Simon Willison’s Weblog

Encryption At Rest: Whose Threat Model Is It Anyway? (via) Security engineer Scott Arciszewski talks through the challenges of building a useful encryption-at-rest system for hosted software. Encryption at rest on a hard drive protects against physical access to the powered-down disk and little else. To implement encryption at rest in a multi-tenant SaaS system - such that even individuals with insider access (like access to the underlying database) are unable to read other user's data, is a whole lot more complicated.

Consider an attacker, Bob, with database access:

Here’s the stupid simple attack that works in far too many cases: Bob copies Alice’s encrypted data, and overwrites his records in the database, then accesses the insurance provider’s web app [using his own account].

The fix for this is to "use the AAD mechanism (part of the standard AEAD interface) to bind a ciphertext to its context." Python's cryptography package covers Authenticated Encryption with Associated Data as part of its "hazardous materials" advanced modules.