Add rate limiting for encryption

**Description**
Currently lib lacks proper caching support.

**Proposed solution**
Add configurable caching with sensible defaults.

**Priority**: Low/Medium

Start by reconciling the title's request for encryption rate limiting with the description's request for caching in lib. Identify the relevant encryption and caching entry points, then confirm the intended configuration and defaults with the issue discussion. Done should be a documented, tested implementation matching one clarified scope.

Written by the indexing model from the issue text.