crypto: new decryption policy "auto"

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.

doc/man1/notmuch-config.rst

@@ -142,9 +142,14 @@ 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
+        set to ``true``, notmuch will try to decrypt the message and
-        the cleartext.  Be aware that the index is likely sufficient
+        index the cleartext.  If ``auto``, it will try to index the
-        to reconstruct the cleartext of the message itself, so please
+        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.
+
+        Be aware that the notmuch 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 ``index.decrypt=true`` without considering the
         security of your index.

lib/index.cc

@@ -548,7 +548,8 @@ _index_encrypted_mime_part (notmuch_message_t *message,
 	}
     }
 #endif
-    clear = _notmuch_crypto_decrypt (message, crypto_ctx, encrypted_data, NULL, &err);
+    clear = _notmuch_crypto_decrypt (notmuch_indexopts_get_decrypt_policy (indexopts),
+				     message, crypto_ctx, encrypted_data, NULL, &err);
     if (err) {
 	_notmuch_database_log (notmuch, "Failed to decrypt during indexing. (%d:%d) [%s]\n",
 			       err->domain, err->code, err->message);

lib/indexopts.c

@@ -33,11 +33,14 @@ notmuch_database_get_default_indexopts (notmuch_database_t *db)
     if (err)
 	return ret;
 
-    if (decrypt_policy &&
+    if (decrypt_policy) {
-	((!(strcasecmp(decrypt_policy, "true"))) ||
+	if ((!(strcasecmp(decrypt_policy, "true"))) ||
-	 (!(strcasecmp(decrypt_policy, "yes"))) ||
+	    (!(strcasecmp(decrypt_policy, "yes"))) ||
-	 (!(strcasecmp(decrypt_policy, "1")))))
+	    (!(strcasecmp(decrypt_policy, "1"))))
-	notmuch_indexopts_set_decrypt_policy (ret, NOTMUCH_DECRYPT_TRUE);
+	    notmuch_indexopts_set_decrypt_policy (ret, NOTMUCH_DECRYPT_TRUE);
+	else if (!strcasecmp(decrypt_policy, "auto"))
+	    notmuch_indexopts_set_decrypt_policy (ret, NOTMUCH_DECRYPT_AUTO);
+    }
 
     free (decrypt_policy);
     return ret;

lib/notmuch.h

@@ -2241,6 +2241,7 @@ notmuch_database_get_default_indexopts (notmuch_database_t *db);
 typedef enum {
     NOTMUCH_DECRYPT_FALSE,
     NOTMUCH_DECRYPT_TRUE,
+    NOTMUCH_DECRYPT_AUTO,
 } notmuch_decryption_policy_t;
 
 /**

mime-node.c

@@ -205,7 +205,8 @@ node_decrypt_and_verify (mime_node_t *node, GMimeObject *part,
 		break;
 
 	node->decrypt_attempted = true;
-	node->decrypted_child = _notmuch_crypto_decrypt (parent ? parent->envelope_file : NULL,
+	node->decrypted_child = _notmuch_crypto_decrypt (node->ctx->crypto->decrypt,
+							 parent ? parent->envelope_file : NULL,
 							 cryptoctx, encrypteddata, &decrypt_result, &err);
     }
     if (! node->decrypted_child) {
@@ -270,7 +271,7 @@ _mime_node_create (mime_node_t *parent, GMimeObject *part)
     }
 
 #if (GMIME_MAJOR_VERSION < 3)
-    if ((GMIME_IS_MULTIPART_ENCRYPTED (part) && (node->ctx->crypto->decrypt == NOTMUCH_DECRYPT_TRUE))
+    if ((GMIME_IS_MULTIPART_ENCRYPTED (part) && (node->ctx->crypto->decrypt != NOTMUCH_DECRYPT_FALSE))
 	|| (GMIME_IS_MULTIPART_SIGNED (part) && node->ctx->crypto->verify)) {
 	GMimeContentType *content_type = g_mime_object_get_content_type (part);
 	const char *protocol = g_mime_content_type_get_parameter (content_type, "protocol");
@@ -286,7 +287,7 @@ _mime_node_create (mime_node_t *parent, GMimeObject *part)
 #endif
 
     /* Handle PGP/MIME parts */
-    if (GMIME_IS_MULTIPART_ENCRYPTED (part) && (node->ctx->crypto->decrypt == NOTMUCH_DECRYPT_TRUE)) {
+    if (GMIME_IS_MULTIPART_ENCRYPTED (part) && (node->ctx->crypto->decrypt != NOTMUCH_DECRYPT_FALSE)) {
 	if (node->nchildren != 2) {
 	    /* this violates RFC 3156 section 4, so we won't bother with it. */
 	    fprintf (stderr, "Error: %d part(s) for a multipart/encrypted "

notmuch-client.h

@@ -415,7 +415,9 @@ struct mime_node {
 /* Construct a new MIME node pointing to the root message part of
  * message. If crypto->verify is true, signed child parts will be
  * verified. If crypto->decrypt is NOTMUCH_DECRYPT_TRUE, encrypted
- * child parts will be decrypted.  If the crypto contexts
+ * child parts will be decrypted using either stored session keys or
+ * asymmetric crypto.  If crypto->decrypt is NOTMUCH_DECRYPT_AUTO,
+ * only session keys will be tried.  If the crypto contexts
  * (crypto->gpgctx or crypto->pkcs7) are NULL, they will be lazily
  * initialized.
  *

notmuch.c

@@ -103,6 +103,7 @@ const notmuch_opt_desc_t  notmuch_shared_indexing_options [] = {
       .present = &indexing_cli_choices.decrypt_policy_set, .keywords =
       (notmuch_keyword_t []){ { "false", NOTMUCH_DECRYPT_FALSE },
 			      { "true", NOTMUCH_DECRYPT_TRUE },
+			      { "auto", NOTMUCH_DECRYPT_AUTO },
 			      { 0, 0 } },
       .name = "decrypt" },
     { }
@@ -128,7 +129,7 @@ notmuch_process_shared_indexing_options (notmuch_database_t *notmuch, g_mime_3_u
 	}
     }
 #if (GMIME_MAJOR_VERSION < 3)
-    if (indexing_cli_choices.opts && notmuch_indexopts_get_decrypt_policy (indexing_cli_choices.opts) == NOTMUCH_DECRYPT_TRUE) {
+    if (indexing_cli_choices.opts && notmuch_indexopts_get_decrypt_policy (indexing_cli_choices.opts) != NOTMUCH_DECRYPT_FALSE) {
 	const char* gpg_path = notmuch_config_get_crypto_gpg_path (config);
 	if (gpg_path && strcmp(gpg_path, "gpg"))
 	    fprintf (stderr, "Warning: deprecated crypto.gpg_path is set to '%s'\n"

test/T357-index-decryption.sh

@@ -140,6 +140,16 @@ test_expect_equal \
     "$output" \
     "$expected"
 
+# ensure no session keys are present:
+test_begin_subtest 'reindex using only session keys'
+test_expect_success 'notmuch reindex --decrypt=auto tag:encrypted and property:index.decryption=success'
+test_begin_subtest "reindexed encrypted messages, decrypting only with session keys"
+output=$(notmuch search wumpus)
+expected=''
+test_expect_equal \
+    "$output" \
+    "$expected"
+
 # and the same search, but by property ($expected is untouched):
 test_begin_subtest "emacs search by property with both messages unindexed"
 output=$(notmuch search property:index.decryption=success)
@@ -180,7 +190,7 @@ notmuch restore <<EOF
 #notmuch-dump batch-tag:3 config,properties,tags
 #= simple-encrypted@crypto.notmuchmail.org session-key=9%3AFC09987F5F927CC0CC0EE80A96E4C5BBF4A499818FB591207705DFDDD6112CF9
 EOF
-notmuch reindex --decrypt=true id:simple-encrypted@crypto.notmuchmail.org
+notmuch reindex --decrypt=auto id:simple-encrypted@crypto.notmuchmail.org
 output=$(notmuch search sekrit)
 expected='thread:0000000000000001   2016-12-22 [1/1] Daniel Kahn Gillmor; encrypted message (encrypted inbox unread)'
 if [ $NOTMUCH_HAVE_GMIME_SESSION_KEYS -eq 0 ]; then

util/crypto.c

@@ -140,13 +140,16 @@ void _notmuch_crypto_cleanup (unused(_notmuch_crypto_t *crypto))
 #endif
 
 GMimeObject *
-_notmuch_crypto_decrypt (notmuch_message_t *message,
+_notmuch_crypto_decrypt (notmuch_decryption_policy_t decrypt,
+			 notmuch_message_t *message,
 			 g_mime_3_unused(GMimeCryptoContext* crypto_ctx),
 			 GMimeMultipartEncrypted *part,
 			 GMimeDecryptResult **decrypt_result,
 			 GError **err)
 {
     GMimeObject *ret = NULL;
+    if (decrypt == NOTMUCH_DECRYPT_FALSE)
+	return NULL;
 
     /* the versions of notmuch that can support session key decryption */
 #if HAVE_GMIME_SESSION_KEYS
@@ -184,6 +187,10 @@ _notmuch_crypto_decrypt (notmuch_message_t *message,
 	g_error_free (*err);
 	*err = NULL;
     }
+
+    if (decrypt == NOTMUCH_DECRYPT_AUTO)
+	return ret;
+
 #if (GMIME_MAJOR_VERSION < 3)
     ret = g_mime_multipart_encrypted_decrypt(part, crypto_ctx,
 					     decrypt_result, err);

util/crypto.h

@@ -16,7 +16,8 @@ typedef struct _notmuch_crypto {
 } _notmuch_crypto_t;
 
 GMimeObject *
-_notmuch_crypto_decrypt (notmuch_message_t *message,
+_notmuch_crypto_decrypt (notmuch_decryption_policy_t decrypt,
+			 notmuch_message_t *message,
 			 GMimeCryptoContext* crypto_ctx,
 			 GMimeMultipartEncrypted *part,
 			 GMimeDecryptResult **decrypt_result,