}
$calculated_sig = $this->getMessageSignature($message);
- return $calculated_sig == $sig;
+
+ return $this->constantTimeCompare($calculated_sig, $sig);
+ }
+
+ /**
+ * String comparison function which will complete in a constant time
+ * for strings of any given matching length, to help prevent an attacker
+ * from distinguishing how much of a signature token they have guessed
+ * correctly.
+ *
+ * For this usage, it's assumed that the length of the string is known,
+ * so we may safely short-circuit on mismatched lengths which will be known
+ * to be invalid by the attacker.
+ *
+ * http://lists.openid.net/pipermail/openid-security/2010-July/001156.html
+ * http://rdist.root.org/2010/01/07/timing-independent-array-comparison/
+ */
+ private function constantTimeCompare($a, $b)
+ {
+ $len = strlen($a);
+ if (strlen($b) !== $len) {
+ // Short-circuit on length mismatch; attackers will already know
+ // the correct target length so this is safe.
+ return false;
+ }
+ if ($len == 0) {
+ // 0-length valid input shouldn't really happen. :)
+ return true;
+ }
+ $result = 0;
+ for ($i = 0; $i < strlen($a); $i++) {
+ // We use scary bitwise operations to avoid logical short-circuits
+ // in lower-level code.
+ $result |= ord($a{$i}) ^ ord($b{$i});
+ }
+ return ($result == 0);
}
}