Paul Boddie's Free Software-related blog

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Python’s email Package and the “PGP/MIME” Question

I vaguely follow the development of Mailpile – the Free Software, Web-based e-mail client – and back in November 2014, there was a blog post discussing problems that the developers had experienced while working with PGP/MIME (or OpenPGP as RFC 3156 calls it). A discussion also took place on the Gnupg-users mailing list, leading to the following observation:

Yes, Mailpile is written in Python and I've had to bend over backwards
in order to validate and generate signatures. I am pretty sure I still
have bugs to work out there, trying to compensate for the Python
library's flaws without rewriting the whole thing is, long term, a
losing game. It is tempting to blame the Python libraries, but the fact
is that they do generate valid MIME - after swearing at Python for
months, it dawned on me that it's probably the PGP/MIME standard that is
just being too picky.

Later, Bjarni notes…

Similarly, when generating messages I had to fork the Python lib's
generator and disable various "helpful" hacks that were randomly
mutating the behavior of the generator if it detected it was generating
an encrypted message!

Coincidentally, while working on PGP/MIME messaging in another context, I also experienced some problems with the Python email package, mentioning them on the Mailman-developers mailing list because I had been reading that list and was aware that a Google Summer of Code project had previously been completed in the realm of message signing, thus offering a potential source of expertise amongst the list members. Although I don’t think I heard anything from the GSoC participant directly, I had the benefit of advice from the same correspondent as Bjarni, even though we have been using different mailing lists!

Here‘s what Bjarni learned about the “helpful” hacks:

This is supposed to be http://bugs.python.org/issue1670765, which is
claimed to be resolved.

Unfortunately, the special-case handling of headers for “multipart/signed” parts is presumably of limited “help”, and other issues remain. As I originally noted

So, where the email module in Python 2.5 likes to wrap headers using tab
character indents, the module in Python 2.7 prefers to use a space for
indentation instead. This means that the module reformats data upon being
asked to provide a string representation of it rather than reporting exactly
what it received.

Why the special-casing wasn’t working for me remains unclear, and so my eventual strategy was to bypass the convenience method in the email API in order to assert some form of control over the serialisation of e-mail messages. It is interesting to note that the “fix” to the Python standard library involved changing the documentation to note the unsatisfactory behaviour and to leave the problem essentially unsolved. This may not have been unreasonable given the design goals of the email package, but it may have been better to bring the code into compliance with user expectations and to remedy what could arguably be labelled a design flaw of the software, even if it was an unintended one.

Contrary to the expectations of Python’s core development community, I still develop using Python 2 and probably won’t see any fixes to the standard library even if they do get made. So, here’s my workaround for message serialisation from my concluding message to the Mailman-developers list:

# given a message...
out = StringIO()
generator = Generator(out, False, 0) # disable reformatting measures
generator.flatten(message)
# out.getvalue() now provides the serialised message

It’s interesting to see such problems occur for different people a few months apart. Maybe I should have been following Mailpile development a bit more closely, but with it all happening at GitHub (with its supposedly amazing but, in my experience, rather sluggish and clumsy user interface), I suppose I haven’t been able to keep up.

Still, I hope that others experiencing similar difficulties become more aware of the issues by seeing this article. And I hope that Bjarni and the Mailpile developers haven’t completely given up on OpenPGP yet. We should all be working more closely together to get usable, Free, PGP-enabled, standards-compliant mail software to as many people as possible.

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