Adding a post-quantum hybrid handshake to a Rust VPN

Short summary

Post-quantum security in practice: hybrid X25519/ML-KEM-768 handshake in Qeli VPN defeats harvest-now-decrypt-later by requiring attackers to break both algorithms. Covers Rust implementation, FFI client integration, wire compatibility, and why mixing both secrets in the KDF—not picking one—is critical.

• •Hybrid approach (X25519 + ML-KEM-768) requires breaking both primitives to recover plaintext
• •Mixing secrets in HKDF-SHA256 KDF binds both classical and post-quantum contributions
• •FFI architecture lets single Rust crypto implementation serve C#/Kotlin clients identically

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