Implements a sophisticated producer/consumer/spawner pattern for backpressure management in async systems. The flow control system provides isolated, reusable components with clear interfaces for managing message rates between workers, demonstrating non-obvious skill in async resource management.

Provides a pluggable cryptographic backend abstraction that cleanly separates interface from implementations. Supports multiple backends (software, AWS KMS, etc.) through well-designed traits, allowing secure channel operations to remain agnostic to underlying cryptographic providers. Demonstrates excellent dependency injection and abstraction skills.

Implements the Noise XX protocol for secure channel establishment with a clear state machine approach. The handshake implementation is well-tested, follows cryptographic best practices, and could be extracted for other secure communication needs. Shows strong cryptographic engineering and protocol implementation skills.

Complete attribute-based access control system featuring a custom expression parser and evaluator, policy storage abstractions, and resource/action-based authorization. Demonstrates non-trivial language implementation skills and provides a reusable policy engine that could be adapted for other domains.