android/vault/VaultProvider.java

/*
 * Copyright (C) 2013 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.example.android.vault;

import static com.example.android.vault.EncryptedDocument.DATA_KEY_LENGTH;
import static com.example.android.vault.EncryptedDocument.MAC_KEY_LENGTH;
import static com.example.android.vault.Utils.closeQuietly;
import static com.example.android.vault.Utils.closeWithErrorQuietly;
import static com.example.android.vault.Utils.readFully;
import static com.example.android.vault.Utils.writeFully;

import android.content.Context;
import android.content.SharedPreferences;
import android.database.Cursor;
import android.database.MatrixCursor;
import android.database.MatrixCursor.RowBuilder;
import android.os.Bundle;
import android.os.CancellationSignal;
import android.os.ParcelFileDescriptor;
import android.provider.DocumentsContract;
import android.provider.DocumentsContract.Document;
import android.provider.DocumentsContract.Root;
import android.provider.DocumentsProvider;
import android.security.KeyChain;
import android.util.Log;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.security.GeneralSecurityException;
import java.security.KeyStore;
import java.security.SecureRandom;

import javax.crypto.Mac;
import javax.crypto.SecretKey;
import javax.crypto.spec.SecretKeySpec;

/**
 * Provider that encrypts both metadata and contents of documents stored inside.
 * Each document is stored as described by {@link EncryptedDocument} with
 * separate metadata and content sections. Directories are just
 * {@link EncryptedDocument} instances without a content section, and a list of
 * child documents included in the metadata section.
 * <p>
 * All content is encrypted/decrypted on demand through pipes, using
 * {@link ParcelFileDescriptor#createReliablePipe()} to detect and recover from
 * remote crashes and errors.
 * <p>
 * Our symmetric encryption key is stored on disk only after using
 * {@link SecretKeyWrapper} to "wrap" it using another public/private key pair
 * stored in the platform {@link KeyStore}. This allows us to protect our
 * symmetric key with hardware-backed keys, if supported. Devices without
 * hardware support still encrypt their keys while at rest, and the platform
 * always requires a user to present a PIN, password, or pattern to unlock the
 * KeyStore before use.
 */
public class VaultProvider extends DocumentsProvider {
    public static final String TAG = "Vault";

    static final String AUTHORITY = "com.example.android.vault.provider";

    static final String DEFAULT_ROOT_ID = "vault";
    static final String DEFAULT_DOCUMENT_ID = "0";

    /** JSON key storing array of all children documents in a directory. */
    private static final String KEY_CHILDREN = "vault:children";

    /** Key pointing to next available document ID. */
    private static final String PREF_NEXT_ID = "next_id";

    /** Blob used to derive {@link #mDataKey} from our secret key. */
    private static final byte[] BLOB_DATA = "DATA".getBytes(StandardCharsets.UTF_8);
    /** Blob used to derive {@link #mMacKey} from our secret key. */
    private static final byte[] BLOB_MAC = "MAC".getBytes(StandardCharsets.UTF_8);

    private static final String[] DEFAULT_ROOT_PROJECTION = new String[] {
            Root.COLUMN_ROOT_ID, Root.COLUMN_FLAGS, Root.COLUMN_ICON, Root.COLUMN_TITLE,
            Root.COLUMN_DOCUMENT_ID, Root.COLUMN_AVAILABLE_BYTES, Root.COLUMN_SUMMARY
    };

    private static final String[] DEFAULT_DOCUMENT_PROJECTION = new String[] {
            Document.COLUMN_DOCUMENT_ID, Document.COLUMN_MIME_TYPE, Document.COLUMN_DISPLAY_NAME,
            Document.COLUMN_LAST_MODIFIED, Document.COLUMN_FLAGS, Document.COLUMN_SIZE,
    };

    private static String[] resolveRootProjection(String[] projection) {
        return projection != null ? projection : DEFAULT_ROOT_PROJECTION;
    }

    private static String[] resolveDocumentProjection(String[] projection) {
        return projection != null ? projection : DEFAULT_DOCUMENT_PROJECTION;
    }

    private final Object mIdLock = new Object();

    /**
     * Flag indicating that the {@link SecretKeyWrapper} public/private key is
     * hardware-backed. A software keystore is more vulnerable to offline
     * attacks if the device is compromised.
     */
    private boolean mHardwareBacked;

    /** File where wrapped symmetric key is stored. */
    private File mKeyFile;
    /** Directory where all encrypted documents are stored. */
    private File mDocumentsDir;

    private SecretKey mDataKey;
    private SecretKey mMacKey;

    @Override
    public boolean onCreate() {
        mHardwareBacked = KeyChain.isBoundKeyAlgorithm("RSA");

        mKeyFile = new File(getContext().getFilesDir(), "vault.key");
        mDocumentsDir = new File(getContext().getFilesDir(), "documents");
        mDocumentsDir.mkdirs();

        try {
            // Load secret key and ensure our root document is ready.
            loadOrGenerateKeys(getContext(), mKeyFile);
            initDocument(Long.parseLong(DEFAULT_DOCUMENT_ID), Document.MIME_TYPE_DIR, null);

        } catch (IOException e) {
            throw new IllegalStateException(e);
        } catch (GeneralSecurityException e) {
            throw new IllegalStateException(e);
        }

        return true;
    }

    /**
     * Used for testing.
     */
    void wipeAllContents() throws IOException, GeneralSecurityException {
        for (File f : mDocumentsDir.listFiles()) {
            f.delete();
        }

        initDocument(Long.parseLong(DEFAULT_DOCUMENT_ID), Document.MIME_TYPE_DIR, null);
    }

    /**
     * Load our symmetric secret key and use it to derive two different data and
     * MAC keys. The symmetric secret key is stored securely on disk by wrapping
     * it with a public/private key pair, possibly backed by hardware.
     */
    private void loadOrGenerateKeys(Context context, File keyFile)
            throws GeneralSecurityException, IOException {
        final SecretKeyWrapper wrapper = new SecretKeyWrapper(context, TAG);

        // Generate secret key if none exists
        if (!keyFile.exists()) {
            final byte[] raw = new byte[DATA_KEY_LENGTH];
            new SecureRandom().nextBytes(raw);

            final SecretKey key = new SecretKeySpec(raw, "AES");
            final byte[] wrapped = wrapper.wrap(key);

            writeFully(keyFile, wrapped);
        }

        // Even if we just generated the key, always read it back to ensure we
        // can read it successfully.
        final byte[] wrapped = readFully(keyFile);
        final SecretKey key = wrapper.unwrap(wrapped);

        final Mac mac = Mac.getInstance("HmacSHA256");
        mac.init(key);

        // Derive two different keys for encryption and authentication.
        final byte[] rawDataKey = new byte[DATA_KEY_LENGTH];
        final byte[] rawMacKey = new byte[MAC_KEY_LENGTH];

        System.arraycopy(mac.doFinal(BLOB_DATA), 0, rawDataKey, 0, rawDataKey.length);
        System.arraycopy(mac.doFinal(BLOB_MAC), 0, rawMacKey, 0, rawMacKey.length);

        mDataKey = new SecretKeySpec(rawDataKey, "AES");
        mMacKey = new SecretKeySpec(rawMacKey, "HmacSHA256");
    }

    @Override
    public Cursor queryRoots(String[] projection) throws FileNotFoundException {
        final MatrixCursor result = new MatrixCursor(resolveRootProjection(projection));
        final RowBuilder row = result.newRow();
        row.add(Root.COLUMN_ROOT_ID, DEFAULT_ROOT_ID);
        row.add(Root.COLUMN_FLAGS, Root.FLAG_SUPPORTS_CREATE | Root.FLAG_LOCAL_ONLY);
        row.add(Root.COLUMN_TITLE, getContext().getString(R.string.app_label));
        row.add(Root.COLUMN_DOCUMENT_ID, DEFAULT_DOCUMENT_ID);
        row.add(Root.COLUMN_ICON, R.drawable.ic_lock_lock);

        // Notify user in storage UI when key isn't hardware-backed
        if (!mHardwareBacked) {
            row.add(Root.COLUMN_SUMMARY, getContext().getString(R.string.info_software));
        }

        return result;
    }

    private EncryptedDocument getDocument(long docId) throws GeneralSecurityException {
        final File file = new File(mDocumentsDir, String.valueOf(docId));
        return new EncryptedDocument(docId, file, mDataKey, mMacKey);
    }

    /**
     * Include metadata for a document in the given result cursor.
     */
    private void includeDocument(MatrixCursor result, long docId)
            throws IOException, GeneralSecurityException {
        final EncryptedDocument doc = getDocument(docId);
        if (!doc.getFile().exists()) {
            throw new FileNotFoundException("Missing document " + docId);
        }

        final JSONObject meta = doc.readMetadata();

        int flags = 0;

        final String mimeType = meta.optString(Document.COLUMN_MIME_TYPE);
        if (Document.MIME_TYPE_DIR.equals(mimeType)) {
            flags |= Document.FLAG_DIR_SUPPORTS_CREATE;
        } else {
            flags |= Document.FLAG_SUPPORTS_WRITE;
        }
        flags |= Document.FLAG_SUPPORTS_DELETE;

        final RowBuilder row = result.newRow();
        row.add(Document.COLUMN_DOCUMENT_ID, meta.optLong(Document.COLUMN_DOCUMENT_ID));
        row.add(Document.COLUMN_DISPLAY_NAME, meta.optString(Document.COLUMN_DISPLAY_NAME));
        row.add(Document.COLUMN_SIZE, meta.optLong(Document.COLUMN_SIZE));
        row.add(Document.COLUMN_MIME_TYPE, mimeType);
        row.add(Document.COLUMN_FLAGS, flags);
        row.add(Document.COLUMN_LAST_MODIFIED, meta.optLong(Document.COLUMN_LAST_MODIFIED));
    }

    @Override
    public String createDocument(String parentDocumentId, String mimeType, String displayName)
            throws FileNotFoundException {
        final long parentDocId = Long.parseLong(parentDocumentId);

        // Allocate the next available ID
        final long childDocId;
        synchronized (mIdLock) {
            final SharedPreferences prefs = getContext()
                    .getSharedPreferences(PREF_NEXT_ID, Context.MODE_PRIVATE);
            childDocId = prefs.getLong(PREF_NEXT_ID, 1);
            if (!prefs.edit().putLong(PREF_NEXT_ID, childDocId + 1).commit()) {
                throw new IllegalStateException("Failed to allocate document ID");
            }
        }

        try {
            initDocument(childDocId, mimeType, displayName);

            // Update parent to reference new child
            final EncryptedDocument parentDoc = getDocument(parentDocId);
            final JSONObject parentMeta = parentDoc.readMetadata();
            parentMeta.accumulate(KEY_CHILDREN, childDocId);
            parentDoc.writeMetadataAndContent(parentMeta, null);

            return String.valueOf(childDocId);

        } catch (IOException e) {
            throw new IllegalStateException(e);
        } catch (GeneralSecurityException e) {
            throw new IllegalStateException(e);
        } catch (JSONException e) {
            throw new IllegalStateException(e);
        }
    }

    /**
     * Create document on disk, writing an initial metadata section. Someone
     * might come back later to write contents.
     */
    private void initDocument(long docId, String mimeType, String displayName)
            throws IOException, GeneralSecurityException {
        final EncryptedDocument doc = getDocument(docId);
        if (doc.getFile().exists()) return;

        try {
            final JSONObject meta = new JSONObject();
            meta.put(Document.COLUMN_DOCUMENT_ID, docId);
            meta.put(Document.COLUMN_MIME_TYPE, mimeType);
            meta.put(Document.COLUMN_DISPLAY_NAME, displayName);
            if (Document.MIME_TYPE_DIR.equals(mimeType)) {
                meta.put(KEY_CHILDREN, new JSONArray());
            }

            doc.writeMetadataAndContent(meta, null);
        } catch (JSONException e) {
            throw new IOException(e);
        }
    }

    @Override
    public void deleteDocument(String documentId) throws FileNotFoundException {
        final long docId = Long.parseLong(documentId);

        try {
            // Delete given document, any children documents under it, and any
            // references to it from parents.
            deleteDocumentTree(docId);
            deleteDocumentReferences(docId);

        } catch (IOException e) {
            throw new IllegalStateException(e);
        } catch (GeneralSecurityException e) {
            throw new IllegalStateException(e);
        }
    }

    /**
     * Recursively delete the given document and any children under it.
     */
    private void deleteDocumentTree(long docId) throws IOException, GeneralSecurityException {
        final EncryptedDocument doc = getDocument(docId);
        final JSONObject meta = doc.readMetadata();
        try {
            if (Document.MIME_TYPE_DIR.equals(meta.getString(Document.COLUMN_MIME_TYPE))) {
                final JSONArray children = meta.getJSONArray(KEY_CHILDREN);
                for (int i = 0; i < children.length(); i++) {
                    final long childDocId = children.getLong(i);
                    deleteDocumentTree(childDocId);
                }
            }
        } catch (JSONException e) {
            throw new IOException(e);
        }

        if (!doc.getFile().delete()) {
            throw new IOException("Failed to delete " + docId);
        }
    }

    /**
     * Remove any references to the given document, usually when included as a
     * child of another directory.
     */
    private void deleteDocumentReferences(long docId) {
        for (String name : mDocumentsDir.list()) {
            try {
                final long parentDocId = Long.parseLong(name);
                final EncryptedDocument parentDoc = getDocument(parentDocId);
                final JSONObject meta = parentDoc.readMetadata();

                if (Document.MIME_TYPE_DIR.equals(meta.getString(Document.COLUMN_MIME_TYPE))) {
                    final JSONArray children = meta.getJSONArray(KEY_CHILDREN);
                    if (maybeRemove(children, docId)) {
                        Log.d(TAG, "Removed " + docId + " reference from " + name);
                        parentDoc.writeMetadataAndContent(meta, null);

                        getContext().getContentResolver().notifyChange(
                                DocumentsContract.buildChildDocumentsUri(AUTHORITY, name), null,
                                false);
                    }
                }
            } catch (NumberFormatException ignored) {
            } catch (IOException e) {
                Log.w(TAG, "Failed to examine " + name, e);
            } catch (GeneralSecurityException e) {
                Log.w(TAG, "Failed to examine " + name, e);
            } catch (JSONException e) {
                Log.w(TAG, "Failed to examine " + name, e);
            }
        }
    }

    @Override
    public Cursor queryDocument(String documentId, String[] projection)
            throws FileNotFoundException {
        final MatrixCursor result = new MatrixCursor(resolveDocumentProjection(projection));
        try {
            includeDocument(result, Long.parseLong(documentId));
        } catch (GeneralSecurityException e) {
            throw new IllegalStateException(e);
        } catch (IOException e) {
            throw new IllegalStateException(e);
        }
        return result;
    }

    @Override
    public Cursor queryChildDocuments(
            String parentDocumentId, String[] projection, String sortOrder)
            throws FileNotFoundException {
        final ExtrasMatrixCursor result = new ExtrasMatrixCursor(
                resolveDocumentProjection(projection));
        result.setNotificationUri(getContext().getContentResolver(),
                DocumentsContract.buildChildDocumentsUri(AUTHORITY, parentDocumentId));

        // Notify user in storage UI when key isn't hardware-backed
        if (!mHardwareBacked) {
            result.putString(DocumentsContract.EXTRA_INFO,
                    getContext().getString(R.string.info_software_detail));
        }

        try {
            final EncryptedDocument doc = getDocument(Long.parseLong(parentDocumentId));
            final JSONObject meta = doc.readMetadata();
            final JSONArray children = meta.getJSONArray(KEY_CHILDREN);
            for (int i = 0; i < children.length(); i++) {
                final long docId = children.getLong(i);
                includeDocument(result, docId);
            }

        } catch (IOException e) {
            throw new IllegalStateException(e);
        } catch (GeneralSecurityException e) {
            throw new IllegalStateException(e);
        } catch (JSONException e) {
            throw new IllegalStateException(e);
        }

        return result;
    }

    @Override
    public ParcelFileDescriptor openDocument(
            String documentId, String mode, CancellationSignal signal)
            throws FileNotFoundException {
        final long docId = Long.parseLong(documentId);

        try {
            final EncryptedDocument doc = getDocument(docId);
            if ("r".equals(mode)) {
                return startRead(doc);
            } else if ("w".equals(mode) || "wt".equals(mode)) {
                return startWrite(doc);
            } else {
                throw new IllegalArgumentException("Unsupported mode: " + mode);
            }
        } catch (IOException e) {
            throw new IllegalStateException(e);
        } catch (GeneralSecurityException e) {
            throw new IllegalStateException(e);
        }
    }

    /**
     * Kick off a thread to handle a read request for the given document.
     * Internally creates a pipe and returns the read end for returning to a
     * remote process.
     */
    private ParcelFileDescriptor startRead(final EncryptedDocument doc) throws IOException {
        final ParcelFileDescriptor[] pipe = ParcelFileDescriptor.createReliablePipe();
        final ParcelFileDescriptor readEnd = pipe[0];
        final ParcelFileDescriptor writeEnd = pipe[1];

        new Thread() {
            @Override
            public void run() {
                try {
                    doc.readContent(writeEnd);
                    Log.d(TAG, "Success reading " + doc);
                    closeQuietly(writeEnd);
                } catch (IOException e) {
                    Log.w(TAG, "Failed reading " + doc, e);
                    closeWithErrorQuietly(writeEnd, e.toString());
                } catch (GeneralSecurityException e) {
                    Log.w(TAG, "Failed reading " + doc, e);
                    closeWithErrorQuietly(writeEnd, e.toString());
                }
            }
        }.start();

        return readEnd;
    }

    /**
     * Kick off a thread to handle a write request for the given document.
     * Internally creates a pipe and returns the write end for returning to a
     * remote process.
     */
    private ParcelFileDescriptor startWrite(final EncryptedDocument doc) throws IOException {
        final ParcelFileDescriptor[] pipe = ParcelFileDescriptor.createReliablePipe();
        final ParcelFileDescriptor readEnd = pipe[0];
        final ParcelFileDescriptor writeEnd = pipe[1];

        new Thread() {
            @Override
            public void run() {
                try {
                    final JSONObject meta = doc.readMetadata();
                    doc.writeMetadataAndContent(meta, readEnd);
                    Log.d(TAG, "Success writing " + doc);
                    closeQuietly(readEnd);
                } catch (IOException e) {
                    Log.w(TAG, "Failed writing " + doc, e);
                    closeWithErrorQuietly(readEnd, e.toString());
                } catch (GeneralSecurityException e) {
                    Log.w(TAG, "Failed writing " + doc, e);
                    closeWithErrorQuietly(readEnd, e.toString());
                }
            }
        }.start();

        return writeEnd;
    }

    /**
     * Maybe remove the given value from a {@link JSONArray}.
     *
     * @return if the array was mutated.
     */
    private static boolean maybeRemove(JSONArray array, long value) throws JSONException {
        boolean mutated = false;
        int i = 0;
        while (i < array.length()) {
            if (value == array.getLong(i)) {
                array.remove(i);
                mutated = true;
            } else {
                i++;
            }
        }
        return mutated;
    }

    /**
     * Simple extension of {@link MatrixCursor} that makes it easy to provide a
     * {@link Bundle} of extras.
     */
    private static class ExtrasMatrixCursor extends MatrixCursor {
        private Bundle mExtras;

        public ExtrasMatrixCursor(String[] columnNames) {
            super(columnNames);
        }

        public void putString(String key, String value) {
            if (mExtras == null) {
                mExtras = new Bundle();
            }
            mExtras.putString(key, value);
        }

        @Override
        public Bundle getExtras() {
            return mExtras;
        }
    }
}