Authenticate user after redirect

I have two separate servers (s1 and s2) with two separate web applications running. The user is authenticated on s1 and by clicking a link gets redirected to s2. Is it possible to authenticate the user on s2 without having the user to enter his/hers credentials again?

My first thought was to encrypt a password with a secret key known by both servers and pass the encrypted string with the request. Is that enough? Is there a smarter way to go about doing this?

  ___开发者_JAVA技巧______                             __________
  |   s1  |  GET (with encrypted pw)?   |   s2   |
  |       |  ----------------------->   |        |
  |_______|                             |________|
Authenticated                   Need to be authenticated

Thanks.

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Yes, you can pass credentials in the querystring or use cross domain cookies.

http://www.15seconds.com/issue/971108.htm

http://anantgarg.com/2010/02/18/cross-domain-cookies-in-safari/