title: How to protect stored user data: phone numbers?
body: <p>I want to build an app that is basically a subscription service for text messages.</p>
  
  <p>User submits a phone number via my site. I send them a confirmation text with a key, and if they correctly enter the key, then they are subscribed, and every day thereafter I will send them a text with a particular message until they unsubscribe.</p>
  
  <p>Question: I want to store these phone numbers in a way that I (the developer or storage layer administrator) can't view these phone numbers, to preserve privacy. I can't just stored them hashed, then how does my app read them to send the text message? How do I do this?</p>
  
  <p>Edit: This is both a security and privacy question... I want someone who hacks into my system to not be able to get any of the phone numbers, even if they have full access to the database.</p>