[PATCH 14/18] cli/reindex: destroy stashed session keys when --try-decrypt=false

[Daniel Kahn Gillmor <dkg@fifthhorseman.net>]

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 --try-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.
---
 doc/man1/notmuch-reindex.rst  |  3 +++
 lib/message.cc                |  5 +++++
 test/T357-index-decryption.sh | 17 +++++++++++++++++
 3 files changed, 25 insertions(+)

diff --git a/doc/man1/notmuch-reindex.rst b/doc/man1/notmuch-reindex.rst
index b15981a2..6f5e48a9 100644
--- a/doc/man1/notmuch-reindex.rst
+++ b/doc/man1/notmuch-reindex.rst
@@ -30,6 +30,9 @@ Supported options for **reindex** include
         the user's secret keys.  If decryption is successful, index
         the cleartext itself.
 
+        If ``false``, notmuch reindex will also delete any stashed
+        session keys for all messages matching the search terms.
+
         Be aware that the index is likely sufficient to reconstruct
         the cleartext of the message itself, so please ensure that the
         notmuch message index is adequately protected. DO NOT USE
diff --git a/lib/message.cc b/lib/message.cc
index 12743460..cfd99130 100644
--- a/lib/message.cc
+++ b/lib/message.cc
@@ -2002,6 +2002,11 @@ notmuch_message_reindex (notmuch_message_t *message,
     ret = notmuch_message_remove_all_properties_with_prefix (message, "index.");
     if (ret)
 	goto DONE; /* XXX TODO: distinguish from other error returns above? */
+    if (indexopts && notmuch_indexopts_get_try_decrypt (indexopts) == NOTMUCH_DECRYPT_FALSE) {
+	ret = notmuch_message_remove_all_properties (message, "session-key");
+	if (ret)
+	    goto DONE;
+    }
 
     /* re-add the filenames with the associated indexopts */
     for (; notmuch_filenames_valid (orig_filenames);
diff --git a/test/T357-index-decryption.sh b/test/T357-index-decryption.sh
index 7fc59f1e..4f5a501a 100755
--- a/test/T357-index-decryption.sh
+++ b/test/T357-index-decryption.sh
@@ -218,6 +218,23 @@ test_expect_equal \
     "$output" \
     "$expected"
 
+test_begin_subtest "purging stashed session keys should lose access to the cleartext"
+notmuch reindex --try-decrypt=false id:simple-encrypted@crypto.notmuchmail.org
+output=$(notmuch search sekrit)
+expected=''
+test_expect_equal \
+    "$output" \
+    "$expected"
+
+test_begin_subtest "and cleartext should be unrecoverable now that there are no stashed session keys"
+notmuch dump
+notmuch reindex --try-decrypt=true id:simple-encrypted@crypto.notmuchmail.org
+output=$(notmuch search sekrit)
+expected=''
+test_expect_equal \
+    "$output" \
+    "$expected"
+
 
 # TODO: test removal of a message from the message store between
 # indexing and reindexing.
-- 
2.14.2