[PATCH 15/18] crypto: actually stash session keys when try-decrypt=true

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

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.
---
 doc/man1/notmuch-config.rst     | 10 ++++++----
 doc/man1/notmuch-insert.rst     | 13 +++++++------
 doc/man1/notmuch-new.rst        | 21 +++++++++++----------
 doc/man1/notmuch-reindex.rst    | 10 +++++-----
 doc/man7/notmuch-properties.rst |  4 ++++
 lib/index.cc                    | 16 +++++++++++++++-
 test/T357-index-decryption.sh   | 18 +++++++++++++++++-
 util/crypto.c                   | 16 +++++++++++++++-
 8 files changed, 80 insertions(+), 28 deletions(-)

diff --git a/doc/man1/notmuch-config.rst b/doc/man1/notmuch-config.rst
index ae7586f3..5adde070 100644
--- a/doc/man1/notmuch-config.rst
+++ b/doc/man1/notmuch-config.rst
@@ -143,10 +143,12 @@ The available configuration items are described below.
         **[STORED IN DATABASE]**
         When indexing an encrypted e-mail message, if this variable is
         set to ``true``, notmuch will try to decrypt the message and
-        index the cleartext.  If ``auto``, it will try to index the
-        cleartext if a stashed session key is already known for the message,
-        but will not try to access your secret keys.  Use ``false`` to
-        avoid decrypting even when a session key is already known.
+        index the cleartext, stashing a copy of any discovered session
+        keys for the message.  If ``auto``, it will try to index the
+        cleartext if a stashed session key is already known for the message
+        (e.g. from a previous copy), but will not try to access your
+        secret keys.  Use ``false`` to avoid decrypting even when a
+        stashed session key is already present.
 
         Be aware that the notmuch index is likely sufficient to
         reconstruct the cleartext of the message itself, so please
diff --git a/doc/man1/notmuch-insert.rst b/doc/man1/notmuch-insert.rst
index a5505b5b..3e6f538d 100644
--- a/doc/man1/notmuch-insert.rst
+++ b/doc/man1/notmuch-insert.rst
@@ -53,12 +53,13 @@ Supported options for **insert** include
     ``--try-decrypt=(true|auto|false)``
 
         If ``true`` and the message is encrypted, try to decrypt the
-        message while indexing.  If ``auto``, and notmuch already
-        knows about a session key for the message, it will try
-        decrypting using that session key but will not try to access
-        the user's secret keys.  If decryption is successful, index
-        the cleartext itself.  Either way, the message is always
-        stored to disk in its original form (ciphertext).
+        message while indexing, storing any session keys discovered.
+        If ``auto``, and notmuch already knows about a session key for
+        the message, it will try decrypting using that session key but
+        will not try to access the user's secret keys.  If decryption
+        is successful, index the cleartext itself.  Either way, the
+        message is always stored to disk in its original form
+        (ciphertext).
 
         Be aware that the index is likely sufficient to reconstruct
         the cleartext of the message itself, so please ensure that the
diff --git a/doc/man1/notmuch-new.rst b/doc/man1/notmuch-new.rst
index d8cb77f5..351b7591 100644
--- a/doc/man1/notmuch-new.rst
+++ b/doc/man1/notmuch-new.rst
@@ -46,16 +46,17 @@ Supported options for **new** include
     ``--try-decrypt=(true|auto|false)``
 
         If ``true``, when encountering an encrypted message, try to
-        decrypt it while indexing.  If decryption is successful, index
-        the cleartext itself.  If ``auto``, try to use any session key
-        already known to belong to this message, but do not attempt to
-        use the user's secret keys.
-
-        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 ``--try-decrypt=true`` without
-        considering the security of your index.
+        decrypt it while indexing, and store any discovered session
+        keys.  If ``auto``, try to use any session key already known
+        to belong to this message, but do not attempt to use the
+        user's secret keys.  If decryption is successful, index the
+        cleartext of the message.
+
+        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
+        ``--try-decrypt=true`` without considering the security of
+        your index.
 
         See also ``index.try_decrypt`` in **notmuch-config(1)**.
 
diff --git a/doc/man1/notmuch-reindex.rst b/doc/man1/notmuch-reindex.rst
index 6f5e48a9..3b99b934 100644
--- a/doc/man1/notmuch-reindex.rst
+++ b/doc/man1/notmuch-reindex.rst
@@ -24,11 +24,11 @@ Supported options for **reindex** include
     ``--try-decrypt=(true|auto|false)``
 
         If ``true``, when encountering an encrypted message, try to
-        decrypt it while reindexing.  If ``auto``, and notmuch already
-        knows about a session key for the message, it will try
-        decrypting using that session key but will not try to access
-        the user's secret keys.  If decryption is successful, index
-        the cleartext itself.
+        decrypt it while reindexing, storing any session keys
+        discovered.  If ``auto``, and notmuch already knows about a
+        session key for the message, it will try decrypting using that
+        session key but will not try to access 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.
diff --git a/doc/man7/notmuch-properties.rst b/doc/man7/notmuch-properties.rst
index e2ab43be..5dcbb4ae 100644
--- a/doc/man7/notmuch-properties.rst
+++ b/doc/man7/notmuch-properties.rst
@@ -98,6 +98,10 @@ of its normal activity.
     message.  This includes attachments, cryptographic signatures, and
     other material that cannot be reconstructed from the index alone.
 
+    See ``index.try_decrypt`` in **notmuch-config(1)** for more
+    details about how to set notmuch's policy on when to store session
+    keys.
+
     The session key should be in the ASCII text form produced by
     GnuPG.  For OpenPGP, that consists of a decimal representation of
     the hash algorithm used (identified by number from RFC 4880,
diff --git a/lib/index.cc b/lib/index.cc
index e2701755..06470919 100644
--- a/lib/index.cc
+++ b/lib/index.cc
@@ -549,11 +549,15 @@ _index_encrypted_mime_part (notmuch_message_t *message,
     }
 #endif
     bool attempted = false;
+    GMimeDecryptResult *decrypt_result = NULL;
+    bool get_sk = (notmuch_indexopts_get_try_decrypt (indexopts) == NOTMUCH_DECRYPT_TRUE);
     clear = _notmuch_crypto_decrypt (&attempted, notmuch_indexopts_get_try_decrypt (indexopts),
-				     message, crypto_ctx, encrypted_data, NULL, &err);
+				     message, crypto_ctx, encrypted_data, get_sk ? &decrypt_result : NULL, &err);
     if (!attempted)
 	return;
     if (err || !clear) {
+	if (decrypt_result)
+	    g_object_unref (decrypt_result);
 	if (err) {
 	    _notmuch_database_log (notmuch, "Failed to decrypt during indexing. (%d:%d) [%s]\n",
 				   err->domain, err->code, err->message);
@@ -568,6 +572,16 @@ _index_encrypted_mime_part (notmuch_message_t *message,
 					  "property (%d)\n", status);
 	return;
     }
+    if (decrypt_result) {
+	if (get_sk) {
+	    status = notmuch_message_add_property (message, "session-key",
+						   g_mime_decrypt_result_get_session_key (decrypt_result));
+	    if (status)
+		_notmuch_database_log (notmuch, "failed to add session-key "
+				       "property (%d)\n", status);
+	}
+	g_object_unref (decrypt_result);
+    }
     _index_mime_part (message, indexopts, clear);
     g_object_unref (clear);
 
diff --git a/test/T357-index-decryption.sh b/test/T357-index-decryption.sh
index 4f5a501a..3ac8c43e 100755
--- a/test/T357-index-decryption.sh
+++ b/test/T357-index-decryption.sh
@@ -48,6 +48,14 @@ test_expect_equal \
     "$output" \
     "$expected"
 
+test_begin_subtest "show the message body of the encrypted message"
+notmuch dump wumpus
+output=$(notmuch show wumpus | awk '/^\014part}/{ f=0 }; { if (f) { print $0 } } /^\014part{ ID: 3/{ f=1 }')
+expected='This is a test encrypted message with a wumpus.'
+test_expect_equal \
+    "$output" \
+    "$expected"
+
 
 test_begin_subtest "message should go away after deletion"
 # cache the message in an env var and remove it:
@@ -129,10 +137,18 @@ test_expect_equal \
     "$output" \
     "$expected"
 
+# try a simple reindexx
+test_begin_subtest 'reindex in auto mode'
+test_expect_success 'notmuch reindex tag:encrypted and property:index.decryption=success'
+test_begin_subtest "reindexed encrypted messages, should not have changed"
+output=$(notmuch search wumpus)
+test_expect_equal \
+    "$output" \
+    "$expected"
 
 # try to remove cleartext indexing
 test_begin_subtest 'reindex without cleartext'
-test_expect_success 'notmuch reindex tag:encrypted and property:index.decryption=success'
+test_expect_success 'notmuch reindex --try-decrypt=false tag:encrypted and property:index.decryption=success'
 test_begin_subtest "reindexed encrypted messages, without cleartext"
 output=$(notmuch search wumpus)
 expected=''
diff --git a/util/crypto.c b/util/crypto.c
index ae6b94be..ba9bfa7b 100644
--- a/util/crypto.c
+++ b/util/crypto.c
@@ -188,10 +188,24 @@ _notmuch_crypto_decrypt (bool *attempted,
     if (attempted)
 	*attempted = true;
 #if (GMIME_MAJOR_VERSION < 3)
+#if (GMIME_MAJOR_VERSION == 2 && GMIME_MINOR_VERSION == 6 && GMIME_MICRO_VERSION >= 21)
+    gboolean oldgetsk = g_mime_crypto_context_get_retrieve_session_key (crypto_ctx);
+    gboolean newgetsk = (decrypt_result);
+    if (newgetsk != oldgetsk)
+	/* This could return an error, but we can't do anything about it, so ignore it */
+	g_mime_crypto_context_set_retrieve_session_key (crypto_ctx, newgetsk, NULL);
+#endif
     ret = g_mime_multipart_encrypted_decrypt(part, crypto_ctx,
 					     decrypt_result, err);
+#if (GMIME_MAJOR_VERSION == 2 && GMIME_MINOR_VERSION == 6 && GMIME_MICRO_VERSION >= 21)
+    if (newgetsk != oldgetsk)
+	g_mime_crypto_context_set_retrieve_session_key (crypto_ctx, oldgetsk, NULL);
+#endif
 #else
-    ret = g_mime_multipart_encrypted_decrypt(part, GMIME_DECRYPT_NONE, NULL,
+    GMimeDecryptFlags flags = GMIME_DECRYPT_NONE;
+    if (decrypt_result)
+	flags |= GMIME_DECRYPT_EXPORT_SESSION_KEY;
+    ret = g_mime_multipart_encrypted_decrypt(part, flags, NULL,
 					     decrypt_result, err);
 #endif
     return ret;
-- 
2.14.2