path: root/lang/python/docs/GPGMEpythonHOWTOen.org

#+TITLE: GNU Privacy Guard (GnuPG)  Made Easy Python Bindings HOWTO (English)
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#+LATEX_HEADER: \author{Ben McGinnes <[email protected]>}


* Introduction
  :PROPERTIES:
  :CUSTOM_ID: intro
  :END:

  | Version:        | 0.0.1-alpha                              |
  | Author:         | Ben McGinnes <[email protected]>             |
  | 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.

** Python 2 versus Python 3
   :PROPERTIES:
   :CUSTOM_ID: py2-vs-py3
   :END:

   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 approprate modifications to support the older
   string and unicode types as opposted 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.


* 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 generate 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:

** Key selection
   :PROPERTIES:
   :CUSTOM_ID: howto-keys-selection
   :END:

   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:

   #+begin_src python
     import gpg

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

   This is passable and very likely to be common:

   #+begin_src python
     import gpg

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

   And this is a really bad idea:

   #+begin_src python
     import gpg

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

   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:

   #+begin_src python
     import gpg

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


*** Counting keys
    :PROPERTIES:
    :CUSTOM_ID: howto-keys-counting
    :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


** Get key
   :PROPERTIES:
   :CUSTOM_ID: howto-get-key
   :END:

   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:

   #+begin_src python
     import gpg

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

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

   #+begin_src python
     import gpg

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

   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.


* 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))
      logrus = []

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

      cipher = c.encrypt(text, recipients=logrus, 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=logrus, 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))
      logrus = []

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

      try:
	  cipher = 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:
	      cipher = c.encrypt(text, recipients=logrus, 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.

**** Encrypting to one key using the second method
     :PROPERTIES:
     :CUSTOM_ID: howto-basic-encryption-monogamous
     :END:

     This example re-creates the first encryption example except it
     uses the same =encrypt= method used in the subsequent examples
     instead of the =op_encrypt= method.  This means that, unlike the
     =op_encrypt= method, it /must/ use byte literal input data.

     #+begin_src python
       import gpg

       rkey = "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)
       rpattern = list(c.keylist(pattern=rkey, secret=False))
       logrus = []

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

       cipher = c.encrypt(text, recipients=logrus, sign=False, always_trust=True)

       afile = open("secret_plans.txt.asc", "wb")
       afile.write(cipher[0])
       afile.close()
     #+end_src

     With one or two exceptions, this method will probably prove to be
     easier to implement than the first method and thus it is the
     recommended encryption method.  Though it is even more likely to
     be used like this:

     #+begin_src python
       import gpg

       rkey = "0x12345678DEADBEEF"

       afile = open("secret_plans.txt", "rb")
       text = afile.read()
       afile.close()

       c = gpg.Context(armor=True)
       rpattern = list(c.keylist(pattern=rkey, secret=False))
       logrus = []

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

       cipher = c.encrypt(text, recipients=logrus, sign=False, always_trust=True)

       afile = open("secret_plans.txt.asc", "wb")
       afile.write(cipher[0])
       afile.close()
     #+end_src


** 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:

   The following sections demonstrate how to specify

*** 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.

    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.

*** Normal or default signing messages or files
    :PROPERTIES:
    :CUSTOM_ID: howto-basic-signing-normal
    :END:

    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.

   #+begin_src python
     import gpg

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

     """
     text = text0.encode("utf-8")

     c = gpg.Context(armor=True, signers=sig_src)
     signed = c.sign(text, mode=0)

     afile = open("/path/to/statement.txt.asc", "w")
     for line in signed[0]:
	 afile.write("{0}\n".format(line.decode("utf-8")))
     afile.close()
   #+end_src

   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 perfprmed more like the way it is done
   in the next example.  Even if the output format is ASCII armoured.

   #+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 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).

    #+begin_src python
      import gpg

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

      """
      text = text0.encode("utf-8")

      c = gpg.Context(armor=True)
      signed = c.sign(text, mode=1)

      afile = open("/path/to/statement.txt.asc", "w")
      for line in signed[0].splitlines()L
	  afile.write("{0}\n".format(line.decode("utf-8")))
      afile.close()
    #+end_src

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

    #+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:

    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.

    #+begin_src python
      import gpg

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

      """
      text = text0.encode("utf-8")

      c = gpg.Context()
      signed = c.sign(text, mode=2)

      afile = open("/path/to/statement.txt.asc", "w")
      for line in signed[0].splitlines():
	  afile.write("{0}\n".format(line.decode("utf-8")))
      afile.close()
    #+end_src

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

    #+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=2)

      afile = open("/path/to/statement.txt.asc", "wb")
      afile.write(signed[0])
      afile.close()
    #+end_src


** Signature verification
   :PROPERTIES:
   :CUSTOM_ID: howto-basic-verification
   :END:

   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:

   #+begin_src python
     import gpg
     import time

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

     c = gpg.Context()

     try:
	 verified = c.verify(open(gpg_file))
     except gpg.errors.BadSignatures as e:
	 verified = None
	 print(e)

     if verified is not None:
	 for i in range(len(verified[1].signatures)):
	     sign = verified[1].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(e)
   #+end_src

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

   #+begin_src python
     import gpg
     import time

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

     c = gpg.Context()

     try:
	 verified = c.verify(open(asc_file))
     except gpg.errors.BadSignatures as e:
	 verified = None
	 print(e)

     if verified is not None:
	 for i in range(len(verified[1].signatures)):
	     sign = verified[1].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
   #+end_src

   In both of the previous examples it is also possible to compare the
   original data that was signed against the signed data in
   =verified[0]= to see if it matches with something like this:

   #+begin_src python
     afile = open(filename, "rb")
     text = afile.read()
     afile.close()

     if text == verified[0]:
	 print("Good signature.")
     else:
	 pass
   #+end_src

   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 =verified[0]= is None and only
   the data in =verified[1]= is available.

   #+begin_src python
     import gpg
     import time

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

     c = gpg.Context()

     try:
	 verified = c.verify(open(filename), open(sig_file))
     except gpg.errors.BadSignatures as e:
	 verified = None
	 print(e)

     if verified is not None:
	 for i in range(len(verified[1].signatures)):
	     sign = verified[1].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
   #+end_src

   #+begin_src python
     import gpg
     import time

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

     c = gpg.Context()

     try:
	 verified = c.verify(open(filename), open(asc_file))
     except gpg.errors.BadSignatures as e:
	 verified = None
	 print(e)

     if verified is not None:
	 for i in range(len(verified[1].signatures)):
	     sign = verified[1].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
   #+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.