What is the content of ASP.NET FormsAuthentication cookie value? How can I see the real value of the hashed string (in case that I have the decrypted key)?
Here you have all necessary information about the content of the authentication cookie
http://msdn.microsoft.com/en-us/library/ff647070.aspx
Check the section "Forms Authentication Cookies"
FormsAuthentication.Decrypt
would decrypt the authentication ticket included on the forms authentication cookie.