index.html

<script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/jshashes/1.0.6/hashes.min.js"></script>

<script>

  // generate seed
  function seed(num_zero_nibbles=32, num_random_nibbles=32) {
    var s = "";
    // first add zeros
    for (var i = 0; i < num_zero_nibbles; i++) { s += "0"; }
    // then add randoms
    var allowed = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
    for (var i = 0; i < num_random_nibbles; i++) {
      s += allowed.charAt(Math.floor(Math.random() * allowed.length));
    }

    return s;
  }

  // hashchain
  function hashchain(seed, num_links) {
    var h = seed
    for (var i = 0; i < num_links; i++) {
      h = new Hashes.SHA256().hex(h)
    }
    return h
  }

  function prove(age) {
    P = hashchain(S, Number(age)+1)
    document.getElementById("display_P").innerHTML = P
    return P
  }

  function challenge(age, age_to_prove) {
    Q = hashchain(S, Number(age)+1-Number(age_to_prove))
    document.getElementById("display_Q").innerHTML = Q
    return Q
  }

  function verify(Q, age_to_prove, P) {
    V = hashchain(Q, Number(age_to_prove))
    document.getElementById("display_V").innerHTML = V
    if (P==V) {
      document.getElementById("verified").innerHTML = "VERIFIED!"
    }
    else {
      document.getElementById("verified").innerHTML = "FAILED!"
    }
    return V
  }

</script>

<button onclick='S=seed();document.getElementById("display_S").innerHTML = S'>
  recreate seed
</button>

<hr/>

<p id="display_S"></p>

<p>Actual Age: <input type="text" name="actual_age" id="actual_age" value="38">
<button onclick='P=prove(document.getElementById("actual_age").value)'>
  create Proof
</button>
<div id="display_P"></div>


<p>Age To Prove: <input type="text" name="age_to_prove" id="age_to_prove" value="19">
<button onclick='Q=challenge(document.getElementById("actual_age").value, document.getElementById("age_to_prove").value)'>
  create Challenge
</button>
<div id="display_Q"></div>

<hr/>

<button onclick='V=verify(Q, document.getElementById("age_to_prove").value, P)'>
  verify challenge
</button>
<br/><br/>
<div id="display_V"></div>
<div id="verified"></div>

<script>
  // start with a seed
  S=seed();
  document.getElementById("display_S").innerHTML = S
</script>