[PATCH 3/6] util/crypto: add _notmuch_crypto_payload_skip_legacy_display

[Daniel Kahn Gillmor]

This is a utility function designed to make it easier to
"fast-forward" past a legacy-display part associated with a
cryptographic envelope, and show the user the intended message body.

The bulk of the ugliness in here is in the test function
_notmuch_crypto_payload_has_legacy_display, which tests all of the
things we'd expect to be true in a a cryptographic payload that
contains a legacy display part.

Signed-off-by: Daniel Kahn Gillmor <dkg at fifthhorseman.net>
---
 util/crypto.c | 96 +++++++++++++++++++++++++++++++++++++++++++++++++++
 util/crypto.h | 17 +++++++++
 2 files changed, 113 insertions(+)

diff --git a/util/crypto.c b/util/crypto.c
index 04dd91d4..6c533f8d 100644
--- a/util/crypto.c
+++ b/util/crypto.c
@@ -209,3 +209,99 @@ _notmuch_message_crypto_successful_decryption (_notmuch_message_crypto_t *msg_cr
 
     return NOTMUCH_STATUS_SUCCESS;
 }
+
+static bool
+_notmuch_crypto_payload_has_legacy_display (GMimeObject *payload)
+{
+    GMimeMultipart *mpayload;
+    const char *protected_header_parameter;
+    GMimeTextPart *legacy_display;
+    char *legacy_display_header_text = NULL;
+    GMimeStream *stream = NULL;
+    GMimeParser *parser = NULL;
+    GMimeObject *legacy_header_object = NULL, *first;
+    GMimeHeaderList *legacy_display_headers = NULL, *protected_headers = NULL;
+    bool ret = false;
+
+    if (! g_mime_content_type_is_type (g_mime_object_get_content_type (payload),
+				       "multipart", "mixed"))
+	return false;
+    protected_header_parameter = g_mime_object_get_content_type_parameter (payload, "protected-headers");
+    if ((! protected_header_parameter) || strcmp (protected_header_parameter, "v1"))
+	return false;
+    if (! GMIME_IS_MULTIPART (payload))
+	return false;
+    mpayload = GMIME_MULTIPART (payload);
+    if (mpayload == NULL)
+	return false;
+    if (g_mime_multipart_get_count (mpayload) != 2)
+	return false;
+    first = g_mime_multipart_get_part (mpayload, 0);
+    if (! g_mime_content_type_is_type (g_mime_object_get_content_type (first),
+				       "text", "rfc822-headers"))
+	return false;
+    protected_header_parameter = g_mime_object_get_content_type_parameter (first, "protected-headers");
+    if ((! protected_header_parameter) || strcmp (protected_header_parameter, "v1"))
+	return false;
+    if (! GMIME_IS_TEXT_PART (first))
+	return false;
+
+    /* ensure that the headers in the first part all match the values
+     * found in the payload's own protected headers!  if they don't,
+     * we should not treat this as a valid "legacy-display" part.
+     *
+     * Crafting a GMimeHeaderList object from the content of the
+     * text/rfc822-headers part is pretty clumsy; we should probably
+     * push something into GMime that makes this a one-shot
+     * operation. */
+    if ((protected_headers = g_mime_object_get_header_list (payload), protected_headers) &&
+	(legacy_display = GMIME_TEXT_PART (first), legacy_display) &&
+	(legacy_display_header_text = g_mime_text_part_get_text (legacy_display), legacy_display_header_text) &&
+	(stream = g_mime_stream_mem_new_with_buffer (legacy_display_header_text, strlen (legacy_display_header_text)), stream) &&
+	(g_mime_stream_write (stream, "\r\n\r\n", 4) == 4) &&
+	(g_mime_stream_seek (stream, 0, GMIME_STREAM_SEEK_SET) == 0) &&
+	(parser = g_mime_parser_new_with_stream (stream), parser) &&
+	(legacy_header_object = g_mime_parser_construct_part (parser, NULL), legacy_header_object) &&
+	(legacy_display_headers = g_mime_object_get_header_list (legacy_header_object), legacy_display_headers)) {
+	/* walk through legacy_display_headers, comparing them against
+	 * their values in the protected_headers: */
+	ret = true;
+	for (int i = 0; i < g_mime_header_list_get_count (legacy_display_headers); i++) {
+	    GMimeHeader *dh = g_mime_header_list_get_header_at (legacy_display_headers, i);
+	    if (dh == NULL) {
+		ret = false;
+		break;
+	    }
+	    GMimeHeader *ph = g_mime_header_list_get_header (protected_headers, g_mime_header_get_name (dh));
+	    if (ph == NULL) {
+		ret = false;
+		break;
+	    }
+	    if (strcmp (g_mime_header_get_value (dh), g_mime_header_get_value (ph))) {
+		ret = false;
+		break;
+	    }
+	}
+    }
+
+    if (legacy_display_header_text)
+	g_free (legacy_display_header_text);
+    if (stream)
+	g_object_unref (stream);
+    if (parser)
+	g_object_unref (parser);
+    if (legacy_header_object)
+	g_object_unref (legacy_header_object);
+
+   return ret;
+}
+
+GMimeObject *
+_notmuch_crypto_payload_skip_legacy_display (GMimeObject *payload)
+{
+    if (_notmuch_crypto_payload_has_legacy_display (payload)) {
+	return g_mime_multipart_get_part (GMIME_MULTIPART (payload), 1);
+    } else {
+	return payload;
+    }
+}
diff --git a/util/crypto.h b/util/crypto.h
index 8e1e6bbd..5ea4fed2 100644
--- a/util/crypto.h
+++ b/util/crypto.h
@@ -97,6 +97,23 @@ _notmuch_message_crypto_successful_decryption (_notmuch_message_crypto_t *msg_cr
 bool
 _notmuch_message_crypto_potential_payload (_notmuch_message_crypto_t *msg_crypto, GMimeObject *payload, GMimeObject *parent, int childnum);
 
+/* If part is a cryptographic payload within an encrypted message, and
+ * it has a "legacy display" part, then we can skip over it and jump
+ * to the actual content, because notmuch already handles protected
+ * headers appropriately.
+ *
+ * This function takes a MIME object that is a cryptographic payload
+ * and either returns it directly (if it does not have a "legacy
+ * display" part), or it returns a pointer to its content-bearing
+ * subpart, with the "legacy display" part and the surrounding
+ * multipart/mixed object removed.
+ *
+ * No new objects are created by calling this function, and the
+ * returned object will only be released when the original part is
+ * disposed of.
+ */
+GMimeObject *
+_notmuch_crypto_payload_skip_legacy_display (GMimeObject *part);
 
 #ifdef __cplusplus
 }
-- 
2.20.1