Python: Closing a for loop by reading stdout

import subprocess


def check_file(dictfile, pgpfile):
    # Command to run, constructed as a list to prevent shell-escaping accidents
    cmd = ["gpg", "--passphrase-fd", "0", pgpfile]

    # Launch process, with stdin/stdout wired up to `p.stdout` and `p.stdin`
    p = subprocess.Popen(cmd, stdin = subprocess.PIPE, stdout = subprocess.PIPE)

    # Read dictfile, and send contents to stdin
    passphrase = open(dictfile).read()
    p.stdin.write(passphrase)

    # Read stdout and check for message
    stdout, stderr = p.communicate()
    for line in stdout.splitlines():
        if line.strip() == "gpg: WARNING: message was not integrity protected":
            # Relevant line was found
            return True

    # Line not found
    return False

Then to use:

not_integrity_protected = check_file("/root/john.txt", "helloworld.txt.gpg")
if not_integrity_protected:
    print "Success!"

If the "gpg: WARNING:" message is actually on stderr (which I would suspect it is), change the subprocess.Popen line to this:

p = subprocess.Popen(cmd, stdin = subprocess.PIPE, stderr = subprocess.PIPE)

..and the for loop from stdout to stderr, like this:

for line in stderr.splitlines():

Use subprocess.check_output to call gpg and break the loop based on its output.

Something like this (untested since I don't know anything about gpg):

import subprocess

dictionaryfile = "/root/john.txt"
pgpencryptedfile = "helloworld.txt.gpg"

with open(dictionaryfile, 'r') as f:
    for line in f:
        x = line.rstrip('\n')
        cmd = ["echo " + x + " | gpg --passphrase-fd 0 " + pgpencryptedfile]
        output = subprocess.check_output(cmd, shell=True)
        if 'gpg: WARNING: message was not integrity protected' in output:
            break

You could use the subprocess module which allows you to use:

subprocess.call(args, *, stdin, stdout, stderr, shell)

(See the Python Documentation for how to use the parameters.)

This is good because you can easily read in the exit code of whatever program you call.

For example if you change 'newstring' to:

"echo " + x + " | gpg --passphrase-fd 0 " + pgpencryptedfile | grep 'gpg: WARNING: message was not integrity protected'

grep will then return 0 if there is a match and a 1 if not matches are found. (Source)

This exit code from grep will be returned from the subprocess.call() function and you can easily store it in a variable and use an if statement.

Edit: As Matthew Adams mentions below, you could also read the exit code of gpg itself.