client_cert_auth.cpp

/*
     This file is part of libhttpserver
     Copyright (C) 2011-2019 Sebastiano Merlino

     This library is free software; you can redistribute it and/or
     modify it under the terms of the GNU Lesser General Public
     License as published by the Free Software Foundation; either
     version 2.1 of the License, or (at your option) any later version.

     This library is distributed in the hope that it will be useful,
     but WITHOUT ANY WARRANTY; without even the implied warranty of
     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
     Lesser General Public License for more details.

     You should have received a copy of the GNU Lesser General Public
     License along with this library; if not, write to the Free Software
     Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301
     USA
*/

/**
 * Example demonstrating client certificate (mTLS) authentication.
 *
 * This example shows how to:
 * 1. Configure the server to request client certificates
 * 2. Extract client certificate information in request handlers
 * 3. Implement certificate-based access control
 *
 * To test this example:
 *
 * 1. Generate server certificate and key:
 *    openssl req -x509 -newkey rsa:2048 -keyout server_key.pem -out server_cert.pem \
 *        -days 365 -nodes -subj "/CN=localhost"
 *
 * 2. Generate a CA certificate for client certs:
 *    openssl req -x509 -newkey rsa:2048 -keyout ca_key.pem -out ca_cert.pem \
 *        -days 365 -nodes -subj "/CN=Test CA"
 *
 * 3. Generate client certificate signed by the CA:
 *    openssl req -newkey rsa:2048 -keyout client_key.pem -out client_csr.pem \
 *        -nodes -subj "/CN=Alice/O=Engineering"
 *    openssl x509 -req -in client_csr.pem -CA ca_cert.pem -CAkey ca_key.pem \
 *        -CAcreateserial -out client_cert.pem -days 365
 *
 * 4. Run the server:
 *    ./client_cert_auth
 *
 * 5. Test with curl using client certificate:
 *    curl -k --cert client_cert.pem --key client_key.pem https://localhost:8443/secure
 *
 *    Or without a certificate (will be denied):
 *    curl -k https://localhost:8443/secure
 */

#include <cstdint>
#include <iostream>
#include <memory>
#include <set>
#include <string>

#include <httpserver.hpp>

// Set of allowed certificate fingerprints (SHA-256, hex-encoded).
// For illustration; production must load allowed fingerprints from a
// database or config file, not an in-process set.
std::set<std::string> allowed_fingerprints;

// Resource that requires client certificate authentication
class secure_resource : public httpserver::http_resource {
 public:
    httpserver::http_response render_get(const httpserver::http_request& req) override {
        // Check if client provided a certificate
        if (!req.has_client_certificate()) {
            return httpserver::http_response::string("Client certificate required").with_status(httpserver::http::http_utils::http_unauthorized);
        }

        // Get certificate information. TASK-019: the four string-typed
        // accessors return string_view aliasing per-request storage; we
        // copy into std::string here so the locals survive the rest of
        // this method (and so the `+` chains below compile).
        std::string cn(req.get_client_cert_cn());
        std::string dn(req.get_client_cert_dn());
        std::string issuer(req.get_client_cert_issuer_dn());
        std::string fingerprint(req.get_client_cert_fingerprint_sha256());
        bool verified = req.is_client_cert_verified();

        // Check if certificate is verified by our CA
        if (!verified) {
            return httpserver::http_response::string("Certificate not verified by trusted CA").with_status(httpserver::http::http_utils::http_forbidden);
        }

        // Optional: Check fingerprint against allowlist
        if (!allowed_fingerprints.empty() &&
            allowed_fingerprints.find(fingerprint) == allowed_fingerprints.end()) {
            return httpserver::http_response::string("Certificate not in allowlist").with_status(httpserver::http::http_utils::http_forbidden);
        }

        // Check certificate validity times. TASK-019 narrows the
        // accessor return type to std::int64_t.
        //
        // IMPORTANT: these time checks are safe only because we have already
        // confirmed has_client_certificate() == true (line 72) and
        // is_client_cert_verified() == true (line 87) above. Without those
        // earlier guards, get_client_cert_not_before() / get_client_cert_not_after()
        // return the sentinel -1 (a large negative int64_t), and any non-negative
        // `now` would satisfy `now > not_after`, producing a misleading
        // "Certificate has expired" response instead of denying the request
        // due to the absence of a certificate. Never call the time accessors
        // without first confirming cert presence and verification.
        // (security-reviewer item 21)
        std::int64_t now = static_cast<std::int64_t>(time(nullptr));
        std::int64_t not_before = req.get_client_cert_not_before();
        std::int64_t not_after = req.get_client_cert_not_after();

        if (now < not_before) {
            return httpserver::http_response::string("Certificate not yet valid").with_status(httpserver::http::http_utils::http_forbidden);
        }

        if (now > not_after) {
            return httpserver::http_response::string("Certificate has expired").with_status(httpserver::http::http_utils::http_forbidden);
        }

        // Build response with certificate info
        std::string response = "Welcome, " + cn + "!\n\n";
        response += "Certificate Details:\n";
        response += "  Subject DN: " + dn + "\n";
        response += "  Issuer DN:  " + issuer + "\n";
        response += "  Fingerprint (SHA-256): " + fingerprint + "\n";
        response += "  Verified: " + std::string(verified ? "Yes" : "No") + "\n";

        return httpserver::http_response::string(response);
    }
};

// Public resource that shows certificate info but doesn't require it
class info_resource : public httpserver::http_resource {
 public:
    httpserver::http_response render_get(const httpserver::http_request& req) override {
        std::string response;

        if (req.has_client_certificate()) {
            response = "Client certificate detected:\n";
            // TASK-019: get_client_cert_cn() returns string_view; copy
            // into std::string for the `+` chain.
            response += "  Common Name: " + std::string(req.get_client_cert_cn()) + "\n";
            response += "  Verified: " + std::string(req.is_client_cert_verified() ? "Yes" : "No") + "\n";
        } else {
            response = "No client certificate provided.\n";
            response += "Use --cert and --key with curl to provide one.\n";
        }

        return httpserver::http_response::string(response);
    }
};

int main() {
    std::cout << "Starting HTTPS server with client certificate authentication on port 8443...\n";
    std::cout << "\nEndpoints:\n";
    std::cout << "  /info   - Shows certificate info (optional cert)\n";
    std::cout << "  /secure - Requires valid client certificate\n\n";

    // Create webserver with SSL and client certificate trust store
    httpserver::webserver ws{httpserver::create_webserver(8443)
        .use_ssl()
        .https_mem_key("server_key.pem")      // Server private key
        .https_mem_cert("server_cert.pem")    // Server certificate
        .https_mem_trust("ca_cert.pem")};      // CA certificate for verifying client certs

    auto secure = std::make_shared<secure_resource>();
    auto info = std::make_shared<info_resource>();

    ws.register_path("/secure", secure);
    ws.register_path("/info", info);

    std::cout << "Server started. Press Ctrl+C to stop.\n\n";
    std::cout << "Test commands:\n";
    std::cout << "  curl -k https://localhost:8443/info\n";
    std::cout << "  curl -k --cert client_cert.pem --key client_key.pem https://localhost:8443/info\n";
    std::cout << "  curl -k --cert client_cert.pem --key client_key.pem https://localhost:8443/secure\n";

    ws.start(true);

    return 0;
}