Python?

by

Given the popularity of Python, at first I was disappointed that there was no complete answer to this question to be found. It took me a fair amount of reading different answers on this board, as well as other resources, to get it right. I thought I might share the result for future reference and perhaps review; I’m by no means a cryptography expert! However, the code below appears to work seamlessly:

from hashlib import md5
from Crypto.Cipher import AES
from Crypto import Random

def derive_key_and_iv(password, salt, key_length, iv_length):
    d = d_i = ''
    while len(d) < key_length + iv_length:
        d_i = md5(d_i + password + salt).digest()
        d += d_i
    return d[:key_length], d[key_length:key_length+iv_length]

def decrypt(in_file, out_file, password, key_length=32):
    bs = AES.block_size
    salt = in_file.read(bs)[len('Salted__'):]
    key, iv = derive_key_and_iv(password, salt, key_length, bs)
    cipher = AES.new(key, AES.MODE_CBC, iv)
    next_chunk = ''
    finished = False
    while not finished:
        chunk, next_chunk = next_chunk, cipher.decrypt(in_file.read(1024 * bs))
        if len(next_chunk) == 0:
            padding_length = ord(chunk[-1])
            chunk = chunk[:-padding_length]
            finished = True
        out_file.write(chunk)

Usage:

with open(in_filename, 'rb') as in_file, open(out_filename, 'wb') as out_file:
    decrypt(in_file, out_file, password)

If you see a chance to improve on this or extend it to be more flexible (e.g. make it work without salt, or provide Python 3 compatibility), please feel free to do so.

Notice

This answer used to also concern encryption in Python using the same scheme. I have since removed that part to discourage anyone from using it. Do NOT encrypt any more data in this way, because it is NOT secure by today’s standards. You should ONLY use decryption, for no other reasons than BACKWARD COMPATIBILITY, i.e. when you have no other choice. Want to encrypt? Use NaCl/libsodium if you possibly can.

More Related Contents:

  1. Encrypt & Decrypt using PyCrypto AES 256
  2. How do I use a X509 certificate with PyCrypto?
  3. RSA encryption and decryption in Python
  4. Encrypt and decrypt using PyCrypto AES-256
  5. What (pure) Python library to use for AES 256 encryption? [closed]
  6. PyCrypto problem using AES+CTR
  7. Encrypting and decrypting a password raises ValueError
  8. python building coder for ceasar cipher
  9. Pip error: Microsoft Visual C++ 14.0 is required
  10. Simple way to encode a string according to a password?
  11. Python referencing old SSL version
  12. Building Python with SSL support in non-standard location
  13. Python Requests requests.exceptions.SSLError: [Errno 8] _ssl.c:504: EOF occurred in violation of protocol
  14. How do I install PyCrypto on Windows?
  15. ImportError: No module named Crypto.Cipher
  16. Python and OpenSSL version reference issue on OS X
  17. How to decode a string encoded with openssl aes-128-cbc using java?
  18. Remove ‘b’ character do in front of a string literal in Python 3 [duplicate]
  19. OpenSSL errors in python requests
  20. Python Encrypting with PyCrypto AES
  21. How to decrypt password from JavaScript CryptoJS.AES.encrypt(password, passphrase) in Python
  22. “ERROR:root:code for hash md5 was not found” when using any hg mercurial commands
  23. Building Python 3.7.1 – SSL module failed
  24. openssl, python requests error: “certificate verify failed”
  25. Getting certificate chain with Python 3.3 SSL module
  26. SSL error unsafe legacy renegotiation disabled
  27. Chrome 80 how to decode cookies
  28. Java AES 128 encrypting differently to openssl
  29. How to do PGP in Python (generate keys, encrypt/decrypt)
  30. How to compile python3 on RHEL with SSL? SSL cannot be imported

Leave a Comment