GNU Privacy Guard (GnuPG) Made Easy Python Bindings HOWTO (English)

Table of Contents

1 Introduction

Version: 0.1.3
Author: Ben McGinnes <ben@gnupg.org>
Author GPG Key: DB4724E6FA4286C92B4E55C4321E4E2373590E5D
Language: Australian English, British English
xml:lang: en-AU, en-GB, en

This document provides basic instruction in how to use the GPGME Python bindings to programmatically leverage the GPGME library.

1.1 Python 2 versus Python 3

Though the GPGME Python bindings themselves provide support for both Python 2 and 3, the focus is unequivocally on Python 3 and specifically from Python 3.4 and above. As a consequence all the examples and instructions in this guide use Python 3 code.

Much of it will work with Python 2, but much of it also deals with Python 3 byte literals, particularly when reading and writing data. Developers concentrating on Python 2.7, and possibly even 2.6, will need to make the appropriate modifications to support the older string and unicode types as opposed to bytes.

There are multiple reasons for concentrating on Python 3; some of which relate to the immediate integration of these bindings, some of which relate to longer term plans for both GPGME and the python bindings and some of which relate to the impending EOL period for Python 2.7. Essentially, though, there is little value in tying the bindings to a version of the language which is a dead end and the advantages offered by Python 3 over Python 2 make handling the data types with which GPGME deals considerably easier.

1.2 Examples

All of the examples found in this document can be found as Python 3 scripts in the lang/python/examples/howto directory.

2 GPGME Concepts

2.1 A C API

Unlike many modern APIs with which programmers will be more familiar with these days, the GPGME API is a C API. The API is intended for use by C coders who would be able to access its features by including the gpgme.h header file with their own C source code and then access its functions just as they would any other C headers.

This is a very effective method of gaining complete access to the API and in the most efficient manner possible. It does, however, have the drawback that it cannot be directly used by other languages without some means of providing an interface to those languages. This is where the need for bindings in various languages stems.

2.2 Python bindings

The Python bindings for GPGME provide a higher level means of accessing the complete feature set of GPGME itself. It also provides a more pythonic means of calling these API functions.

The bindings are generated dynamically with SWIG and the copy of gpgme.h generated when GPGME is compiled.

This means that a version of the Python bindings is fundamentally tied to the exact same version of GPGME used to generate that copy of gpgme.h.

2.3 Difference between the Python bindings and other GnuPG Python packages

There have been numerous attempts to add GnuPG support to Python over the years. Some of the most well known are listed here, along with what differentiates them.

2.3.1 The python-gnupg package maintained by Vinay Sajip

This is arguably the most popular means of integrating GPG with Python. The package utilises the subprocess module to implement wrappers for the gpg and gpg2 executables normally invoked on the command line (gpg.exe and gpg2.exe on Windows).

The popularity of this package stemmed from its ease of use and capability in providing the most commonly required features.

Unfortunately it has been beset by a number of security issues in the past; most of which stemmed from using unsafe methods of accessing the command line via the subprocess calls. While some effort has been made over the last two to three years (as of 2018) to mitigate this, particularly by no longer providing shell access through those subprocess calls, the wrapper is still somewhat limited in the scope of its GnuPG features coverage.

The python-gnupg package is available under the MIT license.

2.3.2 The gnupg package created and maintained by Isis Lovecruft

In 2015 Isis Lovecruft from the Tor Project forked and then re-implemented the python-gnupg package as just gnupg. This new package also relied on subprocess to call the gpg or gpg2 binaries, but did so somewhat more securely.

The naming and version numbering selected for this package, however, resulted in conflicts with the original python-gnupg and since its functions were called in a different manner to python-gnupg, the release of this package also resulted in a great deal of consternation when people installed what they thought was an upgrade that subsequently broke the code relying on it.

The gnupg package is available under the GNU General Public License version 3.0 (or any later version).

2.3.3 The PyME package maintained by Martin Albrecht

This package is the origin of these bindings, though they are somewhat different now. For details of when and how the PyME package was folded back into GPGME itself see the Short History document1 in the Python bindings docs directory.2

The PyME package was first released in 2002 and was also the first attempt to implement a low level binding to GPGME. In doing so it provided access to considerably more functionality than either the python-gnupg or gnupg packages.

The PyME package is only available for Python 2.6 and 2.7.

Porting the PyME package to Python 3.4 in 2015 is what resulted in it being folded into the GPGME project and the current bindings are the end result of that effort.

The PyME package is available under the same dual licensing as GPGME itself: the GNU General Public License version 2.0 (or any later version) and the GNU Lesser General Public License version 2.1 (or any later version).

3 GPGME Python bindings installation

3.1 No PyPI

Most third-party Python packages and modules are available and distributed through the Python Package Installer, known as PyPI.

Due to the nature of what these bindings are and how they work, it is infeasible to install the GPGME Python bindings in the same way.

This is because the bindings use SWIG to dynamically generate C bindings against gpgme.h and gpgme.h is generated from gpgme.h.in at compile time when GPGME is built from source. Thus to include a package in PyPI which actually built correctly would require either statically built libraries for every architecture bundled with it or a full implementation of C for each architecture.

3.2 Requirements

The GPGME Python bindings only have three requirements:

  1. A suitable version of Python 2 or Python 3. With Python 2 that means Python 2.7 and with Python 3 that means Python 3.4 or higher.
  2. SWIG.
  3. GPGME itself. Which also means that all of GPGME's dependencies must be installed too.

3.3 Installation

Installing the Python bindings is effectively achieved by compiling and installing GPGME itself.

Once SWIG is installed with Python and all the dependencies for GPGME are installed you only need to confirm that the version(s) of Python you want the bindings installed for are in your $PATH.

By default GPGME will attempt to install the bindings for the most recent or highest version number of Python 2 and Python 3 it detects in $PATH. It specifically checks for the python and python3 executables first and then checks for specific version numbers.

For Python 2 it checks for these executables in this order: python, python2 and python2.7.

For Python 3 it checks for these executables in this order: python3, python3.6, python3.5, python3.4 and python3.7.3

3.3.1 Installing GPGME

See the GPGME README file for details of how to install GPGME from source.

4 Fundamentals

Before we can get to the fun stuff, there are a few matters regarding GPGME's design which hold true whether you're dealing with the C code directly or these Python bindings.

4.1 No REST

The first part of which is or will be fairly blatantly obvious upon viewing the first example, but it's worth reiterating anyway. That being that this API is not a REST API. Nor indeed could it ever be one.

Most, if not all, Python programmers (and not just Python programmers) know how easy it is to work with a RESTful API. In fact they've become so popular that many other APIs attempt to emulate REST-like behaviour as much as they are able. Right down to the use of JSON formatted output to facilitate the use of their API without having to retrain developers.

This API does not do that. It would not be able to do that and also provide access to the entire C API on which it's built. It does, however, provide a very pythonic interface on top of the direct bindings and it's this pythonic layer with which this HOWTO deals with.

4.2 Context

One of the reasons which prevents this API from being RESTful is that most operations require more than one instruction to the API to perform the task. Sure, there are certain functions which can be performed simultaneously, particularly if the result known or strongly anticipated (e.g. selecting and encrypting to a key known to be in the public keybox).

There are many more, however, which cannot be manipulated so readily: they must be performed in a specific sequence and the result of one operation has a direct bearing on the outcome of subsequent operations. Not merely by generating an error either.

When dealing with this type of persistent state on the web, full of both the RESTful and REST-like, it's most commonly referred to as a session. In GPGME, however, it is called a context and every operation type has one.

5 Working with keys

5.1 Key selection

Selecting keys to encrypt to or to sign with will be a common occurrence when working with GPGMe and the means available for doing so are quite simple.

They do depend on utilising a Context; however once the data is recorded in another variable, that Context does not need to be the same one which subsequent operations are performed.

The easiest way to select a specific key is by searching for that key's key ID or fingerprint, preferably the full fingerprint without any spaces in it. A long key ID will probably be okay, but is not advised and short key IDs are already a problem with some being generated to match specific patterns. It does not matter whether the pattern is upper or lower case.

So this is the best method:

import gpg

k = gpg.Context().keylist(pattern="258E88DCBD3CD44D8E7AB43F6ECB6AF0DEADBEEF")
keys = list(k)

This is passable and very likely to be common:

import gpg

k = gpg.Context().keylist(pattern="0x6ECB6AF0DEADBEEF")
keys = list(k)

And this is a really bad idea:

import gpg

k = gpg.Context().keylist(pattern="0xDEADBEEF")
keys = list(k)

Alternatively it may be that the intention is to create a list of keys which all match a particular search string. For instance all the addresses at a particular domain, like this:

import gpg

ncsc = gpg.Context().keylist(pattern="ncsc.mil")
nsa = list(ncsc)

5.1.1 Counting keys

Counting the number of keys in your public keybox (pubring.kbx), the format which has superseded the old keyring format (pubring.gpg and secring.gpg), or the number of secret keys is a very simple task.

import gpg

c = gpg.Context()
seckeys = c.keylist(pattern=None, secret=True)
pubkeys = c.keylist(pattern=None, secret=False)

seclist = list(seckeys)
secnum = len(seclist)

publist = list(pubkeys)
pubnum = len(publist)

print("""
  Number of secret keys:  {0}
  Number of public keys:  {1}
""".format(secnum, pubnum))

5.2 Get key

An alternative method of getting a single key via its fingerprint is available directly within a Context with Context().get_key. This is the preferred method of selecting a key in order to modify it, sign or certify it and for obtaining relevant data about a single key as a part of other functions; when verifying a signature made by that key, for instance.

By default this method will select public keys, but it can select secret keys as well.

This first example demonstrates selecting the current key of Werner Koch, which is due to expire at the end of 2018:

import gpg

fingerprint = "80615870F5BAD690333686D0F2AD85AC1E42B367"
key = gpg.Context().get_key(fingerprint)

Whereas this example demonstrates selecting the author's current key with the secret key word argument set to True:

import gpg

fingerprint = "DB4724E6FA4286C92B4E55C4321E4E2373590E5D"
key = gpg.Context().get_key(fingerprint, secret=True)

It is, of course, quite possible to select expired, disabled and revoked keys with this function, but only to effectively display information about those keys.

It is also possible to use both unicode or string literals and byte literals with the fingerprint when getting a key in this way.

5.3 Importing keys

Importing keys is possible with the key_import() method and takes one argument which is a bytes literal object containing either the binary or ASCII armoured key data for one or more keys.

The following example retrieves one or more keys from the SKS keyservers via the web using the requests module. Since requests returns the content as a bytes literal object, we can then use that directly to import the resulting data into our keybox.

import gpg
import os.path
import requests

c = gpg.Context()
url = "https://sks-keyservers.net/pks/lookup"
pattern = input("Enter the pattern to search for key or user IDs: ")
payload = { "op": "get", "search": pattern }

r = requests.get(url, verify=True, params=payload)
result = c.key_import(r.content)

if result is not None and hasattr(result, "considered") is False:
    print(result)
elif result is not None and hasattr(result, "considered") is True:
    num_keys = len(result.imports)
    new_revs = result.new_revocations
    new_sigs = result.new_signatures
    new_subs = result.new_sub_keys
    new_uids = result.new_user_ids
    new_scrt = result.secret_imported
    nochange = result.unchanged
    print("""
  The total number of keys considered for import was:  {0}

     Number of keys revoked:  {1}
   Number of new signatures:  {2}
      Number of new subkeys:  {3}
     Number of new user IDs:  {4}
  Number of new secret keys:  {5}
   Number of unchanged keys:  {6}

  The key IDs for all considered keys were:
""".format(num_keys, new_revs, new_sigs, new_subs, new_uids, new_scrt,
           nochange))
    for i in range(num_keys):
        print("{0}\n".format(result.imports[i].fpr))
else:
    pass

NOTE: When searching for a key ID of any length or a fingerprint (without spaces), the SKS servers require the the leading 0x indicative of hexadecimal be included. Also note that the old short key IDs (e.g. 0xDEADBEEF) should no longer be used due to the relative ease by which such key IDs can be reproduced, as demonstrated by the Evil32 Project in 2014 (which was subsequently exploited in 2016).

5.4 Exporting keys

Exporting keys remains a reasonably simple task, but has been separated into three different functions for the OpenPGP cryptographic engine. Two of those functions are for exporting public keys and the third is for exporting secret keys.

5.4.1 Exporting public keys

There are two methods of exporting public keys, both of which are very similar to the other. The default method, key_export(), will export a public key or keys matching a specified pattern as normal. The alternative, the key_export_minimal() method, will do the same thing except producing a minimised output with extra signatures and third party signatures or certifications removed.

import gpg
import os.path
import sys

print("""
This script exports one or more public keys.
""")

c = gpg.Context(armor=True)

if len(sys.argv) >= 4:
    keyfile = sys.argv[1]
    logrus = sys.argv[2]
    homedir = sys.argv[3]
elif len(sys.argv) == 3:
    keyfile = sys.argv[1]
    logrus = sys.argv[2]
    homedir = input("Enter the GPG configuration directory path (optional): ")
elif len(sys.argv) == 2:
    keyfile = sys.argv[1]
    logrus = input("Enter the UID matching the key(s) to export: ")
    homedir = input("Enter the GPG configuration directory path (optional): ")
else:
    keyfile = input("Enter the path and filename to save the secret key to: ")
    logrus = input("Enter the UID matching the key(s) to export: ")
    homedir = input("Enter the GPG configuration directory path (optional): ")

if homedir.startswith("~"):
    if os.path.exists(os.path.expanduser(homedir)) is True:
        c.home_dir = os.path.expanduser(homedir)
    else:
        pass
elif os.path.exists(homedir) is True:
    c.home_dir = homedir
else:
    pass

try:
    result = c.key_export(pattern=logrus)
except:
    result = c.key_export(pattern=None)

if result is not None:
    with open(keyfile, "wb") as f:
        f.write(result)
else:
    pass

It is important to note that the result will only return None when a pattern has been entered for logrus, but it has not matched any keys. When the search pattern itself is set to None this triggers the exporting of the entire public keybox.

import gpg
import os.path
import sys

print("""
This script exports one or more public keys in minimised form.
""")

c = gpg.Context(armor=True)

if len(sys.argv) >= 4:
    keyfile = sys.argv[1]
    logrus = sys.argv[2]
    homedir = sys.argv[3]
elif len(sys.argv) == 3:
    keyfile = sys.argv[1]
    logrus = sys.argv[2]
    homedir = input("Enter the GPG configuration directory path (optional): ")
elif len(sys.argv) == 2:
    keyfile = sys.argv[1]
    logrus = input("Enter the UID matching the key(s) to export: ")
    homedir = input("Enter the GPG configuration directory path (optional): ")
else:
    keyfile = input("Enter the path and filename to save the secret key to: ")
    logrus = input("Enter the UID matching the key(s) to export: ")
    homedir = input("Enter the GPG configuration directory path (optional): ")

if homedir.startswith("~"):
    if os.path.exists(os.path.expanduser(homedir)) is True:
        c.home_dir = os.path.expanduser(homedir)
    else:
        pass
elif os.path.exists(homedir) is True:
    c.home_dir = homedir
else:
    pass

try:
    result = c.key_export_minimal(pattern=logrus)
except:
    result = c.key_export_minimal(pattern=None)

if result is not None:
    with open(keyfile, "wb") as f:
        f.write(result)
else:
    pass

5.4.2 Exporting secret keys

Exporting secret keys is, functionally, very similar to exporting public keys; save for the invocation of pinentry via gpg-agent in order to securely enter the key's passphrase and authorise the export.

The following example exports the secret key to a file which is then set with the same permissions as the output files created by the command line secret key export options.

import gpg
import os
import os.path
import sys

print("""
This script exports one or more secret keys.

The gpg-agent and pinentry are invoked to authorise the export.
""")

c = gpg.Context(armor=True)

if len(sys.argv) >= 4:
    keyfile = sys.argv[1]
    logrus = sys.argv[2]
    homedir = sys.argv[3]
elif len(sys.argv) == 3:
    keyfile = sys.argv[1]
    logrus = sys.argv[2]
    homedir = input("Enter the GPG configuration directory path (optional): ")
elif len(sys.argv) == 2:
    keyfile = sys.argv[1]
    logrus = input("Enter the UID matching the secret key(s) to export: ")
    homedir = input("Enter the GPG configuration directory path (optional): ")
else:
    keyfile = input("Enter the path and filename to save the secret key to: ")
    logrus = input("Enter the UID matching the secret key(s) to export: ")
    homedir = input("Enter the GPG configuration directory path (optional): ")

if homedir.startswith("~"):
    if os.path.exists(os.path.expanduser(homedir)) is True:
        c.home_dir = os.path.expanduser(homedir)
    else:
        pass
elif os.path.exists(homedir) is True:
    c.home_dir = homedir
else:
    pass

try:
    result = c.key_export_secret(pattern=logrus)
except:
    result = c.key_export_secret(pattern=None)

if result is not None:
    with open(keyfile, "wb") as f:
        f.write(result)
    os.chmod(keyfile, 0o600)
else:
    pass

Alternatively the approach of the following script can be used. This longer example saves the exported secret key(s) in files in the GnuPG home directory, in addition to setting the file permissions as only readable and writable by the user. It also exports the secret key(s) twice in order to output both GPG binary (.gpg) and ASCII armoured (.asc) files.

import gpg
import os
import os.path
import subprocess
import sys

print("""
This script exports one or more secret keys as both ASCII armored and binary
file formats, saved in files within the user's GPG home directory.

The gpg-agent and pinentry are invoked to authorise the export.
""")

if sys.platform == "win32":
    gpgconfcmd = "gpgconf.exe --list-dirs homedir"
else:
    gpgconfcmd = "gpgconf --list-dirs homedir"

a = gpg.Context(armor=True)
b = gpg.Context()
c = gpg.Context()

if len(sys.argv) >= 4:
    keyfile = sys.argv[1]
    logrus = sys.argv[2]
    homedir = sys.argv[3]
elif len(sys.argv) == 3:
    keyfile = sys.argv[1]
    logrus = sys.argv[2]
    homedir = input("Enter the GPG configuration directory path (optional): ")
elif len(sys.argv) == 2:
    keyfile = sys.argv[1]
    logrus = input("Enter the UID matching the secret key(s) to export: ")
    homedir = input("Enter the GPG configuration directory path (optional): ")
else:
    keyfile = input("Enter the filename to save the secret key to: ")
    logrus = input("Enter the UID matching the secret key(s) to export: ")
    homedir = input("Enter the GPG configuration directory path (optional): ")

if homedir.startswith("~"):
    if os.path.exists(os.path.expanduser(homedir)) is True:
        c.home_dir = os.path.expanduser(homedir)
    else:
        pass
elif os.path.exists(homedir) is True:
    c.home_dir = homedir
else:
    pass

if c.home_dir is not None:
    if c.home_dir.endswith("/"):
        gpgfile = "{0}{1}.gpg".format(c.home_dir, keyfile)
        ascfile = "{0}{1}.asc".format(c.home_dir, keyfile)
    else:
        gpgfile = "{0}/{1}.gpg".format(c.home_dir, keyfile)
        ascfile = "{0}/{1}.asc".format(c.home_dir, keyfile)
else:
    if os.path.exists(os.environ["GNUPGHOME"]) is True:
        hd = os.environ["GNUPGHOME"]
    else:
        hd = subprocess.getoutput(gpgconfcmd)
    gpgfile = "{0}/{1}.gpg".format(hd, keyfile)
    ascfile = "{0}/{1}.asc".format(hd, keyfile)

try:
    a_result = a.key_export_secret(pattern=logrus)
    b_result = b.key_export_secret(pattern=logrus)
except:
    a_result = a.key_export_secret(pattern=None)
    b_result = b.key_export_secret(pattern=None)

if a_result is not None:
    with open(ascfile, "wb") as f:
        f.write(a_result)
    os.chmod(ascfile, 0o600)
else:
    pass

if b_result is not None:
    with open(gpgfile, "wb") as f:
        f.write(b_result)
    os.chmod(gpgfile, 0o600)
else:
    pass

6 Basic Functions

The most frequently called features of any cryptographic library will be the most fundamental tasks for encryption software. In this section we will look at how to programmatically encrypt data, decrypt it, sign it and verify signatures.

6.1 Encryption

Encrypting is very straight forward. In the first example below the message, text, is encrypted to a single recipient's key. In the second example the message will be encrypted to multiple recipients.

6.1.1 Encrypting to one key

Once the the Context is set the main issues with encrypting data is essentially reduced to key selection and the keyword arguments specified in the gpg.Context().encrypt() method.

Those keyword arguments are: recipients, a list of keys encrypted to (covered in greater detail in the following section); sign, whether or not to sign the plaintext data, see subsequent sections on signing and verifying signatures below (defaults to True); sink, to write results or partial results to a secure sink instead of returning it (defaults to None); passphrase, only used when utilising symmetric encryption (defaults to None); always_trust, used to override the trust model settings for recipient keys (defaults to False); add_encrypt_to, utilises any preconfigured encrypt-to or default-key settings in the user's gpg.conf file (defaults to False); prepare, prepare for encryption (defaults to False); expect_sign, prepare for signing (defaults to False); compress, compresses the plaintext prior to encryption (defaults to True).

import gpg

a_key = "0x12345678DEADBEEF"
text = b"""Some text to test with.

Since the text in this case must be bytes, it is most likely that
the input form will be a separate file which is opened with "rb"
as this is the simplest method of obtaining the correct data format.
"""

c = gpg.Context(armor=True)
rkey = list(c.keylist(pattern=a_key, secret=False))
ciphertext, result, sign_result = c.encrypt(text, recipients=rkey, sign=False)

with open("secret_plans.txt.asc", "wb") as afile:
    afile.write(ciphertext)

Though this is even more likely to be used like this; with the plaintext input read from a file, the recipient keys used for encryption regardless of key trust status and the encrypted output also encrypted to any preconfigured keys set in the gpg.conf file:

import gpg

a_key = "0x12345678DEADBEEF"

with open("secret_plans.txt", "rb") as afile:
    text = afile.read()

c = gpg.Context(armor=True)
rkey = list(c.keylist(pattern=a_key, secret=False))
ciphertext, result, sign_result = c.encrypt(text, recipients=rkey, sign=True,
                                            always_trust=True,
                                            add_encrypt_to=True)

with open("secret_plans.txt.asc", "wb") as afile:
    afile.write(ciphertext)

If the recipients paramater is empty then the plaintext is encrypted symmetrically. If no passphrase is supplied as a parameter or via a callback registered with the Context() then an out-of-band prompt for the passphrase via pinentry will be invoked.

6.1.2 Encrypting to multiple keys

Encrypting to multiple keys essentially just expands upon the key selection process and the recipients from the previous examples.

The following example encrypts a message (text) to everyone with an email address on the gnupg.org domain,4 but does not encrypt to a default key or other key which is configured to normally encrypt to.

import gpg

text = b"""Oh look, another test message.

The same rules apply as with the previous example and more likely
than not, the message will actually be drawn from reading the
contents of a file or, maybe, from entering data at an input()
prompt.

Since the text in this case must be bytes, it is most likely that
the input form will be a separate file which is opened with "rb"
as this is the simplest method of obtaining the correct data
format.
"""

c = gpg.Context(armor=True)
rpattern = list(c.keylist(pattern="@gnupg.org", secret=False))
logrus = []

for i in range(len(rpattern)):
    if rpattern[i].can_encrypt == 1:
        logrus.append(rpattern[i])

ciphertext, result, sign_result = c.encrypt(text, recipients=logrus,
                                            sign=False, always_trust=True)

with open("secret_plans.txt.asc", "wb") as afile:
    afile.write(ciphertext)

All it would take to change the above example to sign the message and also encrypt the message to any configured default keys would be to change the c.encrypt line to this:

ciphertext, result, sign_result = c.encrypt(text, recipients=logrus,
                                            always_trust=True,
                                            add_encrypt_to=True)

The only keyword arguments requiring modification are those for which the default values are changing. The default value of sign is True, the default of always_trust is False, the default of add_encrypt_to is False.

If always_trust is not set to True and any of the recipient keys are not trusted (e.g. not signed or locally signed) then the encryption will raise an error. It is possible to mitigate this somewhat with something more like this:

import gpg

with open("secret_plans.txt.asc", "rb") as afile:
    text = afile.read()

c = gpg.Context(armor=True)
rpattern = list(c.keylist(pattern="@gnupg.org", secret=False))
logrus = []

for i in range(len(rpattern)):
    if rpattern[i].can_encrypt == 1:
        logrus.append(rpattern[i])

    try:
        ciphertext, result, sign_result = c.encrypt(text, recipients=logrus,
                                                    add_encrypt_to=True)
    except gpg.errors.InvalidRecipients as e:
        for i in range(len(e.recipients)):
            for n in range(len(logrus)):
                if logrus[n].fpr == e.recipients[i].fpr:
                    logrus.remove(logrus[n])
                else:
                    pass
        try:
            ciphertext, result, sign_result = c.encrypt(text,
                                                        recipients=logrus,
                                                        add_encrypt_to=True)
            with open("secret_plans.txt.asc", "wb") as afile:
                afile.write(ciphertext)
        except:
            pass

This will attempt to encrypt to all the keys searched for, then remove invalid recipients if it fails and try again.

6.2 Decryption

Decrypting something encrypted to a key in one's secret keyring is fairly straight forward.

In this example code, however, preconfiguring either gpg.Context() or gpg.core.Context() as c is unnecessary because there is no need to modify the Context prior to conducting the decryption and since the Context is only used once, setting it to c simply adds lines for no gain.

import gpg

ciphertext = input("Enter path and filename of encrypted file: ")
newfile = input("Enter path and filename of file to save decrypted data to: ")

with open(ciphertext, "rb") as cfile:
    try:
        plaintext, result, verify_result = gpg.Context().decrypt(cfile)
    except gpg.errors.GPGMEError as e:
        plaintext = None
        print(e)

if plaintext is not None:
    with open(newfile, "wb") as nfile:
        nfile.write(plaintext)
    else:
        pass

The data available in plaintext in this example is the decrypted content as a byte object, the recipient key IDs and algorithms in result and the results of verifying any signatures of the data in verify_result.

6.3 Signing text and files

The following sections demonstrate how to specify keys to sign with.

6.3.1 Signing key selection

By default GPGME and the Python bindings will use the default key configured for the user invoking the GPGME API. If there is no default key specified and there is more than one secret key available it may be necessary to specify the key or keys with which to sign messages and files.

import gpg

logrus = input("Enter the email address or string to match signing keys to: ")
hancock = gpg.Context().keylist(pattern=logrus, secret=True)
sig_src = list(hancock)

The signing examples in the following sections include the explicitly designated signers parameter in two of the five examples; once where the resulting signature would be ASCII armoured and once where it would not be armoured.

While it would be possible to enter a key ID or fingerprint here to match a specific key, it is not possible to enter two fingerprints and match two keys since the patten expects a string, bytes or None and not a list. A string with two fingerprints won't match any single key.

6.3.2 Normal or default signing messages or files

The normal or default signing process is essentially the same as is most often invoked when also encrypting a message or file. So when the encryption component is not utilised, the result is to produce an encoded and signed output which may or may not be ASCII armoured and which may or may not also be compressed.

By default compression will be used unless GnuPG detects that the plaintext is already compressed. ASCII armouring will be determined according to the value of gpg.Context().armor.

The compression algorithm is selected in much the same way as the symmetric encryption algorithm or the hash digest algorithm is when multiple keys are involved; from the preferences saved into the key itself or by comparison with the preferences with all other keys involved.

import gpg

text0 = """Declaration of ... something.

"""
text = text0.encode()

c = gpg.Context(armor=True, signers=sig_src)
signed_data, result = c.sign(text, mode=gpg.constants.sig.mode.NORMAL)

with open("/path/to/statement.txt.asc", "w") as afile:
    afile.write(signed_data.decode())

Though everything in this example is accurate, it is more likely that reading the input data from another file and writing the result to a new file will be performed more like the way it is done in the next example. Even if the output format is ASCII armoured.

import gpg

with open("/path/to/statement.txt", "rb") as tfile:
    text = tfile.read()

c = gpg.Context()
signed_data, result = c.sign(text, mode=gpg.constants.sig.mode.NORMAL)

with open("/path/to/statement.txt.sig", "wb") as afile:
    afile.write(signed_data)

6.3.3 Detached signing messages and files

Detached signatures will often be needed in programmatic uses of GPGME, either for signing files (e.g. tarballs of code releases) or as a component of message signing (e.g. PGP/MIME encoded email).

import gpg

text0 = """Declaration of ... something.

"""
text = text0.encode()

c = gpg.Context(armor=True)
signed_data, result = c.sign(text, mode=gpg.constants.sig.mode.DETACH)

with open("/path/to/statement.txt.asc", "w") as afile:
    afile.write(signed_data.decode())

As with normal signatures, detached signatures are best handled as byte literals, even when the output is ASCII armoured.

import gpg

with open("/path/to/statement.txt", "rb") as tfile:
    text = tfile.read()

c = gpg.Context(signers=sig_src)
signed_data, result = c.sign(text, mode=gpg.constants.sig.mode.DETACH)

with open("/path/to/statement.txt.sig", "wb") as afile:
    afile.write(signed_data)

6.3.4 Clearsigning messages or text

Though PGP/in-line messages are no longer encouraged in favour of PGP/MIME, there is still sometimes value in utilising in-line signatures. This is where clear-signed messages or text is of value.

import gpg

text0 = """Declaration of ... something.

"""
text = text0.encode()

c = gpg.Context()
signed_data, result = c.sign(text, mode=gpg.constants.sig.mode.CLEAR)

with open("/path/to/statement.txt.asc", "w") as afile:
    afile.write(signed_data.decode())

In spite of the appearance of a clear-signed message, the data handled by GPGME in signing it must still be byte literals.

import gpg

with open("/path/to/statement.txt", "rb") as tfile:
    text = tfile.read()

c = gpg.Context()
signed_data, result = c.sign(text, mode=gpg.constants.sig.mode.CLEAR)

with open("/path/to/statement.txt.asc", "wb") as afile:
    afile.write(signed_data)

6.4 Signature verification

Essentially there are two principal methods of verification of a signature. The first of these is for use with the normal or default signing method and for clear-signed messages. The second is for use with files and data with detached signatures.

The following example is intended for use with the default signing method where the file was not ASCII armoured:

import gpg
import time

filename = "statement.txt"
gpg_file = "statement.txt.gpg"

c = gpg.Context()

try:
    data, result = c.verify(open(gpg_file))
    verified = True
except gpg.errors.BadSignatures as e:
    verified = False
    print(e)

if verified is True:
    for i in range(len(result.signatures)):
        sign = result.signatures[i]
        print("""Good signature from:
{0}
with key {1}
made at {2}
""".format(c.get_key(sign.fpr).uids[0].uid, sign.fpr,
           time.ctime(sign.timestamp)))
else:
    pass

Whereas this next example, which is almost identical would work with normal ASCII armoured files and with clear-signed files:

import gpg
import time

filename = "statement.txt"
asc_file = "statement.txt.asc"

c = gpg.Context()

try:
    data, result = c.verify(open(asc_file))
    verified = True
except gpg.errors.BadSignatures as e:
    verified = False
    print(e)

if verified is True:
    for i in range(len(result.signatures)):
        sign = result.signatures[i]
        print("""Good signature from:
{0}
with key {1}
made at {2}
""".format(c.get_key(sign.fpr).uids[0].uid, sign.fpr,
           time.ctime(sign.timestamp)))
else:
    pass

In both of the previous examples it is also possible to compare the original data that was signed against the signed data in data to see if it matches with something like this:

with open(filename, "rb") as afile:
    text = afile.read()

if text == data:
    print("Good signature.")
else:
    pass

The following two examples, however, deal with detached signatures. With his method of verification the data that was signed does not get returned since it is already being explicitly referenced in the first argument of c.verify. So data is None and only the information in result is available.

import gpg
import time

filename = "statement.txt"
sig_file = "statement.txt.sig"

c = gpg.Context()

try:
    data, result = c.verify(open(filename), open(sig_file))
    verified = True
except gpg.errors.BadSignatures as e:
    verified = False
    print(e)

if verified is True:
    for i in range(len(result.signatures)):
        sign = result.signatures[i]
        print("""Good signature from:
{0}
with key {1}
made at {2}
""".format(c.get_key(sign.fpr).uids[0].uid, sign.fpr,
           time.ctime(sign.timestamp)))
else:
    pass
import gpg
import time

filename = "statement.txt"
asc_file = "statement.txt.asc"

c = gpg.Context()

try:
    data, result = c.verify(open(filename), open(asc_file))
    verified = True
except gpg.errors.BadSignatures as e:
    verified = False
    print(e)

if verified is True:
    for i in range(len(result.signatures)):
        sign = result.signatures[i]
        print("""Good signature from:
{0}
with key {1}
made at {2}
""".format(c.get_key(sign.fpr).uids[0].uid, sign.fpr,
           time.ctime(sign.timestamp)))
else:
    pass

7 Creating keys and subkeys

The one thing, aside from GnuPG itself, that GPGME depends on, of course, is the keys themselves. So it is necessary to be able to generate them and modify them by adding subkeys, revoking or disabling them, sometimes deleting them and doing the same for user IDs.

In the following examples a key will be created for the world's greatest secret agent, Danger Mouse. Since Danger Mouse is a secret agent he needs to be able to protect information to SECRET level clearance, so his keys will be 3072-bit keys.

The pre-configured gpg.conf file which sets cipher, digest and other preferences contains the following configuration parameters:

expert
allow-freeform-uid
allow-secret-key-import
trust-model tofu+pgp
tofu-default-policy unknown
enable-large-rsa
enable-dsa2
cert-digest-algo SHA512
default-preference-list TWOFISH CAMELLIA256 AES256 CAMELLIA192 AES192 CAMELLIA128 AES BLOWFISH IDEA CAST5 3DES SHA512 SHA384 SHA256 SHA224 RIPEMD160 SHA1 ZLIB BZIP2 ZIP Uncompressed
personal-cipher-preferences TWOFISH CAMELLIA256 AES256 CAMELLIA192 AES192 CAMELLIA128 AES BLOWFISH IDEA CAST5 3DES
personal-digest-preferences SHA512 SHA384 SHA256 SHA224 RIPEMD160 SHA1
personal-compress-preferences ZLIB BZIP2 ZIP Uncompressed

7.1 Primary key

Generating a primary key uses the create_key method in a Context. It contains multiple arguments and keyword arguments, including: userid, algorithm, expires_in, expires, sign, encrypt, certify, authenticate, passphrase and force. The defaults for all of those except userid, algorithm, expires_in, expires and passphrase is False. The defaults for algorithm and passphrase is None. The default for expires_in is 0. The default for expires is True. There is no default for userid.

If passphrase is left as None then the key will not be generated with a passphrase, if passphrase is set to a string then that will be the passphrase and if passphrase is set to True then gpg-agent will launch pinentry to prompt for a passphrase. For the sake of convenience, these examples will keep passphrase set to None.

import gpg

c = gpg.Context()

c.home_dir = "~/.gnupg-dm"
userid = "Danger Mouse <dm@secret.example.net>"

dmkey = c.create_key(userid, algorithm="rsa3072", expires_in=31536000,
                     sign=True, certify=True)

One thing to note here is the use of setting the c.home_dir parameter. This enables generating the key or keys in a different location. In this case to keep the new key data created for this example in a separate location rather than adding it to existing and active key store data. As with the default directory, ~/.gnupg, any temporary or separate directory needs the permissions set to only permit access by the directory owner. On posix systems this means setting the directory permissions to 700.

The temp-homedir-config.py script in the HOWTO examples directory will create an alternative homedir with these configuration options already set and the correct directory and file permissions.

The successful generation of the key can be confirmed via the returned GenkeyResult object, which includes the following data:

print("""
 Fingerprint:  {0}
 Primary Key:  {1}
  Public Key:  {2}
  Secret Key:  {3}
 Sub Key:  {4}
User IDs:  {5}
""".format(dmkey.fpr, dmkey.primary, dmkey.pubkey, dmkey.seckey, dmkey.sub,
           dmkey.uid))

Alternatively the information can be confirmed using the command line program:

bash-4.4$ gpg --homedir ~/.gnupg-dm -K
~/.gnupg-dm/pubring.kbx
----------------------
sec   rsa3072 2018-03-15 [SC] [expires: 2019-03-15]
      177B7C25DB99745EE2EE13ED026D2F19E99E63AA
uid           [ultimate] Danger Mouse <dm@secret.example.net>

bash-4.4$

As with generating keys manually, to preconfigure expanded preferences for the cipher, digest and compression algorithms, the gpg.conf file must contain those details in the home directory in which the new key is being generated. I used a cut down version of my own gpg.conf file in order to be able to generate this:

bash-4.4$ gpg --homedir ~/.gnupg-dm --edit-key 177B7C25DB99745EE2EE13ED026D2F19E99E63AA showpref quit
Secret key is available.

sec  rsa3072/026D2F19E99E63AA
     created: 2018-03-15  expires: 2019-03-15  usage: SC
     trust: ultimate      validity: ultimate
[ultimate] (1). Danger Mouse <dm@secret.example.net>

[ultimate] (1). Danger Mouse <dm@secret.example.net>
     Cipher: TWOFISH, CAMELLIA256, AES256, CAMELLIA192, AES192, CAMELLIA128, AES, BLOWFISH, IDEA, CAST5, 3DES
     Digest: SHA512, SHA384, SHA256, SHA224, RIPEMD160, SHA1
     Compression: ZLIB, BZIP2, ZIP, Uncompressed
     Features: MDC, Keyserver no-modify

bash-4.4$

7.2 Subkeys

Adding subkeys to a primary key is fairly similar to creating the primary key with the create_subkey method. Most of the arguments are the same, but not quite all. Instead of the userid argument there is now a key argument for selecting which primary key to add the subkey to.

In the following example an encryption subkey will be added to the primary key. Since Danger Mouse is a security conscious secret agent, this subkey will only be valid for about six months, half the length of the primary key.

import gpg

c = gpg.Context()
c.home_dir = "~/.gnupg-dm"

key = c.get_key(dmkey.fpr, secret=True)
dmsub = c.create_subkey(key, algorithm="rsa3072", expires_in=15768000,
                        encrypt=True)

As with the primary key, the results here can be checked with:

print("""
 Fingerprint:  {0}
 Primary Key:  {1}
  Public Key:  {2}
  Secret Key:  {3}
 Sub Key:  {4}
User IDs:  {5}
""".format(dmsub.fpr, dmsub.primary, dmsub.pubkey, dmsub.seckey, dmsub.sub,
           dmsub.uid))

As well as on the command line with:

bash-4.4$ gpg --homedir ~/.gnupg-dm -K
~/.gnupg-dm/pubring.kbx
----------------------
sec   rsa3072 2018-03-15 [SC] [expires: 2019-03-15]
      177B7C25DB99745EE2EE13ED026D2F19E99E63AA
uid           [ultimate] Danger Mouse <dm@secret.example.net>
ssb   rsa3072 2018-03-15 [E] [expires: 2018-09-13]

bash-4.4$

7.3 User IDs

7.3.1 Adding User IDs

By comparison to creating primary keys and subkeys, adding a new user ID to an existing key is much simpler. The method used to do this is key_add_uid and the only arguments it takes are for the key and the new uid.

import gpg

c = gpg.Context()
c.home_dir = "~/.gnupg-dm"

dmfpr = "177B7C25DB99745EE2EE13ED026D2F19E99E63AA"
key = c.get_key(dmfpr, secret=True)
uid = "Danger Mouse <danger.mouse@secret.example.net>"

c.key_add_uid(key, uid)

Unsurprisingly the result of this is:

bash-4.4$ gpg --homedir ~/.gnupg-dm -K
~/.gnupg-dm/pubring.kbx
----------------------
sec   rsa3072 2018-03-15 [SC] [expires: 2019-03-15]
      177B7C25DB99745EE2EE13ED026D2F19E99E63AA
uid           [ultimate] Danger Mouse <danger.mouse@secret.example.net>
uid           [ultimate] Danger Mouse <dm@secret.example.net>
ssb   rsa3072 2018-03-15 [E] [expires: 2018-09-13]

bash-4.4$

7.3.2 Revokinging User IDs

Revoking a user ID is a fairly similar process, except that it uses the key_revoke_uid method.

import gpg

c = gpg.Context()
c.home_dir = "~/.gnupg-dm"

dmfpr = "177B7C25DB99745EE2EE13ED026D2F19E99E63AA"
key = c.get_key(dmfpr, secret=True)
uid = "Danger Mouse <danger.mouse@secret.example.net>"

c.key_revoke_uid(key, uid)

7.4 Key certification

Since key certification is more frequently referred to as key signing, the method used to perform this function is key_sign.

The key_sign method takes four arguments: key, uids, expires_in and local. The default value of uids is None and which results in all user IDs being selected. The default value of both expires_in and local is False; which results in the signature never expiring and being able to be exported.

The key is the key being signed rather than the key doing the signing. To change the key doing the signing refer to the signing key selection above for signing messages and files.

If the uids value is not None then it must either be a string to match a single user ID or a list of strings to match multiple user IDs. In this case the matching of those strings must be precise and it is case sensitive.

To sign Danger Mouse's key for just the initial user ID with a signature which will last a little over a month, do this:

import gpg

c = gpg.Context()
uid = "Danger Mouse <dm@secret.example.net>"

dmfpr = "177B7C25DB99745EE2EE13ED026D2F19E99E63AA"
key = c.get_key(dmfpr, secret=True)
c.key_sign(key, uids=uid, expires_in=2764800)

8 Miscellaneous work-arounds

8.1 Group lines

There is not yet an easy way to access groups configured in the gpg.conf file from within GPGME. As a consequence these central groupings of keys cannot be shared amongst multiple programs, such as MUAs readily.

The following code, however, provides a work-around for obtaining this information in Python.

import subprocess

lines = subprocess.getoutput("gpgconf --list-options gpg").splitlines()

for i in range(len(lines)):
    if lines[i].startswith("group") is True:
        line = lines[i]
    else:
        pass

groups = line.split(":")[-1].replace('"', '').split(',')

group_lines = []
group_lists = []

for i in range(len(groups)):
    group_lines.append(groups[i].split("="))
    group_lists.append(groups[i].split("="))

for i in range(len(group_lists)):
    group_lists[i][1] = group_lists[i][1].split()

The result of that code is that group_lines is a list of lists where group_lines[i][0] is the name of the group and group_lines[i][1] is the key IDs of the group as a string.

The group_lists result is very similar in that it is a list of lists. The first part, group_lists[i][0] matches group_lines[i][0] as the name of the group, but group_lists[i][1] is the key IDs of the group as a string.

A demonstration of using the groups.py module is also available in the form of the executable mutt-groups.py script. This second script reads all the group entries in a user's gpg.conf file and converts them into crypt-hooks suitable for use with the Mutt and Neomutt mail clients.

9.2 License GPL compatible

This file is free software; as a special exception the author gives unlimited permission to copy and/or distribute it, with or without modifications, as long as this notice is preserved.

This file is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY, to the extent permitted by law; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Footnotes:

1

Short_History.org and/or Short_History.html.

2

The lang/python/docs/ directory in the GPGME source.

3

As Python 3.7 is a very recent release, it is not given priority over 3.6 yet, but will probably be prioritised by the release of Python 3.7.2.

4

You probably don't really want to do this. Searching the keyservers for "gnupg.org" produces over 400 results, the majority of which aren't actually at the gnupg.org domain, but just included a comment regarding the project in their key somewhere.

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