[Mailman-Developers] PGP-signed message verification using the email module (and in Mailman)
paul at boddie.org.uk
Wed Jan 8 18:35:02 CET 2014
Sorry if this is too off-topic for the Mailman list and should really be
directed to the email-sig list, but as I noticed that one of the GSOC projects
touched upon message signing and verification, I thought that there might be
more expertise on hand amongst this list's members, and there is a remote
possibility that some of my difficulties can inform that work, too.
I've been looking into using the Python email module for signing and verifying
messages according to RFC 3156, but one difficulty I encountered was where a
message with the following form is received and parsed:
-> Content-Type: ...
This is the content being signed (together with the part's headers).
-> Content-Type: application/pgp-signature
-----BEGIN PGP SIGNATURE-----
-----END PGP SIGNATURE-----
Obviously, the email module permits the inspection of the message, and the
apparent solution in verifying the message is to obtain the signed content
along with the signature (using the get_payload method on the top-level
multipart container) and then to pass them both to gpg for verification.
Something like this...
content, signature = message.get_payload()[:2]
We might also choose to decode the signature, I imagine, since it may employ a
signature = signature.get_payload(decode=True)
However, in an almost accidental discovery involving Python 2.5 and 2.7 being
used for the respective signing and verification operations (and in reverse),
what I found was that the email module would parse the signed content part but
then, upon being asked to provide the string representation of the part, it
would reformat the headers and thus cause the verification to fail.
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.
The behaviour of the module is fairly understandable - no-one wants to keep
the original data around in addition to having a more manageable parsed form
of it - but is there a convenient way of preserving the form of the original
message for future use? It seems that asking for the individual headers will
yield the original formatting, and that the _headers attribute on Message
instances provides the original headers in order, but I wonder if I'm missing
a simple method to get things as they were received and not as the particular
version of the module wants to format them.
Of course, RFC 3156 warns about the pitfalls of encoding the part that is to
be signed, and I have been tempted to use a conservative approach where signed
content is wrapped up inside an opaque message part with headers that are
unlikely to be rewritten, but if I can manage to work with the email module
rather than around it, I can hopefully steer clear of such inelegant measures.
Does anyone have any opinions or experience that they would like to share on
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