Here's the configuration choice for people who want a cleartext index,
but don't want stashed session keys.
Interestingly, this "nostash" decryption policy is actually the same
policy that should be used by "notmuch show" and "notmuch reply",
since they never modify the index or database when they are invoked
with --decrypt.
We take advantage of this parallel to tune the behavior of those
programs so that we're not requesting session keys from GnuPG during
"show" and "reply" that we would then otherwise just throw away.
If you're going to store the cleartext index of an encrypted message,
in most situations you might just as well store the session key.
Doing this storage has efficiency and recoverability advantages.
Combined with a schedule of regular OpenPGP subkey rotation and
destruction, this can also offer security benefits, like "deletable
e-mail", which is the store-and-forward analog to "forward secrecy".
But wait, i hear you saying, i have a special need to store cleartext
indexes but it's really bad for me to store session keys! Maybe
(let's imagine) i get lots of e-mails with incriminating photos
attached, and i want to be able to search for them by the text in the
e-mail, but i don't want someone with access to the index to be
actually able to see the photos themselves.
Fret not, the next patch in this series will support your wacky
uncommon use case.
There are some situations where the user wants to get rid of the
cleartext index of a message. For example, if they're indexing
encrypted messages normally, but suddenly they run across a message
that they really don't want any trace of in their index.
In that case, the natural thing to do is:
notmuch reindex --decrypt=false id:whatever@example.biz
But of course, clearing the cleartext index without clearing the
stashed session key is just silly. So we do the expected thing and
also destroy any stashed session keys while we're destroying the index
of the cleartext.
Note that stashed session keys are stored in the xapian database, but
xapian does not currently allow safe deletion (see
https://trac.xapian.org/ticket/742).
As a workaround, after removing session keys and cleartext material
from the database, the user probably should do something like "notmuch
compact" to try to purge whatever recoverable data is left in the
xapian freelist. This problem really needs to be addressed within
xapian, though, if we want it fixed right.
The new "auto" decryption policy is not only good for "notmuch show"
and "notmuch reindex". It's also useful for indexing messages --
there's no good reason to not try to go ahead and index the cleartext
of a message that we have a stashed session key for.
This change updates the defaults and tunes the test suite to make sure
that they have taken effect.
When showing a message, if the user doesn't specify --decrypt= at all,
but a stashed session key is known to notmuch, notmuch should just go
ahead and try to decrypt the message with the session key (without
bothering the user for access to their asymmetric secret key).
The user can disable this at the command line with --decrypt=false if
they really don't want to look at the e-mail that they've asked
notmuch to show them.
and of course, "notmuch show --decrypt" still works for accessing the
user's secret keys if necessary.
If the user doesn't specify --decrypt= at all, but a stashed session
key is known to notmuch, when replying to an encrypted message,
notmuch should just go ahead and decrypt.
The user can disable this at the command line with --decrypt=false,
though it's not clear why they would ever want to do that.
This new automatic decryption policy should make it possible to
decrypt messages that we have stashed session keys for, without
incurring a call to the user's asymmetric keys.
Future patches in this series will introduce new policies; this merely
readies the way for them.
We also convert --try-decrypt to a keyword argument instead of a boolean.
the command-line interface for indexing (reindex, new, insert) used
--try-decrypt; and the configuration records used index.try_decrypt.
But by comparison with "show" and "reply", there doesn't seem to be
any reason for the "try" prefix.
This changeset adjusts the command-line interface and the
configuration interface.
For the moment, i've left indexopts_{set,get}_try_decrypt alone. The
subsequent changeset will address those.
If the version of GMime we're building against doesn't support session
key extraction or re-use, mark the tests that rely on session key
capabilities as known-broken.
This should resolve test suite failures on ubuntu trusty and debian
jessie and earlier, which have GMime 2.6.20 -- session key support was
introduced in GMime 2.6.21.
If you've got a notmuch dump that includes stashed session keys for
every decrypted message, and you've got your message archive, you
should be able to get back to the same index that you had before.
Here we add a simple test that give some flavor of how that works.
Enable override of the index.try_decrypt setting on a per-run basis
when invoking "notmuch reindex". This allows the possibility of (for
example) an emacs keybinding that adds the cleartext of the currently
shown decrypted message to the index, making it searchable in the
future.
It also enables one-time indexing of all messages matching some query,
like so:
notmuch reindex tag:encrypted and\
not property:index.decryption=success and\
from:alice@example.org
We also update the documentation and tab completion, and add a few
more tests.
Enable override of the index.try_decrypt setting on a per-message
basis when invoking "notmuch insert".
We also update the documentation and tab completion, and add more tests.
Enable override of the index.try_decrypt setting during "notmuch new"
on a per-invocation basis.
We update the documentation and tab completion, and also add a test.
