Inline-encryption, encryption failure when storing sent mails
Daniel Kahn Gillmor
dkg at fifthhorseman.net
Tue Aug 20 10:03:27 PDT 2013
On 08/16/2013 04:02 AM, David Bremner wrote:
> Simon Hirscher <public at simonhirscher.de> writes:
>
>> 1. Support for inline-encryption As far as I can see, so far only
>> encrypted mails with PGP/MIME are supported. Couldn't notmuch also
>> support text/plain messages that contain PGP-encrypted messages by
>> scanning for "^-----BEGIN\ PGP\ (SIGNED\ )?MESSAGE"? – as suggested in
>> a previous message to this mailing list
>> (id:87zl3az8mm.fsf at lillypad.riseup.net; web view:
>
> If someone feels inspired to work on this, maybe
>
> notmuch-wash-convert-inline-patch-to-part
>
> (in notmuch-watch.el) might be a reasonable place to start.
if anyone does feel inclined to work on this, please consider that
dealing cleanly an inline-signed message has a number of serious
problems, not least of which is the Content-Type.
I've been meaning to write this up more cleanly, but a summary here will
have to do for now:
The MIME Content-Type header for an inline-PGP-signed e-mail message is
not signed. This means that an attacker can replay a signed message
while undetectably changing the Content-Type. One example of such an
attack is to leave the base Content-Type as text/plain but to switch
charsets -- the same bytestream can then be interpreted differently.
For example, depending on the charset, the same bytestream can be
represented as:
The rental is €13/week for unit 7.
[charset=big5]
or:
The rental is £ב13/week for unit 7.
[charset=iso-8859-8]
since 1GBP = 1.17EUR, this represents a change of 17% in the value of
the signed message while retaining the signature's validity :P
Given that you don't have cryptographically-reliable Content-Type
information, will you be comfortable indicating that the message is
actually signed?
Also, inline-signed messages may not span the entire part. That is, a
message could have a bit of unsigned text above or below the
inline-signature. The current user-facing UI in notmuch-emacs indicates
whether each part is individually signed or not. How would
notmuch-emacs indicate reliably to the user that only a portion of the
part is signed?
In short: inline PGP is a mess, and existing implementations which try
to cope with it have severe shortcomings. I'd rather avoid introducing
new types of failure to notmuch.
--dkg
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