title: How to find checksum for bitcoin public address without publick key
body: <p>Say that I have an incomplete public address like this one <code>11B6X2JLR3FB6GU7GHUoK7C6FXWL</code>
  as you can see it's <code>28</code> characters only, missing the <code>6</code> characters checksum, however I do not have public key to generate address (and don't have the private key)</p>
  
  <p>For a demonstration purpose of my project at university I want to find the checksum characters so I get an address that looks like <code>11B6X2JLR3FB6GU7GHUoK7C6FXWL[xxxxxx]</code> change <code>[xxxxxx]</code> to the checksum characters.</p>
  
  <p>I have read the bitcoin wiki on how public addresses are generated from <code>private</code>/<code>public</code> keys, but I don't have those two. I know I won't be able to spend the coins sent to this address since I don't have it's <code>private</code> key (and doubt anyone have it, but it's for a university project. vanity gen isn't an option since I need to match all <code>28</code> characters which is mathematically impossible...</p>
  
  <p>Preferable if it's possible to show the code to do it in Ruby, if you don't know Ruby then the steps will be find and I'll work the code my self.</p>