fix for b188985#commitcomment-2393215

see http://php.net/manual/en/function.openssl-error-string.php

library/Zend/Crypt/PublicKey/Rsa.php

@@ -135,6 +135,19 @@ public function getOptions()
         return $this->options;
     }
 
+    /**
+     * Return last openssl error(s)
+     * 
+     * @return string
+     */
+    public function getOpensslErrorString()
+    {
+        $msg='';
+        while ($msg .= openssl_error_string())
+            $msg .= "\n";
+        return $msg;
+    }
+
     /**
      * Sign with private key
      *
@@ -158,7 +171,7 @@ public function sign($data, Rsa\PrivateKey $privateKey = null)
         );
         if (false === $result) {
             throw new Exception\RuntimeException(
-                'Can not generate signature; openssl ' . openssl_error_string()
+                'Can not generate signature; openssl ' . $this->getOpensslErrorString()
             );
         }
 
@@ -221,7 +234,7 @@ public function verify(
         );
         if (-1 === $result) {
             throw new Exception\RuntimeException(
-                'Can not verify signature; openssl ' . openssl_error_string()
+                'Can not verify signature; openssl ' . $this->getOpensslErrorString()
             );
         }