title: How to implement Querystring authentication
body: <p>I’m developing a website of a client and they are sending out newsletters to their customers (through the website administration interface)
  The newsletters are personal to each of the subscribed recipients/customers. 
  Each recipient/ customer is also a user with a username/password that enables them to sign in on the website and manage their newsletter subscriptions and participate in the sites community.</p>
  
  <p>This all works like a charm.
  Now my client want a “Manage my subscriptions” link in the newsletter email that when pressed automatically signs the recipient/customer in on the website with no need to remember username and password.</p>
  
  <p>This could be easily solved be making a link like this: </p>
  
  <p><a href="http://mysite.com/manage.aspx?user=peter&amp;password=hounddog" rel="nofollow noreferrer">http://mysite.com/manage.aspx?user=peter&amp;password=hounddog</a></p>
  
  <p>Of course information should not be clear text but encrypted in some way.</p>
  
  <p>This however poses a problem since the only way a user can be authenticated on the website if by providing a valid username and password.
  In the name of security, passwords are stored as hashed values in the database making it impossible for me to insert the password in the link.</p>
  
  <p>What is the best way to accomplish this without compromising the security?</p>