func md5() -> NSString {

        guard self.length > 0 else {
            return self
        }

        let utf8Buf = self.cString(using: String.Encoding.utf8.rawValue)
        let length = self.lengthOfBytes(using: String.Encoding.utf8.rawValue)

        let buffer = UnsafeMutablePointer<CUnsignedChar>.allocate(capacity: Int(CC_MD5_DIGEST_LENGTH))
        buffer.initialize(to: 0)
        CC_MD5(utf8Buf,CC_LONG(length), buffer)

        let md5String = NSMutableString()

        for i in 0 ..< CC_MD5_DIGEST_LENGTH {
            md5String.appendFormat("%02x", buffer[Int(i)])
        }

        buffer.deallocate()

        return md5String.uppercased as NSString
    }