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Ben McGinnes de11f557a8 docs: python bindings howto 
* Updated groups work around example to make sure it uses the correct
  version of gpgconf for the user invoking it (determined by the
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#+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 <ben@gnupg.org>}
#+HTML_HEAD_EXTRA: <link type="application/rss+xml" href="https://git.gnupg.org/cgi-bin/gitweb.cgi?p=gpgme.git;a=rss;f=lang/python/docs/GPGMEpythonHOWTOen.org"/>
* Introduction
:PROPERTIES:
:CUSTOM_ID: intro
:END:
| Version: | 0.1.0 |
| 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.
** 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 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.
** Examples
:PROPERTIES:
:CUSTOM_ID: howto-python3-examples
:END:
All of the examples found in this document can be found as Python 3
scripts in the =lang/python/examples/howto= directory.
* 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 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.
** 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 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.
*** 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.
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).
*** 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 the 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 General 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.
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.
** 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= 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= 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 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.
* 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 superseded 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 encryption 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:
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=).
#+begin_src python
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)
#+end_src
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:
#+begin_src python
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)
#+end_src
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.
*** Encrypting to multiple keys
:PROPERTIES:
:CUSTOM_ID: howto-basic-encryption-multiple
:END:
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,[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])
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)
#+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
ciphertext, result, sign_result = 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
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)
except:
pass
with open("secret_plans.txt.asc", "wb") as afile:
afile.write(ciphertext)
#+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-decryption
: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 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:
plaintext, result, verify_result = gpg.Context().decrypt(cfile)
with open(newfile, "wb") as nfile:
nfile.write(plaintext)
#+end_src
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=.
** Signing text and files
:PROPERTIES:
:CUSTOM_ID: howto-basic-signing
:END:
The following sections demonstrate how to specify keys to sign with.
*** 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()
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())
#+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 performed more like the way it is done
in the next example. Even if the output format is ASCII armoured.
#+begin_src python
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)
#+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()
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())
#+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
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)
#+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()
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())
#+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
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)
#+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:
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
#+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:
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
#+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 =data= to
see if it matches with something like this:
#+begin_src python
with open(filename, "rb") as afile:
text = afile.read()
if text == data:
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 =data= is =None= and only the
information in =result= is available.
#+begin_src python
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
#+end_src
#+begin_src python
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 not None:
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
#+end_src
* Creating keys and subkeys
:PROPERTIES:
:CUSTOM_ID: key-generation
:END:
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:
#+begin_src conf
expert
allow-freeform-uid
allow-secret-key-import
trust-model tofu+pgp
tofu-default-policy unknown
enable-large-rsa
enable-dsa2
# cert-digest-algo SHA256
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
#+end_src
** Primary key
:PROPERTIES:
:CUSTOM_ID: keygen-primary
:END:
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=.
#+begin_src python
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)
#+end_src
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:
#+begin_src python
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))
#+end_src
Alternatively the information can be confirmed using the command
line program:
#+begin_src shell
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$
#+end_src
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:
#+begin_src shell
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$
#+end_src
** Subkeys
:PROPERTIES:
:CUSTOM_ID: keygen-subkeys
:END:
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.
#+begin_src python
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)
#+end_src
As with the primary key, the results here can be checked with:
#+begin_src python
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))
#+end_src
As well as on the command line with:
#+begin_src shell
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$
#+end_src
** User IDs
:PROPERTIES:
:CUSTOM_ID: keygen-uids
:END:
*** Adding User IDs
:PROPERTIES:
:CUSTOM_ID: keygen-uids-add
:END:
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=.
#+begin_src python
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)
#+end_src
Unsurprisingly the result of this is:
#+begin_src shell
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$
#+end_src
*** Revokinging User IDs
:PROPERTIES:
:CUSTOM_ID: keygen-uids-revoke
:END:
Revoking a user ID is a fairly similar process, except that it
uses the =key_revoke_uid= method.
#+begin_src python
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)
#+end_src
** Key certification
:PROPERTIES:
:CUSTOM_ID: key-sign
:END:
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:
#+begin_src python
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)
#+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 gpg
import subprocess
gpgconf = gpg.core.get_engine_info()[2].file_name
gpgconf_cmd "{0} --list-options gpg".format(gpgconf)
lines = subprocess.getoutput(gpgconf_cmd).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.
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