#+TITLE: GNU Privacy Guard (GnuPG) Made Easy Python Bindings HOWTO (English) #+LATEX_COMPILER: xelatex #+LATEX_CLASS: article #+LATEX_CLASS_OPTIONS: [12pt] #+LATEX_HEADER: \usepackage{xltxtra} #+LATEX_HEADER: \usepackage[margin=1in]{geometry} #+LATEX_HEADER: \setmainfont[Ligatures={Common}]{Times New Roman} #+LATEX_HEADER: \author{Ben McGinnes } * Introduction :PROPERTIES: :CUSTOM_ID: intro :END: | Version: | 0.0.1-alpha | | Author: | Ben McGinnes | | Author GPG Key: | DB4724E6FA4286C92B4E55C4321E4E2373590E5D | | Language: | English | This document provides basic instruction in how to use the GPGME Python bindings to programmatically leverage the GPGME library. * GPGME Concepts :PROPERTIES: :CUSTOM_ID: gpgme-concepts :END: ** A C API :PROPERTIES: :CUSTOM_ID: gpgme-c-api :END: 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 eith 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. ** Python bindings :PROPERTIES: :CUSTOM_ID: gpgme-python-bindings :END: 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 gemerate that copy of =gpgme.h=. ** Difference between the Python bindings and other GnuPG Python packages :PROPERTIES: :CUSTOM_ID: gpgme-python-bindings-diffs :END: 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. *** The python-gnupg package maintained by Vinay Sajip :PROPERTIES: :CUSTOM_ID: diffs-python-gnupg :END: 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, most of which stemmed from using unsafe methods of accessing the command line via the =subprocess= calls. The python-gnupg package is available under the MIT license. *** The gnupg package created and maintained by Isis Lovecruft :PROPERTIES: :CUSTOM_ID: diffs-isis-gnupg :END: 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. However the naming and version numbering selected for this package resulted in conflicts with the original python-gnupg and since its functions were called in a different manner, 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 later). *** The PyME package maintained by Martin Albrecht :PROPERTIES: :CUSTOM_ID: diffs-pyme :END: 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/ document[fn:1] in this Python bindings =docs= directory.[fn: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 Public License version 2.1 (or any later version). * GPGME Python bindings installation :PROPERTIES: :CUSTOM_ID: gpgme-python-install :END: ** No PyPI :PROPERTIES: :CUSTOM_ID: do-not-use-pypi :END: 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. ** Requirements :PROPERTIES: :CUSTOM_ID: gpgme-python-requirements :END: 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. ** Installation :PROPERTIES: :CUSTOM_ID: installation :END: 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= executabled 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= and =python3.4=. *** Installing GPGME :PROPERTIES: :CUSTOM_ID: install-gpgme :END: See the GPGME =README= file for details of how to install GPGME from source. * Fundamentals :PROPERTIES: :CUSTOM_ID: howto-fund-a-mental :END: 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. ** No REST :PROPERTIES: :CUSTOM_ID: no-rest-for-the-wicked :END: 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. ** Context :PROPERTIES: :CUSTOM_ID: howto-get-context :END: 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 persistant 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. * Working with keys :PROPERTIES: :CUSTOM_ID: howto-keys :END: ** Counting keys :PROPERTIES: :CUSTOM_ID: howto-basic-verification :END: Counting the number of keys in your public keybox (=pubring.kbx=), the format which has superceded the old keyring format (=pubring.gpg= and =secring.gpg=), or the number of secret keys is a very simple task. #+begin_src python 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) #+end_src * Basic Functions :PROPERTIES: :CUSTOM_ID: howto-the-basics :END: The most frequently called features of any cryptographic library will be the most fundamental tasks for enxryption software. In this section we will look at how to programmatically encrypt data, decrypt it, sign it and verify signatures. ** Encryption :PROPERTIES: :CUSTOM_ID: howto-basic-encryption :END: 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. *** Encrypting to one key :PROPERTIES: :CUSTOM_ID: howto-basic-encryption-single :END: The text is then encapsulated in a GPGME Data object as =plain= and the =cipher= object is created with another Data object. Then we create the Context as =c= and set it to use the ASCII armoured OpenPGP format. In later examples there will be alternative methods of setting the OpenPGP output to be ASCII armoured. Next we prepare a keylist object in our Context and follow it with specifying the recipients as =r=. Note that the configuration in one's =gpg.conf= file is honoured, so if you have the options set to encrypt to one key or to a default key, that will be included with this operation. This is followed by a quick check to be sure that the recipient is actually selected and that the key is available. Assuming it is, the encryption can proceed, but if not a message will print stating the key was not found. The encryption operation is invoked within the Context with the =c.op_encrypt= function, loading the recipien (=r=), the message (=plain=) and the =cipher=. The =cipher.seek= uses =os.SEEK_SET= to set the data to the correct byte format for GPGME to use it. At this point we no longer need the plaintext material, so we delete both the =text= and the =plain= objects. Then we write the encrypted data out to a file, =secret_plans.txt.asc=. #+begin_src python import gpg import os rkey = "0x12345678DEADBEEF" text = """ Some plain text to test with. Obtained from any input source Python can read. It makes no difference whether it is string or bytes, but the bindings always produce byte output data. Which is useful to know when writing out either the encrypted or decrypted results. """ plain = gpg.core.Data(text) cipher = gpg.core.Data() c = gpg.core.Context() c.set_armor(1) c.op_keylist_start(rkey, 0) r = c.op_keylist_next() if r == None: print("""The key for user "{0}" was not found""".format(rkey)) else: try: c.op_encrypt([r], 1, plain, cipher) cipher.seek(0, os.SEEK_SET) del(text) del(plain) afile = open("secret_plans.txt.asc", "wb") afile.write(cipher.read()) afile.close() except gpg.errors.GPGMEError as ex: print(ex.getstring()) #+end_src *** Encrypting to multiple keys :PROPERTIES: :CUSTOM_ID: howto-basic-encryption-multiple :END: Encrypting to multiple keys, in addition to a default key or a key configured to always encrypt to, is a little different and uses a slightly different call to the =op_encrypt call= demonstrated in the previous section. The following example encrypts a message (=text=) to everyone with an email address on the =gnupg.org= domain,[fn:3] but does /not/ encrypt to a default key or other key which is configured to normally encrypt to. #+begin_src python 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)) rlogrus = [] for i in range(len(rpattern)): if rpattern[i].can_encrypt == 1: rlogrus.append(rpattern[i]) cipher = c.encrypt(text, recipients=rlogrus, sign=False, always_trust=True) afile = open("secret_plans.txt.asc", "wb") afile.write(cipher[0]) afile.close() #+end_src 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: #+begin_src python cipher = c.encrypt(text, recipients=rlogrus, always_trust=True, add_encrypt_to=True) #+end_src 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: #+begin_src python import gpg afile = open("secret_plans.txt", "rb") text = afile.read() afile.close() c = gpg.Context(armor=True) rpattern = list(c.keylist(pattern="@gnupg.org", secret=False)) rlogrus = [] for i in range(len(rpattern)): if rpattern[i].can_encrypt == 1: rlogrus.append(rpattern[i]) try: cipher = c.encrypt(text, recipients=rlogrus, add_encrypt_to=True) except gpg.errors.InvalidRecipients as e: for i in range(len(e.recipients)): for n in range(len(rlogrus)): if rlogrus[n].fpr == e.recipients[i].fpr: rlogrus.remove(rlogrus[n]) else: pass try: cipher = c.encrypt(text, recipients=rlogrus, add_encrypt_to=True) except: pass afile = open("secret_plans.txt.asc", "wb") afile.write(cipher[0]) afile.close() #+end_src This will attempt to encrypt to all the keys searched for, then remove invalid recipients if it fails and try again. ** Decryption :PROPERTIES: :CUSTOM_ID: howto-basic-encryption :END: 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. #+begin_src python import os.path import gpg if os.path.exists("/path/to/secret_plans.txt.asc") is True: ciphertext = "/path/to/secret_plans.txt.asc" elif os.path.exists("/path/to/secret_plans.txt.gpg") is True: ciphertext = "/path/to/secret_plans.txt.gpg" else: ciphertext = None if ciphertext is not None: afile = open(ciphertext, "rb") plaintext = gpg.Context().decrypt(afile) afile.close() newfile = open("/path/to/secret_plans.txt", "wb") newfile.write(plaintext[0]) newfile.close() print(plaintext[0]) plaintext[1] plaintext[2] del(plaintext) else: pass #+end_src The data available in plaintext in this example is the decrypted content as a byte object in =plaintext[0]=, the recipient key IDs and algorithms in =plaintext[1]= and the results of verifying any signatures of the data in =plaintext[0]=. ** Signing text and files :PROPERTIES: :CUSTOM_ID: howto-basic-signing :END: X *** Signing key selection :PROPERTIES: :CUSTOM_ID: howto-basic-signing-signers :END: 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. #+begin_src python 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) #+end_src 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. *** Normal or default signing messages or files :PROPERTIES: :CUSTOM_ID: howto-basic-signing-normal :END: #+begin_src python import gpg text = b"""Declaration of ... something. """ c = gpg.Context(armor=True, signers=sig_src) signed = c.sign(text, mode=0) afile = open("/path/to/statement.txt.asc", "wb") for i in range(len(signed[0].splitlines())): afile.write("{0}\n".format(signed[0].splitlines()[i])) afile.close() #+end_src #+begin_src python import gpg tfile = open("/path/to/statement.txt", "rb") text = tfile.read() tfile.close() c = gpg.Context() signed = c.sign(text, mode=0) afile = open("/path/to/statement.txt.sig", "wb") afile.write(signed[0]) afile.close() #+end_src *** Detached signing messages and files :PROPERTIES: :CUSTOM_ID: howto-basic-signing-detached :END: Detached ASCII Armoured signing: #+begin_src python import gpg text = b"""Declaration of ... something. """ c = gpg.Context(armor=True) signed = c.sign(text, mode=1) afile = open("/path/to/statement.txt.asc", "wb") for i in range(len(signed[0].splitlines())): afile.write("{0}\n".format(signed[0].splitlines()[i])) afile.close() #+end_src Detached binary signing of a file. #+begin_src python import gpg tfile = open("/path/to/statement.txt", "rb") text = tfile.read() tfile.close() c = gpg.Context(signers=sig_src) signed = c.sign(text, mode=1) afile = open("/path/to/statement.txt.sig", "wb") afile.write(signed[0]) afile.close() #+end_src *** Clearsigning messages or text :PROPERTIES: :CUSTOM_ID: howto-basic-signing-clear :END: #+begin_src python import gpg text = """Declaration of ... something. """ c = gpg.Context() signed = c.sign(text, mode=2) afile = open("/path/to/statement.txt.asc", "w") for i in range(len(signed[0].splitlines())): afile.write("{0}\n".format(signed[0].splitlines()[i].decode('utf-8'))) afile.close() #+end_src ** Signature verification :PROPERTIES: :CUSTOM_ID: howto-basic-verification :END: Verify a signed file, both detached and not: #+begin_src python import gpg import sys import time c = gpg.Context() data, result = c.verify(open(filename), open(detached_sig_filename) if detached_sig_filename else None) for index, sign in enumerate(result.signatures): print("signature", index, ":") print(" summary: %#0x" % (sign.summary)) print(" status: %#0x" % (sign.status)) print(" timestamp: ", sign.timestamp) print(" timestamp: ", time.ctime(sign.timestamp)) print(" fingerprint:", sign.fpr) print(" uid: ", c.get_key(sign.fpr).uids[0].uid) if data: sys.stdout.buffer.write(data) #+end_src * Miscellaneous work-arounds :PROPERTIES: :CUSTOM_ID: cheats-and-hacks :END: ** Group lines :PROPERTIES: :CUSTOM_ID: group-lines :END: 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. #+begin_src 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 = groups for i in range(len(group_lines)): group_lines[i] = group_lines[i].split("=") group_lists = group_lines for i in range(len(group_lists)): group_lists[i][1] = group_lists[i][1].split() #+end_src 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. * Copyright and Licensing :PROPERTIES: :CUSTOM_ID: copyright-and-license :END: ** Copyright (C) The GnuPG Project, 2018 :PROPERTIES: :CUSTOM_ID: copyright :END: Copyright © The GnuPG Project, 2018. ** License GPL compatible :PROPERTIES: :CUSTOM_ID: license :END: 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 [fn:1] =Short_History.org= and/or =Short_History.html=. [fn:2] The =lang/python/docs/= directory in the GPGME source. [fn:3] 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.