title: Generating public/private key pair with limitations on the form of valid keys
body: <p>I need a way to create public/private key pairs with a special pattern for valid pairs, like for example all valid pairs must start with 3, only much more sophisticated than this example. I'm doing this in order to make it hard for another party to generate valid pairs. </p>
  
  <p><strong>edit</strong> 
  I'm trying to use anonymous authentication where users are able to self-generate public/private key pairs based on an initial key. the public key is also used as a pseudonym for the connection to a server. I'm working on a location privacy protocol where I have to authenticate users but they have to remain anonymous. here there is a problem that pseudonyms change often so I can't prevent a dos attack so I thought of this solution to prevent it </p>