mtp/docs/NATIVE-HOST.md

MTP Native Host

The native host is a Rust library (mtp-host) that runs a QUIC server, accepts MTP client connections, negotiates protocol versions, and optionally performs a mutual-authentication handshake (login/register) using Ed25519 and ML-DSA-65 signatures.

Cargo Dependency

[dependencies]
mtp = { path = "/path/to/mtp", features = ["host"] }

# Add crypto for authenticated connections:
mtp = { path = "/path/to/mtp", features = ["host", "crypto"] }

HostConfig

use mtp::host::HostConfig;
use std::net::{IpAddr, Ipv4Addr};

let config = HostConfig::new(
    IpAddr::V4(Ipv4Addr::UNSPECIFIED),
    4433,
    std::fs::read("cert.pem")?,
    std::fs::read("key.pem")?,
)
.with_authentication(
    /* Keyring */,
    |client_id: u64| {
        let db = CLIENT_DB.clone();
        Box::pin(async move { db.lock().unwrap().get(&client_id).cloned() })
    },
    |bundle: PublicKeyBundle| {
        let mut db = CLIENT_DB.lock().unwrap();
        let id = next_id();
        db.insert(id, bundle);
        Box::pin(async move { id })
    },
);

Field	Type	Description
ip	IpAddr	Bind address
port	u16	Listen port
tls_fullchain	Vec<u8>	PEM-encoded TLS certificate chain
tls_key	Vec<u8>	PEM-encoded TLS private key
authentication_policy	AuthenticationPolicy (crypto)	ForceAuthentication, AllowAuthentication, or Unauthenticated
host_keyring	Keyring (crypto)	Host's signing and KEM keys
get_existing_user	Fn(u64) -> Pin<Box<dyn Future<Output = Option<PublicKeyBundle>> + Send>> + Send + Sync (crypto)	Async lookup callback for login
complete_register	Fn(PublicKeyBundle) -> Pin<Box<dyn Future<Output = u64> + Send>> + Send + Sync (crypto)	Async registration callback, returns new client ID

AuthenticationPolicy

ForceAuthentication requires every client to complete the login/register handshake. AllowAuthentication accepts both authenticated and unauthenticated connections — unauthenticated clients get a random ID and AuthState::Unauthenticated. Unauthenticated rejects any client that tries to authenticate and is the default.

use mtp::host::AuthenticationPolicy;

// Force authentication (default was `require_authentication: true`):
let config = HostConfig::new(ip, port, cert, key)
    .with_authentication(host_keyring, get_user, register);

// Allow both authenticated and unauthenticated:
let config = HostConfig::new(ip, port, cert, key)
    .with_allow_authentication(host_keyring, get_user, register);

// Unauthenticated only (default):
let config = HostConfig::new(ip, port, cert, key);

TLS

The host requires a TLS certificate. For development, generate a self-signed certificate using rcgen. For production, use a CA-signed certificate.

Accepting Connections

use mtp::host::MTPHost;

let mut host = MTPHost::new(config).await?;
println!("Listening on {}", host.local_addr());

while let Some(conn) = host.accept().await? {
    // conn is an MTPConnection ready for I/O
}

MTPConnection

Returned by accept() after version negotiation (and authentication if enabled):

pub struct MTPConnection {
    pub version: Version,
    pub codec: VersionedCodec,
    pub sender: Sender,
    pub receiver: Receiver,
    pub description: Option<String>,
    #[cfg(feature = "crypto")]
    pub auth_state: AuthState,
    #[cfg(feature = "crypto")]
    pub client_id: u64,
    #[cfg(feature = "crypto")]
    pub client_public_key: Option<PublicKeyBundle>,
}

• version -- the negotiated protocol version
• codec -- a VersionedCodec scoped to the negotiated version (use for version-aware encode/decode)
• sender / receiver -- for message I/O
• description -- optional client-provided label (e.g. "phone", "desktop")
• client_id -- the authenticated client's ID
• client_public_key -- the client's public key bundle (for signature verification of subsequent messages)

Version Negotiation

When a client connects, accept() performs the following sequence:

1. Accept the QUIC connection
2. Read the client's first CommunicationValue (always encoded with reserved type IDs)
3. Extract the protocol version from DataType::Version (reserved data type ID 0) as a DataValue::Str("major.minor")
4. Call registry.negotiate(&[client_version]) to find the highest mutually supported version
5. Return an AcceptError (closing the connection) if no compatible version exists
6. Return Ok(Some(MTPConnection)) with the negotiated version

The Registry is built automatically from all type maps defined in your type-maps.yaml via Registry::builtin().

Registry

use mtp::codec::registry::Registry;

let registry = host.registry();
assert!(registry.supports(&Version(2, 0)));

let negotiated = registry.negotiate(&[Version(1, 0), Version(2, 0)]);
// -> Some(Version(2, 0)) if both versions are registered

Authentication Flow

When authentication_policy is ForceAuthentication, accept() runs a mutually-authenticated challenge-response handshake before returning the connection. The host issues a fresh, random server_challenge that the client must sign, which is what makes the client's proof unreplayable: a captured proof is bound to a one-time challenge the host generates per connection and will never reissue. The challenge lives only on the accepting task's stack; there is no replay database or shared state.

All signed payloads begin with a one-byte domain-separation tag (see mtp::crypto::auth) so a signature for one step can never be reused as another.

Login

Client                                    Host
  |                                         |
  |  QUIC connect                           |
  |---------------------------------------->|
  |                                         |
  |  Identification { Version, Id }         |  (unsigned hello)
  |---------------------------------------->|
  |                                         |  lookup get_existing_user(id)
  |                                         |  generate random server_challenge
  |  Challenge {                            |
  |    ServerNonce(server_challenge),       |
  |    Signature, [PqSignature]             |  host signs the challenge
  |  }                                      |
  |<----------------------------------------|
  |  ChallengeResponse {                    |
  |    ClientNonce, Signature, [PqSignature]|  client signs the challenge
  |  }                                      |
  |---------------------------------------->|
  |                                         |  verify proof over server_challenge
  |  IdentificationResponse {              |
  |    Connected=true, Id,                  |
  |    ClientNonce(echoed),                 |
  |    Signature, [PqSignature]             |
  |  }                                      |
  |<----------------------------------------|

Payloads (|| is concatenation, integers big-endian; DS_* are domain tags):

• Host challenge: DS_CHALLENGE || id (8) || server_challenge (16)
• Client proof: DS_LOGIN_PROOF || version_string || id (8) || server_challenge (16) || client_nonce (16)
• Host final: DS_HOST_FINAL || assigned_id (8) || client_nonce (16) || server_challenge (16)

Register

Client                                    Host
  |                                         |
  |  QUIC connect                           |
  |---------------------------------------->|
  |                                         |
  |  Register {                             |
  |    Version,                             |  (unsigned hello)
  |    PublicKeys (serialized PublicKeyBundle)
  |  }                                      |
  |---------------------------------------->|
  |                                         |  generate random server_challenge
  |  Challenge {                            |
  |    ServerNonce(server_challenge),       |
  |    Signature, [PqSignature]             |  (challenge binds id = 0)
  |  }                                      |
  |<----------------------------------------|
  |  ChallengeResponse {                    |
  |    ClientNonce, Signature, [PqSignature]|
  |  }                                      |
  |---------------------------------------->|
  |                                         |  verify proof over server_challenge
  |                                         |  call complete_register(bundle) -> new_id
  |  RegisterResponse {                    |
  |    Connected=true, Id(new_id),          |
  |    ClientNonce(echoed),                 |
  |    Signature, [PqSignature]             |
  |  }                                      |
  |<----------------------------------------|

The register client proof is: DS_REGISTER_PROOF || version_string || server_challenge (16) || client_nonce (16) || public_key_bytes

After a successful handshake, accept() returns an MTPConnection with auth_state = Authenticated, client_id set, and client_public_key available for verifying subsequent signed messages from the client.

Rejection

If verification fails or the client is not found (login), the host sends a rejection response with Connected=false and closes the send stream, returning AcceptError::AuthenticationFailed from accept().

Handling Messages

Use conn.sender and conn.receiver for bidirectional message exchange:

while let Some(conn) = host.accept().await? {
    tokio::spawn(async move {
        loop {
            match conn.receiver.receive().await {
                Ok(msg) => {
                    let response = process_message(&msg, &conn);
                    conn.sender.send(&response).await.ok();
                }
                Err(_) => break,
            }
        }
    });
}

Versioned Codec

The conn.codec is a VersionedCodec pre-configured with the negotiated version. Use it to encode/decode with version-specific type maps:

let tm = conn.codec.registry().get(&conn.version).unwrap();

// Look up type IDs for the negotiated version
let desc_id = DataTypeId(tm.data_id_enum(DataType::Description).unwrap());
let value = msg.get_data(desc_id);

Host Callbacks

get_existing_user

Called during login to retrieve a client's public key bundle for signature verification. Must return Some(PublicKeyBundle) if the client ID is known, or None to reject.

let get_existing_user = |id: u64| {
    let db = db.clone();
    Box::pin(async move { db.lock().unwrap().get(&id).cloned() })
};

complete_register

Called during registration to persist a new client's public key bundle and assign a client ID. The returned u64 becomes the client's permanent identifier.

let complete_register = |bundle: PublicKeyBundle| {
    let db = db.clone();
    let id = next_id.fetch_add(1, Ordering::SeqCst);
    Box::pin(async move {
        db.lock().unwrap().insert(id, bundle);
        id
    })
};

Both callbacks are called from within accept() and must be Send + Sync. They are async (returning Pin<Box<dyn Future<...>>) and are .awaited by the host, so they can perform I/O or other async work as needed.

Host Key Generation

Generate a host keyring once and persist it:

use mtp::crypto::{Ed25519Signer, Keyring, MlDsaSigner};
use mtp::crypto::kem::HybridKem;

let (_ed_signer, sig_sk, sig_pk) = Ed25519Signer::generate();
let (_pq_signer, sig_pq_sk, sig_pq_pk) = MlDsaSigner::generate();
let (kem_sk, kem_pk) = HybridKem::generate_keypair();

let host_keyring = Keyring::new(kem_pk, kem_sk, sig_pq_pk, sig_pq_sk, sig_pk, sig_sk);

// Save to disk
let bytes = host_keyring.to_bytes();
std::fs::write("host_keys.bin", bytes)?;

Export the public key bundle so clients can verify the host identity:

let bundle = host_keyring.public_key_bundle();
std::fs::write("host_enc_kem_pk.bin", bundle.kem_public_key.as_bytes())?;
std::fs::write("host_sig_pk.bin", bundle.sig_cl_public_key.as_bytes())?;
std::fs::write("host_sig_pq_pk.bin", bundle.sig_pq_public_key.as_bytes())?;

Policy

The transport Policy is set to defaults internally. To customise (timeouts, send mode, etc.), use mtp_transport::host() directly instead of MTPHost:

use mtp_transport::{host, Policy};

let transport = host(ip, port, cert, key, custom_policy).await?;
// Then build version negotiation on top:
// - accept transport.next()
// - read first frame
// - registry.negotiate()
// - return MTPConnection

Graceful Shutdown

Drop the MTPHost to stop accepting new connections. Active connections continue until their Sender/Receiver are dropped or the peer disconnects.