feature #893 Added suport for encrypted private keys (by a passprase)…

… in DKIM Signer (dariogz)

This PR was submitted for the 5.x branch but it was merged into the 6.0-dev branch instead (closes #893).

Discussion
----------

Added suport for encrypted private keys (by a passprase) in DKIM Signer

## **Problem**
I noticed that when I want to use a private key, encrypted with a passprase, it was not possible to pass this passprase as a parameter, causing the **openssl_get_privatekey**() method to throw an exception.

This method accepts as an optional parameter a string passphrase
Documentation: http://php.net/manual/es/function.openssl-pkey-get-private.php

## **Added**
- passphrase attribute
- passphrase parameter as optional
- second parameter passphrase to openssl_get_privatekey()

Commits
-------

2ec5736 Added suport for encrypted private keys (by a passprase) in DKIM Signer

lib/classes/Swift/Signers/DKIMSigner.php

@@ -36,6 +36,8 @@ class Swift_Signers_DKIMSigner implements Swift_Signers_HeaderSigner
      */
     protected $selector;
 
+    private $passphrase = '';
+
     /**
      * Hash algorithm used.
      *
@@ -169,13 +171,15 @@ class Swift_Signers_DKIMSigner implements Swift_Signers_HeaderSigner
      * @param string $privateKey
      * @param string $domainName
      * @param string $selector
+     * @param string $passphrase
      */
-    public function __construct($privateKey, $domainName, $selector)
+    public function __construct($privateKey, $domainName, $selector, $passphrase = '')
     {
         $this->privateKey = $privateKey;
         $this->domainName = $domainName;
         $this->signerIdentity = '@'.$domainName;
         $this->selector = $selector;
+        $this->passphrase = $passphrase;
     }
 
     /**
@@ -670,7 +674,7 @@ private function getEncryptedHash()
                 $algorithm = OPENSSL_ALGO_SHA256;
                 break;
         }
-        $pkeyId = openssl_get_privatekey($this->privateKey);
+        $pkeyId = openssl_get_privatekey($this->privateKey, $this->passphrase);
         if (!$pkeyId) {
             throw new Swift_SwiftException('Unable to load DKIM Private Key ['.openssl_error_string().']');
         }