asp.net mvc身份验证cookie问题

Web.config:

    <authentication mode="Forms">
        <forms loginUrl="/Account/Login" defaultUrl="/Home/MyAccount" timeout="43200"/>
    </authentication>

持久化cookie的代码:

public void SignIn(string userName, bool createPersistentCookie) {
    int timeout = createPersistentCookie ? 525600 : 120; // Timeout in minutes, 525600 = 365 days.
    FormsAuthenticationTicket ticket = new FormsAuthenticationTicket(userName, createPersistentCookie, timeout);
    string encrypted = FormsAuthentication.Encrypt(ticket);
    HttpCookie cookie = new HttpCookie(FormsAuthentication.FormsCookieName, encrypted);
    cookie.Expires = System.DateTime.Now.AddMinutes(timeout);

    HttpContext.Current.Response.Cookies.Add(cookie);
    FormsAuthentication.SetAuthCookie(userName, createPersistentCookie);
}

检索cookie的代码:

        if (System.Web.HttpContext.Current.Request.Cookies.AllKeys.Contains(FormsAuthentication.FormsCookieName)) {
            cookie = System.Web.HttpContext.Current.Request.Cookies[FormsAuthentication.FormsCookieName];
        }

当前代码检查会话的身份验证。我还想添加从cookie获取用户名的功能。我有两个问题:

2. 如何解密cookie以获取用户名?