dkimpy

dkimpy - DKIM (DomainKeys Identified Mail) https://launchpad.net/dkimpy/

Friendly fork of: http://hewgill.com/pydkim/

INTRODUCTION

dkimpy is a library that implements DKIM (DomainKeys Identified Mail) email signing and verification. Basic DKIM requirements are defined in RFC 6376:

https://tools.ietf.org/html/rfc6376

VERSION

This is dkimpy 1.1.8.

REQUIREMENTS

Dependencies will be automatically included for normal DKIM usage. The extras_requires feature 'ed25519' will add the dependencies needed for signing and verifying using the new DCRUP ed25519-sha256 algorithm. The extras_requires feature 'ARC' will add the extra dependencies needed for ARC. Similarly, extras_requires feature 'asyncio' will add the extra dependencies needed for asyncio.

• Python 3.x >= 3.5. Recent versions have not been tested on python3 < 3.4, but may still work on earlier python3 versions.
• dnspython or py3dns. dnspython is preferred if both are present and installed to satisfy the DNS module requirement if neither are installed.
• authres. Needed for ARC.
• PyNaCl. Needed for use of ed25519 capability.
• aiodns. Needed for asycnio (Requires python3.5 or later)

INSTALLATION

This package includes a scripts and man pages. For those to be installed when installing using setup.py, the following incantation is required because setuptools developers decided not being able to do this by default is a feature:

python3 setup.py install --single-version-externally-managed --record=/dev/null

DOCUMENTATION

An online version of the package documentation for the most recent release can be found at:

https://pymilter.org/pydkim/

TESTING

To run dkimpy's test suite:

PYTHONPATH=. python3 dkim

or

python3 test.py

or

PYTHONPATH=. python3 -m unittest dkim.tests.test_suite

Alternatively, if you have testrepository installed:

testr init

testr run

You should install all optional dependencies required for the test suite, e.g. by creating a virtualenv and using:

pip install -e '.[testing]'

The included ARC tests are very limited. The primary testing method for ARC is using the ARC test suite: https://github.com/ValiMail/arc_test_suite

As of 0.6.0, all tests pass for both python2.7 and python3. The test suite ships with test runners for dkimpy. After downloading the test suite, you can run the signing and validation tests like this:

python3 ./testarc.py sign runners/arcsigntest.py python3 ./testarc.py validate runners/arcverifytest.py

As ov version 1.1.0, python2.7 is no longer supported.

USAGE

The dkimpy library offers one module called dkim. The sign() function takes an RFC822 formatted message, along with some signing options, and returns a DKIM-Signature header line that can be prepended to the message. The verify() function takes an RFC822 formatted message, and returns True or False depending on whether the signature verifies correctly. There is also a DKIM class which can be used to perform these functions in a more modern way.

In version 0.9.0, the default set of header fields that are oversigned was changed from 'from', 'subject', 'date' to 'from' to reduce fragility of signatures. To restore the previous behavior, you can add them back after instantiating your DKIM class using the add_frozen function as shown in the following example:

>>> dkim = DKIM()
>>> dkim.add_frozen((b'date',b'subject'))
>>> [text(x) for x in sorted(dkim.frozen_sign)]
['date', 'from', 'subject']

DKIM RSA MODERNIZATION (RFC 8301)

RFC8301 updated DKIM requirements in two ways:

1. It set the minimum valid RSA key size to 1024 bits.
2. It removed use of rsa-sha1.

As of version 0.7, the dkimpy defaults largely support these requirements.

It is possible to override the minimum key size to a lower value, but this is strongly discouraged. As of 2018, keys much smaller than the minimum are not difficult to factor.

The code for rsa-sha1 signing and verification is retained, but not used for signing by default. Future releases will raise warnings and then errors when verifying rsa-sha1 signatures. There are still some significant users of rsa-sha1 signatures, so operationally it's premature to disable verification of rsa-sha1.

ED25519 (RFC 8463) SUPPORT

As of version 0.7, experimental signing and verifying of DKIM Ed25519 signatures is supported as described in draft-ietf-dcrup-dkim-crypto:

https://datatracker.ietf.org/doc/draft-ietf-dcrup-dkim-crypto/

The RFC that documents ed25519 DKIM signatures, RFC 8463, has been released and dkimpy 0.7 and later are aligned to its requirements. As of 0.8, ed25519 need not be considered experimental. The dkimpy implementation has successfully interoperated with three other implementations and the technical parameters for ed25519-sha256 are defined and stable.

To install from pypi with the required optional depenencies, use the ed25519 option:

pip install -e '.[ed25519]'

DKIM SCRIPTS

Three helper programs are also supplied: dknewkey, dkimsign and dkimverify

dknewkey is s script that produces private and public key pairs suitable for use with DKIM. Note that the private key file format used for ed25519 is not standardized (there is no standard) and is unique to dkimpy. Creation of keys should be done in a secure environment. If an unauthorized entity gains access to current private keys they can generate signed email that will pass DKIM checkes and will be difficult to repudiate.

dkimsign is a filter that reads an RFC822 message on standard input, and writes the same message on standard output with a DKIM-Signature line prepended. The signing options are specified on the command line:

dkimsign selector domain privatekeyfile [identity]

The identity is optional and defaults to "@domain".

dkimverify reads an RFC822 message on standard input, and returns with exit code 0 if the signature verifies successfully. Otherwise, it returns with exit code 1.

ARC (Authenticated Receive Chain)

As of version 0.6.0, dkimpy provides experimental support for ARC (Authenticated Received Chain). See RFC 8617 for the current version of ARC:

https://tools.ietf.org/html/rfc8617

In addition to arcsign and arcverify, the dkim module now provides arc_sign and arc_verify functions as well as an ARC class.

If an invalid authentication results header field is included in the set for ARC, it is ignored and no error is raised.

Both DKIM ed25519 and ARC are now considered stable (no longer experimantal).

ASYNC SUPPORT

As of version 1.0, an alternative to dkim.verify for use in an async environment is provied. It requires aiodns, https://pypi.org/project/aiodns/. Here is a simple example of dkim.verify_async usage:

>>> sys.stdin = sys.stdin.detach()
>>>
>>> async def main():
>>>     res = await dkim.verify_async(message)
>>>     return res
>>>
>>> if __name__ == "__main__":
>>>     res = asyncio.run(main())

This feature requires python3.5 or newer.

If aiodns is available, the async functions will be used. To avoide async when aiodns is availale, set dkim.USE_ASYNC = False.

TLSRPT (TLS Report)

As of version 1.0, the RFC 8460 tlsrpt service type is supported:

https://tools.ietf.org/html/rfc8460

A non-tlsrpt signed with a key record with s=tlsrpt won't verify. Since the service type (s=) is optional in the DKIM public key record, it is not required by RFC 8460. When checking for a tlsrpt signature, set the tlsrpt= flag when verifying the signature:

>>> res = dkim.verify(smessage, tlsrpt='strict')

If tlsrpt='strict', only public key records with s=tlsrpt will be considered valid. If set to tlsrpt=True, the service type is not required, but other RFC 8460 requirements are applied.

LIMITATIONS

Dkimpy will correctly sign/verify messages with ASCII or UTF-8 content. Messages that contain other types of content will not verify correctly. It does not yet implement RFC 8616, Email Authentication for Internationalized Mail.

FEEDBACK

Bug reports may be submitted to the bug tracker for the dkimpy project on launchpad.