How to verify gnupg signature with Python gnupg module?

Question

I have problem with verification signature with Python gnupg module. With this module I can encrypt and sign file:

gpg.encrypt_file(stream, encrypt_for, sign=sign_by, passphrase=key_passwd, output=file_out)

Such encrypted file can be decrypted by command line gpg, output:

gpg: encrypted with 2048-bit ELG-E key, ID 518CD1AD, created 2011-04-14
            "client"
gpg: Signature made 04/14/11 13:36:14 using DSA key ID C7C006DD
gpg: Good signature from "server"

It can be decrypted by Python gnupg module too, output file has decrypted content, but I cannot verify signature. Code that decrypts and verifies:

def decrypt_file(file_in, file_out, key_passwd):
    gpg = gnupg.GPG()
    f = open(file_in, "rb")
    data = f.read()
    f.close()
    gpg.decrypt(data, passphrase=key_passwd, output=file_out)
    verified = gpg.verify(data)
    if not verified:
        raise ValueError("Signature could not be verified!")

Exception I got:

decrypting file...
Exception in thread Thread-12:
Traceback (most recent call last):
    File "c:\Python26\lib\threading.py", line 534, in __bootstrap_inner
        self.run()
    File "c:\Python26\lib\threading.py", line 486, in run
        self.__target(*self.__args, **self.__kwargs)
    File "c:\Python26\lib\site-packages\gnupg.py", line 202, in _read_response
        result.handle_status(keyword, value)
    File "c:\Python26\lib\site-packages\gnupg.py", line 731, in handle_status
        raise ValueError("Unknown status message: %r" % key)
ValueError: Unknown status message: u'UNEXPECTED'

Traceback (most recent call last):
    File "ht_gnupg.py", line 32, in <module>
        test()
    File "ht_gnupg.py", line 27, in test
        decrypt_file('test_p.enc', 'test_p.txt', 'client')
    File "ht_gnupg.py", line 18, in decrypt_file
        raise ValueError("Signature could not be verified!")
ValueError: Signature could not be verified!

I use gnupg-0.2.7 from python-gnupg-0.2.7.win32.exe with ActiveStatus Python 2.6.

I also tried gpg.verify_file() but I got the same error. File is ASCII armoured and looks like:

-----BEGIN PGP MESSAGE-----
Version: GnuPG v1.4.9 (MingW32)

hQIOA0EAndRRjNGtEAf/YxMQaFMnBwT3Per6ypoMYaO1AKQikRgJJMJ90a/EoZ44
...
=G6Ai
-----END PGP MESSAGE-----

How to verify signature like command line gpg?

Answer 1

See this gist for an example script showing you how to verify signatures when decrypting.

Code (as of 2011-04-05) follows:

from cStringIO import StringIO
import gnupg
import logging
import os
import shutil

def generate_key(gpg, first_name, last_name, domain, passphrase=None):
    "Generate a key"
    params = {
        'Key-Type': 'DSA',
        'Key-Length': 1024,
        'Subkey-Type': 'ELG-E',
        'Subkey-Length': 2048,
        'Name-Comment': 'A test user',
        'Expire-Date': 0,
    }
    params['Name-Real'] = '%s %s' % (first_name, last_name)
    params['Name-Email'] = ("%s.%s@%s" % (first_name, last_name, domain)).lower()
    if passphrase is None:
        passphrase = ("%s%s" % (first_name[0], last_name)).lower()
    params['Passphrase'] = passphrase
    cmd = gpg.gen_key_input(**params)
    return gpg.gen_key(cmd)

def init_logging():
    logging.basicConfig(level=logging.DEBUG, filename="gpg.log",
                        filemode="w", format="%(asctime)s %(levelname)-5s %(name)-10s %(threadName)-10s %(message)s")

def print_info(decrypted):
    print('User name: %s' % decrypted.username)
    print('Key id: %s' % decrypted.key_id)
    print('Signature id: %s' % decrypted.signature_id)
    #print('Signature timestamp: %s' % decrypted.sig_timestamp)
    print('Fingerprint: %s' % decrypted.fingerprint)

def main():
    init_logging()
    if os.path.exists('keys'):
        shutil.rmtree('keys')
    gpg = gnupg.GPG(gnupghome='keys')
    key = generate_key(gpg, "Andrew", "Able", "alpha.com",
                            passphrase="andy")
    andrew = key.fingerprint
    key = generate_key(gpg, "Barbara", "Brown", "beta.com")
    barbara = key.fingerprint
    #First - without signing
    data = 'Top secret'
    encrypted = gpg.encrypt_file(StringIO(data), barbara,
                                 #sign=andrew, passphrase='andy',
                                 output='encrypted.txt')
    assert encrypted.status == 'encryption ok'
    # Data is in encrypted.txt. Read it in and verify/decrypt it.
    data = open('encrypted.txt', 'r').read()
    decrypted = gpg.decrypt(data, passphrase='bbrown', output='decrypted.txt')
    print_info(decrypted)
    #Now with signing
    data = 'Top secret'
    encrypted = gpg.encrypt_file(StringIO(data), barbara,
                                 sign=andrew, passphrase='andy',
                                 output='encrypted.txt')
    assert encrypted.status == 'encryption ok'
    # Data is in encrypted.txt. Read it in and verify/decrypt it.
    data = open('encrypted.txt', 'r').read()
    decrypted = gpg.decrypt(data, passphrase='bbrown', output='decrypted.txt')
    print_info(decrypted)

if __name__ == '__main__':
    main()