android/remoteprovisioner/ServerInterface.java

/**
 * Copyright (C) 2021 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.android.remoteprovisioner;

import android.content.Context;
import android.util.Base64;
import android.util.Log;

import java.io.BufferedInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.SocketTimeoutException;
import java.net.URL;
import java.util.List;

/**
 * Provides convenience methods for interfacing with the remote provisioning server.
 */
public class ServerInterface {

    private static final int TIMEOUT_MS = 5000;

    private static final String TAG = "ServerInterface";
    private static final String GEEK_URL = ":fetchEekChain";
    private static final String CERTIFICATE_SIGNING_URL = ":signCertificates?challenge=";

    /**
     * Ferries the CBOR blobs returned by KeyMint to the provisioning server. The data sent to the
     * provisioning server contains the MAC'ed CSRs and encrypted bundle containing the MAC key and
     * the hardware unique public key.
     *
     * @param context The application context which is required to use SettingsManager.
     * @param csr The CBOR encoded data containing the relevant pieces needed for the server to
     *                    sign the CSRs. The data encoded within comes from Keystore / KeyMint.
     * @param challenge The challenge that was sent from the server. It is included here even though
     *                    it is also included in `cborBlob` in order to allow the server to more
     *                    easily reject bad requests.
     * @return A List of byte arrays, where each array contains an entire DER-encoded certificate
     *                    chain for one attestation key pair.
     */
    public static List<byte[]> requestSignedCertificates(Context context, byte[] csr,
                                                         byte[] challenge) {
        try {
            URL url = new URL(SettingsManager.getUrl(context) + CERTIFICATE_SIGNING_URL
                              + Base64.encodeToString(challenge, Base64.URL_SAFE));
            HttpURLConnection con = (HttpURLConnection) url.openConnection();
            con.setRequestMethod("POST");
            con.setDoOutput(true);
            con.setConnectTimeout(TIMEOUT_MS);

            // May not be able to use try-with-resources here if the connection gets closed due to
            // the output stream being automatically closed.
            try (OutputStream os = con.getOutputStream()) {
                os.write(csr, 0, csr.length);
            }

            if (con.getResponseCode() != HttpURLConnection.HTTP_OK) {
                int failures = SettingsManager.incrementFailureCounter(context);
                Log.e(TAG, "Server connection for signing failed, response code: "
                        + con.getResponseCode() + "\nRepeated failure count: " + failures);
                return null;
            }
            SettingsManager.clearFailureCounter(context);
            BufferedInputStream inputStream = new BufferedInputStream(con.getInputStream());
            ByteArrayOutputStream cborBytes = new ByteArrayOutputStream();
            byte[] buffer = new byte[1024];
            int read = 0;
            while ((read = inputStream.read(buffer, 0, buffer.length)) != -1) {
                cborBytes.write(buffer, 0, read);
            }
            return CborUtils.parseSignedCertificates(cborBytes.toByteArray());
        } catch (SocketTimeoutException e) {
            SettingsManager.incrementFailureCounter(context);
            Log.e(TAG, "Server timed out", e);
            return null;
        } catch (IOException e) {
            SettingsManager.incrementFailureCounter(context);
            Log.e(TAG, "Failed to request signed certificates from the server", e);
            return null;
        }
    }

    /**
     * Calls out to the specified backend servers to retrieve an Endpoint Encryption Key and
     * corresponding certificate chain to provide to KeyMint. This public key will be used to
     * perform an ECDH computation, using the shared secret to encrypt privacy sensitive components
     * in the bundle that the server needs from the device in order to provision certificates.
     *
     * A challenge is also returned from the server so that it can check freshness of the follow-up
     * request to get keys signed.
     *
     * @param context The application context which is required to use SettingsManager.
     * @return A GeekResponse object which optionally contains configuration data.
     */
    public static GeekResponse fetchGeek(Context context) {
        try {
            URL url = new URL(SettingsManager.getUrl(context) + GEEK_URL);
            HttpURLConnection con = (HttpURLConnection) url.openConnection();
            con.setRequestMethod("POST");
            con.setConnectTimeout(TIMEOUT_MS);
            con.setDoOutput(true);

            byte[] config = CborUtils.buildProvisioningInfo(context);
            try (OutputStream os = con.getOutputStream()) {
                os.write(config, 0, config.length);
            }

            if (con.getResponseCode() != HttpURLConnection.HTTP_OK) {
                int failures = SettingsManager.incrementFailureCounter(context);
                Log.e(TAG, "Server connection for GEEK failed, response code: "
                        + con.getResponseCode() + "\nRepeated failure count: " + failures);
                return null;
            }
            SettingsManager.clearFailureCounter(context);

            BufferedInputStream inputStream = new BufferedInputStream(con.getInputStream());
            ByteArrayOutputStream cborBytes = new ByteArrayOutputStream();
            byte[] buffer = new byte[1024];
            int read = 0;
            while ((read = inputStream.read(buffer, 0, buffer.length)) != -1) {
                cborBytes.write(buffer, 0, read);
            }
            inputStream.close();
            return CborUtils.parseGeekResponse(cborBytes.toByteArray());
        } catch (SocketTimeoutException e) {
            SettingsManager.incrementFailureCounter(context);
            Log.e(TAG, "Server timed out", e);
        } catch (IOException e) {
            // This exception will trigger on a completely malformed URL.
            SettingsManager.incrementFailureCounter(context);
            Log.e(TAG, "Failed to fetch GEEK from the servers.", e);
        }
        return null;
    }
}