mitigate efail

This commit prevents the gpg backend from decrypting non integrity
protected messages. The efail [0] vurneability relies on the attacker
being able to inject content into an encrypted mail.

According to [1], the correct way of detecting if decryption succeeded
is not to check the return code. Instead the `--status-fd` should be
checked for DECRYPTION_OKAY.

Imp currently displays the decrypted body (including the gpg warning)
in the message pane. This opens up decryption oracle attacks.

[0] https://efail.de/
[1] https://git.gnupg.org/cgi-bin/gitweb.cgi?p=gnupg.git;a=blob_plain;f=doc/DETAILS

lib/Horde/Crypt/Pgp/Backend/Binary.php

@@ -567,6 +567,7 @@ public function decryptMessage($text, $params)
             true
         );
         $this->_ensureResult($result);
+        $this->_ensureDecryptionOk($result);
 
         return $this->_checkSignatureResult($result->stderr, $result->output);
     }
@@ -685,12 +686,16 @@ public function _callGpg(
         $data->output = null;
         $data->stderr = null;
         $data->stdout = null;
+        $data->status = null;
 
         /* Verbose output? */
         if (!$verbose) {
             array_unshift($options, '--quiet');
         }
 
+        $status_file = $this->_createTempFile('horde-pgp-stat', false);
+        array_unshift($options, '--status-file ' . $status_file);
+
         /* Create temp files for output. */
         if ($output) {
             $output_file = $this->_createTempFile('horde-pgp', false);
@@ -751,6 +756,9 @@ public function _callGpg(
         }
         pclose($fp);
 
+        $data->status = file_get_contents($status_file);
+        unlink($status_file);
+
         if ($output) {
             $data->output = file_get_contents($output_file);
             unlink($output_file);
@@ -850,6 +858,27 @@ protected function _ensureResult($result)
         }
     }
 
+    /**
+     * Checks whether the decryption succeeded.
+     *
+     * @param object $result  A result from _callGpg().
+     *
+     * @throws Horde_Crypt_Exception with messages from stderr if the result
+     *                               is not a successful decryption.
+     */
+    protected function _ensureDecryptionOk($result)
+    {
+        if (!preg_match("/DECRYPTION_OKAY/", $result->status)) {
+            throw new Horde_Crypt_Exception(
+                preg_replace(
+                    array('/^gpg: /', '/\n/'),
+                    array('', '. '),
+                    $result->stderr
+                )
+            );
+        }
+    }
+
     /**
      * Create a temporary file that will be deleted at the end of this
      * process.