fix for mbstring func overload

Author: Minishlink

composer.json

@@ -12,14 +12,21 @@
       "homepage": "https://github.com/Minishlink"
     }
   ],
+  "repositories": [
+    {
+      "type": "vcs",
+      "url": "https://github.com/Minishlink/PHPASN1"
+    }
+  ],
   "require": {
     "php": ">=5.6",
     "mdanter/ecc": "^0.4.0",
     "lib-openssl": "*",
     "spomky-labs/base64url": "^1.0",
     "spomky-labs/php-aes-gcm": "^1.2",
     "spomky-labs/jose": "^6.0",
-    "guzzlehttp/guzzle": "^6.2"
+    "guzzlehttp/guzzle": "^6.2",
+    "fgrosse/phpasn1": "dev-fix_mbstring_overload as 1.5.2"
   },
   "require-dev": {
     "phpunit/phpunit": "4.8.*"

src/Encryption.php

@@ -121,7 +121,7 @@ private static function hkdf($salt, $ikm, $info, $length)
         $prk = hash_hmac('sha256', $ikm, $salt, true);
 
         // expand
-        return substr(hash_hmac('sha256', $info.chr(1), $prk, true), 0, $length);
+        return mb_substr(hash_hmac('sha256', $info.chr(1), $prk, true), 0, $length, '8bit');
     }
 
     /**

src/VAPID.php

@@ -58,7 +58,7 @@ public static function validate(array $vapid)
             $posStartKey += 30; // length of '-----BEGIN EC PRIVATE KEY-----'
 
             $pemSerializer = new PemPrivateKeySerializer(new DerPrivateKeySerializer());
-            $keys = self::getUncompressedKeys($pemSerializer->parse(substr($pem, $posStartKey, $posEndKey - $posStartKey)));
+            $keys = self::getUncompressedKeys($pemSerializer->parse(mb_substr($pem, $posStartKey, $posEndKey - $posStartKey, '8bit')));
             $vapid['publicKey'] = $keys['publicKey'];
             $vapid['privateKey'] = $keys['privateKey'];
         }