# Protocol Layers Overview

quicprochat composes four distinct protocol layers into a single security stack. Each layer addresses a specific class of threat and delegates everything else to the layers above or below it. No single layer is sufficient on its own; the composition is what delivers end-to-end confidentiality, server authentication, forward secrecy, post-compromise security, and post-quantum resistance.

This page provides a high-level comparison and a suggested reading order. The deep-dive pages that follow contain implementation details drawn directly from the source code.

## Layer comparison

| Layer | Standard / Spec | Crate(s) | Security Properties |
|---|---|---|---|
| **QUIC + TLS 1.3** | RFC 9000, RFC 9001 | `quinn 0.11`, `rustls 0.23` | Transport confidentiality, server authentication, 0-RTT resumption |
| **Protobuf framing** | Custom binary header + [Protocol Buffers](https://protobuf.dev/) | `quicprochat-rpc`, `prost 0.13` | Typed length-prefixed frames, method dispatch, push events, status codes |
| **MLS** | [RFC 9420](https://www.rfc-editor.org/rfc/rfc9420.html) | `openmls 0.5` | Group key agreement, forward secrecy, post-compromise security (PCS) |
| **Hybrid KEM** | [draft-ietf-tls-hybrid-design](https://datatracker.ietf.org/doc/draft-ietf-tls-hybrid-design/) | `ml-kem 0.2`, `x25519-dalek 2` | Post-quantum resistance via ML-KEM-768 combined with X25519 |

## How the layers compose

Data flows through the stack from top to bottom on send and from bottom to top on receive:

```text
Application plaintext
       |
       v
  +-----------+
  |    MLS    |   RFC 9420 group encryption (PrivateMessage)
  +-----------+
       |
       v
  +-----------+
  | Protobuf  |   Typed serialisation into Protobuf messages
  | framing   |   + binary header [method_id/event_type][req_id][len]
  +-----------+
       |
       v
  +-----------+
  | QUIC+TLS  |   QUIC transport encryption (TLS 1.3)
  +-----------+
       |
       v
    Network
```

The Hybrid KEM layer operates orthogonally: it wraps MLS payloads in an outer post-quantum encryption envelope before they enter the transport layer. It is implemented and tested but not yet integrated into the MLS ciphersuite (planned for a future milestone).

## Suggested reading order

The pages in this section are ordered to build understanding incrementally:

1. **[QUIC + TLS 1.3](quic-tls.md)** -- Start here. This is the transport layer that every client-server connection uses. Understanding QUIC stream multiplexing and the TLS 1.3 handshake is prerequisite to understanding how the Protobuf framing protocol rides on top.

2. **[MLS (RFC 9420)](mls.md)** -- The core cryptographic innovation. MLS provides the group key agreement that makes quicprochat an E2E encrypted group messenger rather than just a transport-encrypted relay. This is the longest and most detailed page.

3. **[Protobuf Framing](capn-proto.md)** -- The framing and RPC layer that bridges MLS application data with the transport. Understanding the three frame types (Request, Response, Push), the method ID dispatch table, and status codes is essential for reading the server and client source code.

4. **[Hybrid KEM: X25519 + ML-KEM-768](hybrid-kem.md)** -- The post-quantum encryption layer. Read this last because it builds on concepts from all other layers: key encapsulation (from MLS), wire format conventions (from Protobuf framing), and AEAD encryption.

## Cross-cutting concerns

Several topics span multiple layers and have their own dedicated pages elsewhere in this book:

- **Forward secrecy**: Provided by MLS epoch ratcheting. See [Forward Secrecy](../cryptography/forward-secrecy.md).
- **Post-compromise security**: Provided by MLS Update proposals. See [Post-Compromise Security](../cryptography/post-compromise-security.md).
- **Post-quantum readiness**: Currently provided by the standalone Hybrid KEM module; integration into MLS is planned. See [Post-Quantum Readiness](../cryptography/post-quantum-readiness.md).
- **Key lifecycle and zeroization**: Private key material is zeroized after use across all layers. See [Key Lifecycle and Zeroization](../cryptography/key-lifecycle.md).
- **Wire format details**: The Protobuf schema definitions are documented in the [Wire Format Reference](../wire-format/overview.md) section.
- **Design rationale**: The ADR pages explain *why* each layer was chosen. See [Design Decisions Overview](../design-rationale/overview.md).

## Crate mapping

Each protocol layer maps to one or more workspace crates:

| Layer | Primary Crate | Source File(s) |
|---|---|---|
| QUIC + TLS 1.3 | `quicprochat-server`, `quicprochat-client` | Server and client entry points |
| Protobuf framing | `quicprochat-rpc` | `src/framing.rs`, `src/server.rs`, `src/client.rs` |
| Protobuf types + method IDs | `quicprochat-proto` | `src/lib.rs` (method_ids), `proto/qpc/v1/*.proto` |
| MLS | `quicprochat-core` | `src/group.rs`, `src/keystore.rs` |
| Hybrid KEM | `quicprochat-core` | `src/hybrid_kem.rs` |

For a full crate responsibility breakdown, see [Crate Responsibilities](../architecture/crate-responsibilities.md).