As we prepare to handle S/MIME-encrypted PKCS#7 EnvelopedData (which
is not multipart), we don't want to be limited to passing only
GMimeMultipartEncrypted MIME parts to _notmuch_crypto_decrypt.
There is no functional change here, just a matter of adjusting how we
pass arguments internally.
Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
Our _notmuch_message_crypto_potential_payload implementation could
only return a failure if bad arguments were passed to it. It is an
internal function, so if that happens it's an entirely internal bug
for notmuch.
It will be more useful for this function to return whether or not the
part is in fact a cryptographic payload, so we dispense with the
status return.
If some future change suggests adding a status return back, there are
only a handful of call sites, and no pressure to retain a stable API,
so it could be changed easily. But for now, go with the simpler
function.
We will use this return value in future patches, to make different
decisions based on whether a part is the cryptographic payload or not.
But for now, we just leave the places where it gets invoked marked
with (void) to show that the result is ignored.
Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
_notmuch_message_crypto_potential_payload is called on a GMimeObject
while walking the MIME tree of a message to determine whether that
object is the payload. It doesn't make sense to name the argument
"payload" if it might not be the payload, so we rename it to "part"
for clarity.
This is a non-functional change, just semantic cleanup.
Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
When walking the MIME tree, if we discover that we are at the
cryptographic payload, then we would like to record at least the
Subject header of the current MIME part.
In the future, we might want to record many other headers as well, but
for now we will stick with just the Subject.
See
https://dkg.fifthhorseman.net/blog/e-mail-cryptography.html#cryptographic-envelope
for more description of the Cryptographic Payload vs. the
Cryptographic Envelope.
Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
E-mail encryption and signatures reported by notmuch are at the MIME
part level. This makes sense in the dirty details, but for users we
need to have a per-message conception of the cryptographic state of
the e-mail. (see
https://dkg.fifthhorseman.net/blog/e-mail-cryptography.html for more
discussion of why this is important).
The object created in this patch is a useful for tracking the
cryptographic state of the underlying message as a whole, based on a
depth-first search of the message's MIME structure.
This object stores a signature list of the message, but we don't
handle it yet. Further patches in this series will make use of the
signature list.
The comment line here lingers from when we were using some fancy
version checking about session keys. Correct it to match the current
state.
Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
This means dropping GMimeCryptoContext and notmuch_config arguments.
All the argument changes are to internal functions, so this is not an
API or ABI break.
We also get to drop the #define for g_mime_3_unused.
signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
Note that we do keep ignoring the gpg_path configuration option,
though, to avoid breakage of existing installations. It is ignored
like any other unknown configuration option, but we at least document
that it is ignored so that people who find it in their legacy configs
can know that it's safe to drop.
signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
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.
In our consolidation of _notmuch_crypto_decrypt, the callers lost
track a little bit of whether any actual decryption was attempted.
Now that we have the more-subtle "auto" policy, it's possible that
_notmuch_crypto_decrypt could be called without having any actual
decryption take place.
This change lets the callers be a little bit smarter about whether or
not any decryption was actually attempted.
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.
When doing any decryption, if the notmuch database knows of any
session keys associated with the message in question, try them before
defaulting to using default symmetric crypto.
This changeset does the primary work in _notmuch_crypto_decrypt, which
grows some new parameters to handle it.
The primary advantage this patch offers is a significant speedup when
rendering large encrypted threads ("notmuch show") if session keys
happen to be cached.
Additionally, it permits message composition without access to
asymmetric secret keys ("notmuch reply"); and it permits recovering a
cleartext index when reindexing after a "notmuch restore" for those
messages that already have a session key stored.
Note that we may try multiple decryptions here (e.g. if there are
multiple session keys in the database), but we will ignore and throw
away all the GMime errors except for those that come from last
decryption attempt. Since we don't necessarily know at the time of
the decryption that this *is* the last decryption attempt, we'll ask
for the errors each time anyway.
This does nothing if no session keys are stashed in the database,
which is fine. Actually stashing session keys in the database will
come as a subsequent patch.
We will use this centralized function to consolidate the awkward
behavior around different gmime versions.
It's only invoked from two places: mime-node.c's
node_decrypt_and_verify() and lib/index.cc's
_index_encrypted_mime_part().
However, those two places have some markedly distinct logic, so the
interface for this _notmuch_crypto_decrypt function is going to get a
little bit clunky. It's worthwhile, though, for the sake of keeping
these #if directives reasonably well-contained.
