Feature suggestion. Indexing encrypted mail?

[Jameson Graef Rollins]

On Sat, Apr 05 2014, David Bremner <david at tethera.net> wrote:
> john.wyzer at gmx.de writes:
>
>> Would it be possible to add the configurable option to also decrypt
>> encrypted messages on the fly while indexing to make them searchable,
>> too?
>>
>> That would be really great for people that consider gnupg  mainly an
>> encryption for transport or have their complete hard drive encrypted...
>
> As far I understand an attacker could reconstruct the message from the
> index, so one question is whether the extra complexity in notmuch is
> worth the minimal extra security over decrypting on delivery and storing
> plaintext on the (presumably encrypted) disk. Of course decrypting on
> delivery may be inconvenient (or impossible). I have CCed the two people
> who have implemented most of the crypto related stuff in notmuch so they
> can comment.

Indexing encrypted email is a bit of a foot-gun, since, as David
mentions, it is apparently possible to reconstruct encrypted messages
From the index.  It therefore needs to be approached with care.

I think decrypting on "delivery" (or mail fetch or whatever) sounds
difficult and unwieldy.  In either event, it seems out of the scope of
notmuch.  If a user figured out how to have that done, no changes to
notmuch would be needed afaict.
 
I don't think it would be difficult modify notmuch new to decrypt at
indexing time.  Given that gnupg agent would be used for accessing the
users private key for decryption, the interface would be fairly
straightforward.

A couple of decryption options could be added to notmuch new:

* don't decrypt: don't attempt to decrypt and index any encrypted
  message (default)

* decrypt always: fail if any encrypted message could not be decrypted

* decrypt opportunistically: attempt to decrypt, but continue indexing
  if an encrypted message could not be decrypted

If something like this is enabled, we should make sure we make the
dangers clear to the users.

jamie.
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