ded/cpp-httplib @ 86abc9a0eaa531308d82d5e72d32b94f828302a1

title: "T04. Configure mTLS" order: 45

status: "draft"

Regular TLS verifies the server certificate only. mTLS (mutual TLS) adds the other direction: the client presents a certificate too, and the server verifies it. It's common for zero-trust API-to-API traffic and internal system authentication.

Server side

Pass the CA used to verify client certificates as the third (and fourth) argument to SSLServer.

httplib::SSLServer svr(
  "server-cert.pem",    // server certificate
  "server-key.pem",     // server private key
  "client-ca.pem",      // CA that signs valid client certs
  nullptr               // CA directory (none)
);

svr.Get("/", [](const httplib::Request &req, httplib::Response &res) {
  res.set_content("authenticated", "text/plain");
});

svr.listen("0.0.0.0", 443);

With this, any connection whose client certificate isn't signed by client-ca.pem is rejected at the handshake. By the time a handler runs, the client is already authenticated.

Configure with in-memory PEM

httplib::SSLServer::PemMemory pem{};
pem.cert_pem = server_cert.data();
pem.cert_pem_len = server_cert.size();
pem.key_pem = server_key.data();
pem.key_pem_len = server_key.size();
pem.client_ca_pem = client_ca.data();
pem.client_ca_pem_len = client_ca.size();

httplib::SSLServer svr(pem);

This is the clean way when you load certificates from environment variables or a secrets manager.

Client side

On the client side, pass the client certificate and key to SSLClient.

httplib::SSLClient cli("api.example.com", 443,
                       "client-cert.pem",
                       "client-key.pem");

auto res = cli.Get("/");

Note you're using SSLClient directly, not Client. If the private key has a password, pass it as the fifth argument.

WebSocket clients

ws::WebSocketClient has the same PemMemory struct, so wss:// connections can present a client certificate too.

httplib::ws::WebSocketClient::PemMemory pem{};
pem.cert_pem = client_cert.data();
pem.cert_pem_len = client_cert.size();
pem.key_pem = client_key.data();
pem.key_pem_len = client_key.size();

httplib::ws::WebSocketClient ws("wss://api.example.com/ws", pem);

if (ws.connect()) {
  ws.send("hello");
}

Passing PemMemory to a ws:// (non-TLS) URL is silently ignored. There's no constructor that reads the cert files directly, so unlike SSLClient you always load the PEM into memory yourself before passing it in.

Read client info from a handler

To see which client connected from inside a handler, use req.peer_cert(). Details in T05. Access the peer certificate on the server.

Use cases

Microservice-to-microservice calls: Issue a cert per service, use the cert as identity
IoT device management: Burn a cert into each device and use it to gate API access
An alternative to internal VPN: Put cert-based auth in front of public endpoints so internal resources can be reached safely

Note: Issuing and revoking client certificates is more operational work than password-based auth. You'll need either an internal PKI setup or an automated flow using ACME-family tools.

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