Python pexpect with gpg

Question

I'm trying to get Python pexpect to work with GNU gpg. We do not have the gnupg module loaded and trying to do so would be nearly impossible.

I can get pexepct to work doing FTP and SSH, but with gpg I'm having a slight problem. Below is the command I run to decrypt the file from the command line. I enter the passphrase at the prompt and it works perfectly.

# gpg --decrypt encKey_20141009_b7489540_36.00.tar.gpg > encKey_20141009_b7489540_36.00.tar

You need a passphrase to unlock the secret key for user: "XXXXXXXXXXXXXXXXXXXXXXXXXXX" 2048-bit RSA key, ID AD22F55F, created 2013-07-18 (main key ID 0D56620B)

Enter passphrase:

Yet when I try to use pexpect I have a problem. Below is the code that I'm trying to get to work:

#!/usr/bin/python

import pexpect
Password = raw_input('Enter passphrase:  ')

child = pexpect.spawn('gpg --decrypt encKey_20141009.tar.gpg > encKey_20141009_.tar')
child.expect('Enter passphrase: ')
child.sendline(Password)

Below is the error that I receive:

Traceback (most recent call last):
  File "./pexepect.py", line 9, in ?
child.expect('Enter passphrase: ')
 File "/usr/lib/python2.4/site-packages/pexpect.py", line 1311, in expect
return self.expect_list(compiled_pattern_list, timeout, searchwindowsize)
File "/usr/lib/python2.4/site-packages/pexpect.py", line 1325, in expect_list
  return self.expect_loop(searcher_re(pattern_list), timeout, searchwindowsize)
File "/usr/lib/python2.4/site-packages/pexpect.py", line 1396, in expect_loop
raise EOF (str(e) + '\n' + str(self))
pexpect.EOF: End Of File (EOF) in read_nonblocking(). Exception style platform.
<pexpect.spawn object at 0x2b2dfe13f350>
version: 2.3 ($Revision: 399 $)
command: /usr/bin/gpg
args: ['/usr/bin/gpg', '--decrypt', 'encKey_20141009.tar.gpg', '>', 'encKey_20141009.tar']
searcher: searcher_re:
    0: re.compile("Enter passphrase: ")
buffer (last 100 chars):
before (last 100 chars): usage: gpg [options] --decrypt [filename]

after: pexpect.EOF
match: None
match_index: None
exitstatus: 2
flag_eof: True
pid: 14399
child_fd: 3
closed: False
timeout: 30
delimiter: pexpect.EOF
logfile: None
logfile_read: None
logfile_send: None
maxread: 2000
ignorecase: False
searchwindowsize: None
delaybeforesend: 0.05
delayafterclose: 0.1
delayafterterminate: 0.1

Answer 1

> is a redirection in shell. pexpect doesn't run shell, it runs gpg executable. Specify the output file using gpg options instead ( --output ).

Here's how to run gpg using subprocess module:

#!/usr/bin/env python3
import os
from subprocess import Popen

passphrase = input('Enter passphrase: ')
file_to_decrypt = 'encKey_20141009.tar.gpg'
in_fd, out_fd = os.pipe()
cmd = ['gpg', '--passphrase-fd', str(in_fd)]
cmd += ['--output', os.path.splitext(file_to_decrypt)[0]]
cmd += ['--decrypt', file_to_decrypt]
with Popen(cmd, pass_fds=[in_fd]):
    os.close(in_fd) # unused in the parent
    with open(out_fd, 'w') as out_file:
        out_file.write(passphrase)