Update JWT.php

Add a method to retrieve the header. OpenIdConnect tends to add a "kid" to the header that is used to determine which key to use for verification. Getting header information allows you to figure out which key to pass to "decode". Other possible ways to handle this, allow $key passed to decode to be an array of keys. For now, let's just let people get to the header.

Author: jlesueur

Authentication/JWT.php

@@ -26,6 +26,28 @@
  */
 class JWT
 {
+    
+    /**
+     * Returns just the header portion of the jwt. This allows
+     * you to determine which key should be used to verify
+     * the jwt, using the "kid" field
+     * 
+     * @param string      $jwt
+     * 
+     * @return object     The JWT's header object, with fields "typ","alg", and optionally "kid"
+     */
+    public static function decodeHeader($jwt) {
+        $tks = explode('.', $jwt);
+        if (count($tks) != 3) {
+            throw new UnexpectedValueException('Wrong number of segments');
+        }
+        list($headb64, $bodyb64, $cryptob64) = $tks;
+        if (null === ($header = JWT::jsonDecode(JWT::urlsafeB64Decode($headb64)))) {
+            throw new UnexpectedValueException('Invalid segment encoding');
+        }
+        return $header;
+    }
+    
     /**
      * Decodes a JWT string into a PHP object.
      *
@@ -117,6 +139,7 @@ public static function sign($msg, $key, $method = 'HS256')
         if (empty($methods[$method])) {
             throw new DomainException('Algorithm not supported');
         }
+        
         return hash_hmac($methods[$method], $msg, $key, true);
     }