Web SSO SAML Assertion login to saleforce.com

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assertion

secure

SAML

We have MVC 4 application from that we are receiving Auth token from saleforce.com to use their API using following code

requestData.Append("grant_type=password&");
requestData.Append("client_id=" + ConfigurationManager.AppSettings["SfdcClientId"] + "&");
requestData.Append("client_secret=" + ConfigurationManager.AppSettings["SfdcClientSecret"] + "&");
requestData.Append("username=" + login.Email + "&");
requestData.Append("password=" + login.Password);
Everything is working fine so far

Now we are implementing SSO using Siteminder which will do all authentication process and return user id on successful Authentication

So now we have only User ID with us and want Access token from Saleforce.com using saml assertion

We form Saml request and posting it to saleforce.com as per their documentation https://developer.salesforce.com/page/Digging_Deeper_into_OAuth_2.0_on_Force.com but its giving invalid_Assertion error we try to figure out but still not get any solution to this

Does any one know have to do this please assist me