Mutipass SSO is common protocol to use SSO (single sign on) among two or more websites. This protocol uses by various big companies to implement SSO. Desk.com and Tenderapp.
To implement in your asp.net site. you can use this libarary just in simple steps.
###for transmitter
//Data is simple class example class, use whatever you want
var _transmitter = new Transmitter<Data>(_data);
_transmitter.ApiKey = "any-api-key-which-you-want"; // just a key, its good to use strong key
_transmitter.InitVector = "OpenSSL for Ruby"; // Don't chnage
_transmitter.SiteKey = "your-site-key"; // any site key (friendly name)
var multipass = _transmitter.CreateMultipass();
//Redirect user to destination website
Response.Redirect(string.Format("http://www.mysite.com?multipass={0}&sig={1}",multipass.MultipassText,multipass.Signature));
###for receiver var _receiver = new Receiver();
_receiver.ApiKey = "any-api-key-which-you-want"; // same as transmitter
_receiver.InitVector = "OpenSSL for Ruby"; // Don't change
_receiver.SiteKey = "your-site-key"; // same as transmitter
string multipass = Request.QueryString["multipass"];
string signature = Request.QueryString["sig"];
//get your which is transmitted from transmitter
var data = _receiver.GetData(multipass, signature);