Don't use "phutil_hashes_are_identical()" to compare public keys

Summary:
Ref T13436. There's no real security value to doing this comparison, it just wards off evil "security researchers" who get upset if you ever compare two strings with a non-constant-time algorithm.

In practice, SSH public keys are pretty long, pretty public, and have pretty similar lengths. This leads to a relatively large amount of work to do constant-time comparisons on them (we frequently can't abort early after identifying differing string length).

Test Plan: Ran `bin/ssh-auth --sshd-key ...` on `secure` with ~1K keys, saw runtime drop by ~50% (~400ms to ~200ms) with `===`.

Maniphest Tasks: T13436

Differential Revision: https://secure.phabricator.com/D20875

Author: epriestley

scripts/ssh/ssh-auth.php

@@ -109,7 +109,7 @@
 if ($sshd_key !== null) {
   $matches = array();
   foreach ($authstruct['keys'] as $key => $key_struct) {
-    if (phutil_hashes_are_identical($key_struct['key'], $sshd_key)) {
+    if ($key_struct['key'] === $sshd_key) {
       $matches[$key] = $key_struct;
     }
   }