content: /*
   * Copyright (C) 2011 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 android.security;
  
  import android.annotation.NonNull;
  import android.annotation.Nullable;
  import android.annotation.SdkConstant;
  import android.annotation.SdkConstant.SdkConstantType;
  import android.annotation.WorkerThread;
  import android.app.Activity;
  import android.app.PendingIntent;
  import android.app.Service;
  import android.content.ComponentName;
  import android.content.Context;
  import android.content.Intent;
  import android.content.ServiceConnection;
  import android.net.Uri;
  import android.os.Binder;
  import android.os.IBinder;
  import android.os.Looper;
  import android.os.Process;
  import android.os.RemoteException;
  import android.os.UserHandle;
  import android.security.keystore.AndroidKeyStoreProvider;
  import android.security.keystore.KeyPermanentlyInvalidatedException;
  import android.security.keystore.KeyProperties;
  
  import com.android.org.conscrypt.TrustedCertificateStore;
  
  import java.io.ByteArrayInputStream;
  import java.io.Closeable;
  import java.io.Serializable;
  import java.security.KeyPair;
  import java.security.Principal;
  import java.security.PrivateKey;
  import java.security.UnrecoverableKeyException;
  import java.security.cert.Certificate;
  import java.security.cert.CertificateException;
  import java.security.cert.CertificateFactory;
  import java.security.cert.X509Certificate;
  import java.util.ArrayList;
  import java.util.Collection;
  import java.util.List;
  import java.util.Locale;
  import java.util.concurrent.BlockingQueue;
  import java.util.concurrent.LinkedBlockingQueue;
  
  import javax.security.auth.x500.X500Principal;
  
  /**
   * The {@code KeyChain} class provides access to private keys and
   * their corresponding certificate chains in credential storage.
   *
   * <p>Applications accessing the {@code KeyChain} normally go through
   * these steps:
   *
   * <ol>
   *
   * <li>Receive a callback from an {@link javax.net.ssl.X509KeyManager
   * X509KeyManager} that a private key is requested.
   *
   * <li>Call {@link #choosePrivateKeyAlias
   * choosePrivateKeyAlias} to allow the user to select from a
   * list of currently available private keys and corresponding
   * certificate chains. The chosen alias will be returned by the
   * callback {@link KeyChainAliasCallback#alias}, or null if no private
   * key is available or the user cancels the request.
   *
   * <li>Call {@link #getPrivateKey} and {@link #getCertificateChain} to
   * retrieve the credentials to return to the corresponding {@link
   * javax.net.ssl.X509KeyManager} callbacks.
   *
   * </ol>
   *
   * <p>An application may remember the value of a selected alias to
   * avoid prompting the user with {@link #choosePrivateKeyAlias
   * choosePrivateKeyAlias} on subsequent connections. If the alias is
   * no longer valid, null will be returned on lookups using that value
   *
   * <p>An application can request the installation of private keys and
   * certificates via the {@code Intent} provided by {@link
   * #createInstallIntent}. Private keys installed via this {@code
   * Intent} will be accessible via {@link #choosePrivateKeyAlias} while
   * Certificate Authority (CA) certificates will be trusted by all
   * applications through the default {@code X509TrustManager}.
   */
  // TODO reference intent for credential installation when public
  public final class KeyChain {
  
      /**
       * @hide Also used by KeyChainService implementation
       */
      public static final String ACCOUNT_TYPE = "com.android.keychain";
  
      /**
       * Package name for KeyChain chooser.
       */
      private static final String KEYCHAIN_PACKAGE = "com.android.keychain";
  
      /**
       * Action to bring up the KeyChainActivity
       */
      private static final String ACTION_CHOOSER = "com.android.keychain.CHOOSER";
  
      /**
       * Package name for the Certificate Installer.
       */
      private static final String CERT_INSTALLER_PACKAGE = "com.android.certinstaller";
  
      /**
       * Extra for use with {@link #ACTION_CHOOSER}
       * @hide Also used by KeyChainActivity implementation
       */
      public static final String EXTRA_RESPONSE = "response";
  
      /**
       * Extra for use with {@link #ACTION_CHOOSER}
       * @hide Also used by KeyChainActivity implementation
       */
      public static final String EXTRA_URI = "uri";
  
      /**
       * Extra for use with {@link #ACTION_CHOOSER}
       * @hide Also used by KeyChainActivity implementation
       */
      public static final String EXTRA_ALIAS = "alias";
  
      /**
       * Extra for use with {@link #ACTION_CHOOSER}
       * @hide Also used by KeyChainActivity implementation
       */
      public static final String EXTRA_SENDER = "sender";
  
      /**
       * Extra for use with {@link #ACTION_CHOOSER}
       * @hide Also used by KeyChainActivity implementation
       */
      public static final String EXTRA_KEY_TYPES = "key_types";
  
      /**
       * Extra for use with {@link #ACTION_CHOOSER}
       * @hide Also used by KeyChainActivity implementation
       */
      public static final String EXTRA_ISSUERS = "issuers";
  
      /**
       * Action to bring up the CertInstaller.
       */
      private static final String ACTION_INSTALL = "android.credentials.INSTALL";
  
      /**
       * Optional extra to specify a {@code String} credential name on
       * the {@code Intent} returned by {@link #createInstallIntent}.
       */
      // Compatible with old com.android.certinstaller.CredentialHelper.CERT_NAME_KEY
      public static final String EXTRA_NAME = "name";
  
      /**
       * Optional extra to specify an X.509 certificate to install on
       * the {@code Intent} returned by {@link #createInstallIntent}.
       * The extra value should be a PEM or ASN.1 DER encoded {@code
       * byte[]}. An {@link X509Certificate} can be converted to DER
       * encoded bytes with {@link X509Certificate#getEncoded}.
       *
       * <p>{@link #EXTRA_NAME} may be used to provide a default alias
       * name for the installed certificate.
       */
      // Compatible with old android.security.Credentials.CERTIFICATE
      public static final String EXTRA_CERTIFICATE = "CERT";
  
      /**
       * Optional extra for use with the {@code Intent} returned by
       * {@link #createInstallIntent} to specify a PKCS#12 key store to
       * install. The extra value should be a {@code byte[]}. The bytes
       * may come from an external source or be generated with {@link
       * java.security.KeyStore#store} on a "PKCS12" instance.
       *
       * <p>The user will be prompted for the password to load the key store.
       *
       * <p>The key store will be scanned for {@link
       * java.security.KeyStore.PrivateKeyEntry} entries and both the
       * private key and associated certificate chain will be installed.
       *
       * <p>{@link #EXTRA_NAME} may be used to provide a default alias
       * name for the installed credentials.
       */
      // Compatible with old android.security.Credentials.PKCS12
      public static final String EXTRA_PKCS12 = "PKCS12";
  
      /**
       * Broadcast Action: Indicates the trusted storage has changed. Sent when
       * one of this happens:
       *
       * <ul>
       * <li>a new CA is added,
       * <li>an existing CA is removed or disabled,
       * <li>a disabled CA is enabled,
       * <li>trusted storage is reset (all user certs are cleared),
       * <li>when permission to access a private key is changed.
       * </ul>
       *
       * @deprecated Use {@link #ACTION_KEYCHAIN_CHANGED}, {@link #ACTION_TRUST_STORE_CHANGED} or
       * {@link #ACTION_KEY_ACCESS_CHANGED}. Apps that target a version higher than
       * {@link android.os.Build.VERSION_CODES#N_MR1} will only receive this broadcast if they
       * register for it at runtime.
       */
      @SdkConstant(SdkConstantType.BROADCAST_INTENT_ACTION)
      public static final String ACTION_STORAGE_CHANGED = "android.security.STORAGE_CHANGED";
  
      /**
       * Broadcast Action: Indicates the contents of the keychain has changed. Sent when a KeyChain
       * entry is added, modified or removed.
       */
      @SdkConstant(SdkConstantType.BROADCAST_INTENT_ACTION)
      public static final String ACTION_KEYCHAIN_CHANGED = "android.security.action.KEYCHAIN_CHANGED";
  
      /**
       * Broadcast Action: Indicates the contents of the trusted certificate store has changed. Sent
       * when one the following occurs:
       *
       * <ul>
       * <li>A pre-installed CA is disabled or re-enabled</li>
       * <li>A CA is added or removed from the trust store</li>
       * </ul>
       */
      @SdkConstant(SdkConstantType.BROADCAST_INTENT_ACTION)
      public static final String ACTION_TRUST_STORE_CHANGED =
              "android.security.action.TRUST_STORE_CHANGED";
  
      /**
       * Broadcast Action: Indicates that the access permissions for a private key have changed.
       *
       */
      @SdkConstant(SdkConstantType.BROADCAST_INTENT_ACTION)
      public static final String ACTION_KEY_ACCESS_CHANGED =
              "android.security.action.KEY_ACCESS_CHANGED";
  
      /**
       * Used as a String extra field in {@link #ACTION_KEY_ACCESS_CHANGED} to supply the alias of
       * the key.
       */
      public static final String EXTRA_KEY_ALIAS = "android.security.extra.KEY_ALIAS";
  
      /**
       * Used as a boolean extra field in {@link #ACTION_KEY_ACCESS_CHANGED} to supply if the key is
       * accessible to the application.
       */
      public static final String EXTRA_KEY_ACCESSIBLE = "android.security.extra.KEY_ACCESSIBLE";
  
      /**
       * Indicates that a call to {@link #generateKeyPair} was successful.
       * @hide
       */
      public static final int KEY_GEN_SUCCESS = 0;
  
      /**
       * An alias was missing from the key specifications when calling {@link #generateKeyPair}.
       * @hide
       */
      public static final int KEY_GEN_MISSING_ALIAS = 1;
  
      /**
       * A key attestation challenge was provided to {@link #generateKeyPair}, but it shouldn't
       * have been provided.
       * @hide
       */
      public static final int KEY_GEN_SUPERFLUOUS_ATTESTATION_CHALLENGE = 2;
  
      /**
       * Algorithm not supported by {@link #generateKeyPair}
       * @hide
       */
      public static final int KEY_GEN_NO_SUCH_ALGORITHM = 3;
  
      /**
       * Invalid algorithm parameters when calling {@link #generateKeyPair}
       * @hide
       */
      public static final int KEY_GEN_INVALID_ALGORITHM_PARAMETERS = 4;
  
      /**
       * Keystore is not available when calling {@link #generateKeyPair}
       * @hide
       */
      public static final int KEY_GEN_NO_KEYSTORE_PROVIDER = 5;
  
      /**
       * StrongBox unavailable when calling {@link #generateKeyPair}
       * @hide
       */
      public static final int KEY_GEN_STRONGBOX_UNAVAILABLE = 6;
  
      /**
       * General failure while calling {@link #generateKeyPair}
       * @hide
       */
      public static final int KEY_GEN_FAILURE = 7;
  
      /**
       * Successful call to {@link #attestKey}
       * @hide
       */
      public static final int KEY_ATTESTATION_SUCCESS = 0;
  
      /**
       * Attestation challenge missing when calling {@link #attestKey}
       * @hide
       */
      public static final int KEY_ATTESTATION_MISSING_CHALLENGE = 1;
  
      /**
       * The caller requested Device ID attestation when calling {@link #attestKey}, but has no
       * permissions to get device identifiers.
       * @hide
       */
      public static final int KEY_ATTESTATION_CANNOT_COLLECT_DATA = 2;
  
      /**
       * The underlying hardware does not support Device ID attestation or cannot attest to the
       * identifiers that are stored on the device. This indicates permanent inability
       * to get attestation records on the device.
       * @hide
       */
      public static final int KEY_ATTESTATION_CANNOT_ATTEST_IDS = 3;
  
      /**
       * General failure when calling {@link #attestKey}
       * @hide
       */
      public static final int KEY_ATTESTATION_FAILURE = 4;
  
      /**
       * Returns an {@code Intent} that can be used for credential
       * installation. The intent may be used without any extras, in
       * which case the user will be able to install credentials from
       * their own source.
       *
       * <p>Alternatively, {@link #EXTRA_CERTIFICATE} or {@link
       * #EXTRA_PKCS12} maybe used to specify the bytes of an X.509
       * certificate or a PKCS#12 key store for installation. These
       * extras may be combined with {@link #EXTRA_NAME} to provide a
       * default alias name for credentials being installed.
       *
       * <p>When used with {@link Activity#startActivityForResult},
       * {@link Activity#RESULT_OK} will be returned if a credential was
       * successfully installed, otherwise {@link
       * Activity#RESULT_CANCELED} will be returned.
       */
      @NonNull
      public static Intent createInstallIntent() {
          Intent intent = new Intent(ACTION_INSTALL);
          intent.setClassName(CERT_INSTALLER_PACKAGE,
                              "com.android.certinstaller.CertInstallerMain");
          return intent;
      }
  
      /**
       * Launches an {@code Activity} for the user to select the alias
       * for a private key and certificate pair for authentication. The
       * selected alias or null will be returned via the
       * KeyChainAliasCallback callback.
       *
       * <p>A device policy controller (as a device or profile owner) can
       * intercept the request before the activity is shown, to pick a
       * specific private key alias by implementing
       * {@link android.app.admin.DeviceAdminReceiver#onChoosePrivateKeyAlias
       * onChoosePrivateKeyAlias}.
       *
       * <p>{@code keyTypes} and {@code issuers} may be used to
       * narrow down suggested choices to the user. If either {@code keyTypes}
       * or {@code issuers} is specified and non-empty, and there are no
       * matching certificates in the KeyChain, then the certificate
       * selection prompt would be suppressed entirely.
       *
       * <p>{@code host} and {@code port} may be used to give the user
       * more context about the server requesting the credentials.
       *
       * <p>{@code alias} allows the caller to preselect an existing
       * alias which will still be subject to user confirmation.
       *
       * @param activity The {@link Activity} context to use for
       *     launching the new sub-Activity to prompt the user to select
       *     a private key; used only to call startActivity(); must not
       *     be null.
       * @param response Callback to invoke when the request completes;
       *     must not be null.
       * @param keyTypes The acceptable types of asymmetric keys such as
       *     "RSA", "EC" or null.
       * @param issuers The acceptable certificate issuers for the
       *     certificate matching the private key, or null.
       * @param host The host name of the server requesting the
       *     certificate, or null if unavailable.
       * @param port The port number of the server requesting the
       *     certificate, or -1 if unavailable.
       * @param alias The alias to preselect if available, or null if
       *     unavailable.
       */
      public static void choosePrivateKeyAlias(@NonNull Activity activity,
              @NonNull KeyChainAliasCallback response,
              @Nullable @KeyProperties.KeyAlgorithmEnum String[] keyTypes,
              @Nullable Principal[] issuers,
              @Nullable String host, int port, @Nullable String alias) {
          Uri uri = null;
          if (host != null) {
              uri = new Uri.Builder()
                      .authority(host + (port != -1 ? ":" + port : ""))
                      .build();
          }
          choosePrivateKeyAlias(activity, response, keyTypes, issuers, uri, alias);
      }
  
      /**
       * Launches an {@code Activity} for the user to select the alias
       * for a private key and certificate pair for authentication. The
       * selected alias or null will be returned via the
       * KeyChainAliasCallback callback.
       *
       * <p>A device policy controller (as a device or profile owner) can
       * intercept the request before the activity is shown, to pick a
       * specific private key alias by implementing
       * {@link android.app.admin.DeviceAdminReceiver#onChoosePrivateKeyAlias
       * onChoosePrivateKeyAlias}.
       *
       * <p>{@code keyTypes} and {@code issuers} may be used to
       * narrow down suggested choices to the user. If either {@code keyTypes}
       * or {@code issuers} is specified and non-empty, and there are no
       * matching certificates in the KeyChain, then the certificate
       * selection prompt would be suppressed entirely.
       *
       * <p>{@code uri} may be used to give the user more context about
       * the server requesting the credentials.
       *
       * <p>{@code alias} allows the caller to preselect an existing
       * alias which will still be subject to user confirmation.
       *
       * @param activity The {@link Activity} context to use for
       *     launching the new sub-Activity to prompt the user to select
       *     a private key; used only to call startActivity(); must not
       *     be null.
       * @param response Callback to invoke when the request completes;
       *     must not be null.
       * @param keyTypes The acceptable types of asymmetric keys such as
       *     "RSA", "EC" or null.
       * @param issuers The acceptable certificate issuers for the
       *     certificate matching the private key, or null.
       * @param uri The full URI the server is requesting the certificate
       *     for, or null if unavailable.
       * @param alias The alias to preselect if available, or null if
       *     unavailable.
       * @throws IllegalArgumentException if the specified issuers are not
       *     of type {@code X500Principal}.
       */
      public static void choosePrivateKeyAlias(@NonNull Activity activity,
              @NonNull KeyChainAliasCallback response,
              @Nullable @KeyProperties.KeyAlgorithmEnum String[] keyTypes,
              @Nullable Principal[] issuers,
              @Nullable Uri uri, @Nullable String alias) {
          /*
           * Specifying keyTypes excludes certificates with different key types
           * from the list of certificates presented to the user.
           * In practice today, most servers would require RSA or EC
           * certificates.
           *
           * Specifying issuers narrows down the list by filtering out
           * certificates with issuers which are not matching the provided ones.
           * This has been reported to Chrome several times (crbug.com/731769).
           * There's no concrete description on what to do when the client has no
           * certificates that match the provided issuers.
           * To be conservative, Android will not present the user with any
           * certificates to choose from.
           * If the list of issuers is empty then the client may send any
           * certificate, see:
           * https://tools.ietf.org/html/rfc5246#section-7.4.4
           */
          if (activity == null) {
              throw new NullPointerException("activity == null");
          }
          if (response == null) {
              throw new NullPointerException("response == null");
          }
          Intent intent = new Intent(ACTION_CHOOSER);
          intent.setPackage(KEYCHAIN_PACKAGE);
          intent.putExtra(EXTRA_RESPONSE, new AliasResponse(response));
          intent.putExtra(EXTRA_URI, uri);
          intent.putExtra(EXTRA_ALIAS, alias);
          intent.putExtra(EXTRA_KEY_TYPES, keyTypes);
          ArrayList<byte[]> issuersList = new ArrayList();
          if (issuers != null) {
              for (Principal issuer: issuers) {
                  // In a TLS client context (like Chrome), issuers would only
                  // be specified as X500Principals. No other use cases for
                  // specifying principals have been brought up. Under these
                  // circumstances, only allow issuers specified as
                  // X500Principals.
                  if (!(issuer instanceof X500Principal)) {
                      throw new IllegalArgumentException(String.format(
                              "Issuer %s is of type %s, not X500Principal",
                              issuer.toString(), issuer.getClass()));
                  }
                  // Pass the DER-encoded issuer as that's the most accurate
                  // representation and what is sent over the wire.
                  issuersList.add(((X500Principal) issuer).getEncoded());
              }
          }
          intent.putExtra(EXTRA_ISSUERS, (Serializable) issuersList);
          // the PendingIntent is used to get calling package name
          intent.putExtra(EXTRA_SENDER, PendingIntent.getActivity(activity, 0, new Intent(), 0));
          activity.startActivity(intent);
      }
  
      private static class AliasResponse extends IKeyChainAliasCallback.Stub {
          private final KeyChainAliasCallback keyChainAliasResponse;
          private AliasResponse(KeyChainAliasCallback keyChainAliasResponse) {
              this.keyChainAliasResponse = keyChainAliasResponse;
          }
          @Override public void alias(String alias) {
              keyChainAliasResponse.alias(alias);
          }
      }
  
      /**
       * Returns the {@code PrivateKey} for the requested alias, or null if the alias does not exist
       * or the caller has no permission to access it (see note on exceptions below).
       *
       * <p> This method may block while waiting for a connection to another process, and must never
       * be called from the main thread.
       * <p> As {@link Activity} and {@link Service} contexts are short-lived and can be destroyed
       * at any time from the main thread, it is safer to rely on a long-lived context such as one
       * returned from {@link Context#getApplicationContext()}.
       *
       * <p> If the caller provides a valid alias to which it was not granted access, then the
       * caller must invoke {@link #choosePrivateKeyAlias} again to get another valid alias
       * or a grant to access the same alias.
       * <p>On Android versions prior to Q, when a key associated with the specified alias is
       * unavailable, the method will throw a {@code KeyChainException} rather than return null.
       * If the exception's cause (as obtained by calling {@code KeyChainException.getCause()})
       * is a throwable of type {@code IllegalStateException} then the caller lacks a grant
       * to access the key and certificates associated with this alias.
       *
       * @param alias The alias of the desired private key, typically returned via
       *              {@link KeyChainAliasCallback#alias}.
       * @throws KeyChainException if the alias was valid but there was some problem accessing it.
       * @throws IllegalStateException if called from the main thread.
       */
      @Nullable @WorkerThread
      public static PrivateKey getPrivateKey(@NonNull Context context, @NonNull String alias)
              throws KeyChainException, InterruptedException {
          KeyPair keyPair = getKeyPair(context, alias);
          if (keyPair != null) {
              return keyPair.getPrivate();
          }
  
          return null;
      }
  
      /** @hide */
      @Nullable @WorkerThread
      public static KeyPair getKeyPair(@NonNull Context context, @NonNull String alias)
              throws KeyChainException, InterruptedException {
          if (alias == null) {
              throw new NullPointerException("alias == null");
          }
          if (context == null) {
              throw new NullPointerException("context == null");
          }
  
          final String keyId;
          try (KeyChainConnection keyChainConnection = bind(context.getApplicationContext())) {
              keyId = keyChainConnection.getService().requestPrivateKey(alias);
          } catch (RemoteException e) {
              throw new KeyChainException(e);
          } catch (RuntimeException e) {
              // only certain RuntimeExceptions can be propagated across the IKeyChainService call
              throw new KeyChainException(e);
          }
  
          if (keyId == null) {
              return null;
          } else {
              try {
                  return AndroidKeyStoreProvider.loadAndroidKeyStoreKeyPairFromKeystore(
                          KeyStore.getInstance(), keyId, KeyStore.UID_SELF);
              } catch (RuntimeException | UnrecoverableKeyException | KeyPermanentlyInvalidatedException e) {
                  throw new KeyChainException(e);
              }
          }
      }
  
      /**
       * Returns the {@code X509Certificate} chain for the requested alias, or null if the alias
       * does not exist or the caller has no permission to access it (see note on exceptions
       * in {@link #getPrivateKey}).
       *
       * <p>
       * <strong>Note:</strong> If a certificate chain was explicitly specified when the alias was
       * installed, this method will return that chain. If only the client certificate was specified
       * at the installation time, this method will try to build a certificate chain using all
       * available trust anchors (preinstalled and user-added).
       *
       * <p> This method may block while waiting for a connection to another process, and must never
       * be called from the main thread.
       * <p> As {@link Activity} and {@link Service} contexts are short-lived and can be destroyed
       * at any time from the main thread, it is safer to rely on a long-lived context such as one
       * returned from {@link Context#getApplicationContext()}.
       * <p> In case the caller specifies an alias for which it lacks a grant, it must call
       * {@link #choosePrivateKeyAlias} again. See {@link #getPrivateKey} for more details on
       * coping with this scenario.
       *
       * @param alias The alias of the desired certificate chain, typically
       * returned via {@link KeyChainAliasCallback#alias}.
       * @throws KeyChainException if the alias was valid but there was some problem accessing it.
       * @throws IllegalStateException if called from the main thread.
       */
      @Nullable @WorkerThread
      public static X509Certificate[] getCertificateChain(@NonNull Context context,
              @NonNull String alias) throws KeyChainException, InterruptedException {
          if (alias == null) {
              throw new NullPointerException("alias == null");
          }
  
          final byte[] certificateBytes;
          final byte[] certChainBytes;
          try (KeyChainConnection keyChainConnection = bind(context.getApplicationContext())) {
              IKeyChainService keyChainService = keyChainConnection.getService();
              certificateBytes = keyChainService.getCertificate(alias);
              if (certificateBytes == null) {
                  return null;
              }
              certChainBytes = keyChainService.getCaCertificates(alias);
          } catch (RemoteException e) {
              throw new KeyChainException(e);
          } catch (RuntimeException e) {
              // only certain RuntimeExceptions can be propagated across the IKeyChainService call
              throw new KeyChainException(e);
          }
  
          try {
              X509Certificate leafCert = toCertificate(certificateBytes);
              // If the keypair is installed with a certificate chain by either
              // DevicePolicyManager.installKeyPair or CertInstaller, return that chain.
              if (certChainBytes != null && certChainBytes.length != 0) {
                  Collection<X509Certificate> chain = toCertificates(certChainBytes);
                  ArrayList<X509Certificate> fullChain = new ArrayList<>(chain.size() + 1);
                  fullChain.add(leafCert);
                  fullChain.addAll(chain);
                  return fullChain.toArray(new X509Certificate[fullChain.size()]);
              } else {
                  // If there isn't a certificate chain, either due to a pre-existing keypair
                  // installed before N, or no chain is explicitly installed under the new logic,
                  // fall back to old behavior of constructing the chain from trusted credentials.
                  //
                  // This logic exists to maintain old behaviour for already installed keypair, at
                  // the cost of potentially returning extra certificate chain for new clients who
                  // explicitly installed only the client certificate without a chain. The latter
                  // case is actually no different from pre-N behaviour of getCertificateChain(),
                  // in that sense this change introduces no regression. Besides the returned chain
                  // is still valid so the consumer of the chain should have no problem verifying it.
                  TrustedCertificateStore store = new TrustedCertificateStore();
                  List<X509Certificate> chain = store.getCertificateChain(leafCert);
                  return chain.toArray(new X509Certificate[chain.size()]);
              }
          } catch (CertificateException | RuntimeException e) {
              throw new KeyChainException(e);
          }
      }
  
      /**
       * Returns {@code true} if the current device's {@code KeyChain} supports a
       * specific {@code PrivateKey} type indicated by {@code algorithm} (e.g.,
       * "RSA").
       */
      public static boolean isKeyAlgorithmSupported(
              @NonNull @KeyProperties.KeyAlgorithmEnum String algorithm) {
          final String algUpper = algorithm.toUpperCase(Locale.US);
          return KeyProperties.KEY_ALGORITHM_EC.equals(algUpper)
                  || KeyProperties.KEY_ALGORITHM_RSA.equals(algUpper);
      }
  
      /**
       * Returns {@code true} if the current device's {@code KeyChain} binds any
       * {@code PrivateKey} of the given {@code algorithm} to the device once
       * imported or generated. This can be used to tell if there is special
       * hardware support that can be used to bind keys to the device in a way
       * that makes it non-exportable.
       *
       * @deprecated Whether the key is bound to the secure hardware is known only
       * once the key has been imported. To find out, use:
       * <pre>{@code
       * PrivateKey key = ...; // private key from KeyChain
       *
       * KeyFactory keyFactory =
       *     KeyFactory.getInstance(key.getAlgorithm(), "AndroidKeyStore");
       * KeyInfo keyInfo = keyFactory.getKeySpec(key, KeyInfo.class);
       * if (keyInfo.isInsideSecureHardware()) {
       *     // The key is bound to the secure hardware of this Android
       * }}</pre>
       */
      @Deprecated
      public static boolean isBoundKeyAlgorithm(
              @NonNull @KeyProperties.KeyAlgorithmEnum String algorithm) {
          if (!isKeyAlgorithmSupported(algorithm)) {
              return false;
          }
  
          return KeyStore.getInstance().isHardwareBacked(algorithm);
      }
  
      /** @hide */
      @NonNull
      public static X509Certificate toCertificate(@NonNull byte[] bytes) {
          if (bytes == null) {
              throw new IllegalArgumentException("bytes == null");
          }
          try {
              CertificateFactory certFactory = CertificateFactory.getInstance("X.509");
              Certificate cert = certFactory.generateCertificate(new ByteArrayInputStream(bytes));
              return (X509Certificate) cert;
          } catch (CertificateException e) {
              throw new AssertionError(e);
          }
      }
  
      /** @hide */
      @NonNull
      public static Collection<X509Certificate> toCertificates(@NonNull byte[] bytes) {
          if (bytes == null) {
              throw new IllegalArgumentException("bytes == null");
          }
          try {
              CertificateFactory certFactory = CertificateFactory.getInstance("X.509");
              return (Collection<X509Certificate>) certFactory.generateCertificates(
                      new ByteArrayInputStream(bytes));
          } catch (CertificateException e) {
              throw new AssertionError(e);
          }
      }
  
      /**
       * @hide for reuse by CertInstaller and Settings.
       * @see KeyChain#bind
       */
      public static class KeyChainConnection implements Closeable {
          private final Context context;
          private final ServiceConnection serviceConnection;
          private final IKeyChainService service;
          protected KeyChainConnection(Context context,
                                       ServiceConnection serviceConnection,
                                       IKeyChainService service) {
              this.context = context;
              this.serviceConnection = serviceConnection;
              this.service = service;
          }
          @Override public void close() {
              context.unbindService(serviceConnection);
          }
          public IKeyChainService getService() {
              return service;
          }
      }
  
      /**
       * @hide for reuse by CertInstaller and Settings.
       *
       * Caller should call unbindService on the result when finished.
       */
      @WorkerThread
      public static KeyChainConnection bind(@NonNull Context context) throws InterruptedException {
          return bindAsUser(context, Process.myUserHandle());
      }
  
      /**
       * @hide
       */
      @WorkerThread
      public static KeyChainConnection bindAsUser(@NonNull Context context, UserHandle user)
              throws InterruptedException {
          if (context == null) {
              throw new NullPointerException("context == null");
          }
          ensureNotOnMainThread(context);
          final BlockingQueue<IKeyChainService> q = new LinkedBlockingQueue<IKeyChainService>(1);
          ServiceConnection keyChainServiceConnection = new ServiceConnection() {
              volatile boolean mConnectedAtLeastOnce = false;
              @Override public void onServiceConnected(ComponentName name, IBinder service) {
                  if (!mConnectedAtLeastOnce) {
                      mConnectedAtLeastOnce = true;
                      try {
                          q.put(IKeyChainService.Stub.asInterface(Binder.allowBlocking(service)));
                      } catch (InterruptedException e) {
                          // will never happen, since the queue starts with one available slot
                      }
                  }
              }
              @Override public void onServiceDisconnected(ComponentName name) {}
          };
          Intent intent = new Intent(IKeyChainService.class.getName());
          ComponentName comp = intent.resolveSystemService(context.getPackageManager(), 0);
          intent.setComponent(comp);
          if (comp == null || !context.bindServiceAsUser(
                  intent, keyChainServiceConnection, Context.BIND_AUTO_CREATE, user)) {
              throw new AssertionError("could not bind to KeyChainService");
          }
          return new KeyChainConnection(context, keyChainServiceConnection, q.take());
      }
  
      private static void ensureNotOnMainThread(@NonNull Context context) {
          Looper looper = Looper.myLooper();
          if (looper != null && looper == context.getMainLooper()) {
              throw new IllegalStateException(
                      "calling this from your main thread can lead to deadlock");
          }
      }
  }
  