Added Encryption to Caching

- Access Token is encrypted with Secret Key, and stored in file
- Code to auto-update existing cache files to use encrypted token
- Tests fixes accordingly

lib/PayPal/Security/Cipher.php

@@ -0,0 +1,57 @@
+<?php
+
+namespace PayPal\Security;
+
+/**
+ * Class Cipher
+ *
+ * Helper class to encrypt/decrypt data with secret key
+ *
+ * @package PayPal\Security
+ */
+class Cipher
+{
+    private $secretKey;
+
+    /**
+     * Fixed IV Size
+     */
+    const IV_SIZE = 16;
+
+    function __construct($secretKey)
+    {
+        $this->$secretKey = $secretKey;
+    }
+
+    /**
+     * Encrypts the input text using the cipher key
+     *
+     * @param $input
+     * @return string
+     */
+    function encrypt($input)
+    {
+        // Create a random IV. Not using mcrypt to generate one, as to not have a dependency on it.
+        $iv = substr(uniqid("", true), 0, Cipher::IV_SIZE);
+        // Encrypt the data
+        $encrypted = openssl_encrypt($input, "AES-256-CBC", $this->secretKey, 0, $iv);
+        // Encode the data with IV as prefix
+        return base64_encode($iv . $encrypted);
+    }
+
+    /**
+     * Decrypts the input text from the cipher key
+     *
+     * @param $input
+     * @return string
+     */
+    function decrypt($input)
+    {
+        // Decode the IV + data
+        $input = base64_decode($input);
+        // Remove the IV
+        $iv = substr($input, 0, Cipher::IV_SIZE);
+        // Return Decrypted Data
+        return openssl_decrypt(substr($input, Cipher::IV_SIZE), "AES-256-CBC", $this->secretKey, 0, $iv);
+    }
+}