title: How to validate the email address in Firebase ssecurity rules
body: <p>I have been saving user data under a reference ID, and some user info containing email address. I want to make the data available, but prevent people guessing ID's and seeing if there is info behind it. </p>
  
  <p>I only want to give the data if they provide me the ID AND the email address they registered with. </p>
  
  <p>So I have this data stored in Firebase:</p>
  
  <pre><code>{ "data" : {
     "ms12345678" : { 
         "name" : "John Doe",
         "age"  : 40,
         "email" : "johndoe@domain.com"
      }
  }
  </code></pre>
  
  <p>So doing <code>var ref = firebase.database().ref("data/ms12345678")</code> should fail, unless I provide <code>johndoe@domain.com</code> as well.</p>
  
  <p>I am not sure what approach I should take here. I don't want users to authenticate, but provide them with a link.</p>