title: Crypting mail addresses - funny design problem
body: <p>In my web project, I am storing mail addresses. These addresses may be used by the system to throw mails to the recipients. It is also important to say that these mail addresses have expiration time.</p>
  
  <p>But the critical point is trustness: for this very service, people must be sure that the mail addresses wil not be given to somebody else (especially to authorities, for example).</p>
  
  <p>To resume:</p>
  
  <ul>
  <li>the system has to "know" the mail address.</li>
  <li>the webmaster (or somebody else) has to be unable to find the true mail addresses.</li>
  </ul>
  
  <p>By doing this, the webmaster will not be able to give information (even by force :)).</p>
  
  <p>Intermediate solution: I already know how to do this as soon as the information has expired. E.g. The mail address is encrypted with gnupg (GPG / PGP algorithms). The system (or anybody) can decrypt if he/it has the password. But as soon as the mail address has expired, let's revoke the secret key :arrow: one cannot decrypt the mail address anymore.</p>
  
  <p>But this raises a performance problem (to create the private key)...</p>
  
  <p>Any help would be most appreciated !</p>