Question #1: Is setAuthCookie any less safe than FormsAuthentication.Encrypt(ticketVariable)?
I mean if anyone tries to modify the cookie created by setAuthCookie, by modifying the username, I suppose that'll violate the authentication on subsequent calls?
Question #2: for those using iphones and tablets to access the site, I suppose FormsAuthentication will fail? Given that I don't want to use cookieless option, is there another approach to make the site secure on both smart phones web browsers and ummm none-smartphone web browsers?
cheers
SetAuthCookie basically creates a new FormsAuthenticationTicket with the supplied username & persistence options, serializes it, FormsAuthentication.Encrypt()'s it, and sets it in the Response.Cookies collection. SetAuthCookie and GetAuthCookie both call FormsAuthentication.Encrypt indirectly.
On subsequent requests, the FormsAuthentiationModule handles the AuthenticateRequest event. If it sees a cookie (it may have expired), it attempts to decrypt it's value with the machineKey (it may have been tampered with) and deserialize it back into a FormsAuthenticationTicket (it may be corrupt). If none of that (bad stuff) happens, the ticket contains the username, issue date, expiration info, etc.. If the ticket hasn't expired, an IIdentity and IPrincipal are created and assigned to HttpContext.Current.User and Thread.CurrentThread.Principal. In .NET 4.5 and later (I think), this is Claims-based (ClaimsIdentity, ClaimsPrincipal). Prior to that, it was a (GenericPrincipal, FormsIdentity) I think.
Any tampering at all on the user side will cause the request to be treated as anonymous. It will fail to decrypt. The only things that would compromise this validation would be if the machineKey in web.config/machine.config somehow got into the hands of an attacker or if there was a bug in the framework code (search for Padding Oracle for a historical example of this).
Aside from that, the other thing to watch out for would be session hijacking. If someone steals your cookie on a public wifi for example, they can present it to the server and the server will behave as if it's you. This generally involves network traffic sniffing. For these reasons, best practice is to use SSL for your entire site and set the cookie to HTTP only and Secure (only presented over https connections) in web.config/system.web/authorization/forms. HTTP only means that it will not be available to client-side Javascript. HTTP Only and Secure effectively means HTTPS only. This will only work if you use SSL on your entire site.
FormsAuthentication will work fine on mobile web browsers. It simply requires the client to accept cookies. As far as I know, all mobile devices will allow this.