Android

What Android's Keystore Actually Does with AES Keys (and What It Doesn't)

There is a question that comes up constantly in Android security engineering, usually phrased something along the lines of: “how do I prove to my server that this AES key is hardware-backed?” The common answers to this question are often wrong in that they make you think you have a guarantee that you do not have. The short answer is: you cannot prove an AES key’s hardware provenance to a server directly. The certificate chain mechanism that makes remote attestation possible for asymmetric keys does not exist for symmetric keys. If you are building a system that depends upon being able to remotely attest an AES key’s provenance, you need to redesign the system, not find a better API call.