Merge branch 'dev' into feature/sync

This reverts commit da018fc64d.

uhabits-android/build.gradle.kts

@@ -122,6 +122,7 @@ dependencies {
     implementation("androidx.legacy:legacy-preference-v14:1.0.0")
     implementation("androidx.legacy:legacy-support-v4:1.0.0")
     implementation("com.google.android.material:material:1.4.0")
+    implementation("com.google.zxing:core:3.4.1")
     implementation("com.opencsv:opencsv:5.5.1")
     implementation(project(":uhabits-core"))
     kapt("com.google.dagger:dagger-compiler:$daggerVersion")

uhabits-android/src/androidTest/java/org/isoron/uhabits/sync/RemoteSyncServerTest.kt

@@ -0,0 +1,154 @@
+/*
+ * Copyright (C) 2016-2021 Álinson Santos Xavier <git@axavier.org>
+ *
+ * This file is part of Loop Habit Tracker.
+ *
+ * Loop Habit Tracker is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by the
+ * Free Software Foundation, either version 3 of the License, or (at your
+ * option) any later version.
+ *
+ * Loop Habit Tracker 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 General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package org.isoron.uhabits.sync
+
+import androidx.test.filters.MediumTest
+import com.fasterxml.jackson.databind.ObjectMapper
+import io.ktor.client.HttpClient
+import io.ktor.client.engine.mock.MockEngine
+import io.ktor.client.engine.mock.MockRequestHandleScope
+import io.ktor.client.engine.mock.respond
+import io.ktor.client.engine.mock.respondError
+import io.ktor.client.engine.mock.respondOk
+import io.ktor.client.features.json.JsonFeature
+import io.ktor.client.request.HttpRequestData
+import io.ktor.client.request.HttpResponseData
+import io.ktor.http.HttpStatusCode
+import io.ktor.http.fullPath
+import io.ktor.http.headersOf
+import kotlinx.coroutines.runBlocking
+import org.isoron.uhabits.BaseAndroidTest
+import org.isoron.uhabits.core.sync.AbstractSyncServer
+import org.isoron.uhabits.core.sync.GetDataVersionResponse
+import org.isoron.uhabits.core.sync.KeyNotFoundException
+import org.isoron.uhabits.core.sync.RegisterReponse
+import org.isoron.uhabits.core.sync.ServiceUnavailable
+import org.isoron.uhabits.core.sync.SyncData
+import org.junit.Test
+
+@MediumTest
+class RemoteSyncServerTest : BaseAndroidTest() {
+
+    private val mapper = ObjectMapper()
+    val data = SyncData(1, "Hello world")
+
+    @Test
+    fun when_register_succeeds_should_return_key() = runBlocking {
+        val server = server("/register") {
+            respondWithJson(RegisterReponse("ABCDEF"))
+        }
+        assertEquals("ABCDEF", server.register())
+    }
+
+    @Test(expected = ServiceUnavailable::class)
+    fun when_register_fails_should_raise_correct_exception() = runBlocking {
+        val server = server("/register") {
+            respondError(HttpStatusCode.ServiceUnavailable)
+        }
+        server.register()
+        return@runBlocking
+    }
+
+    @Test
+    fun when_get_data_version_succeeds_should_return_version() = runBlocking {
+        server("/db/ABC/version") {
+            respondWithJson(GetDataVersionResponse(5))
+        }.apply {
+            assertEquals(5, getDataVersion("ABC"))
+        }
+        return@runBlocking
+    }
+
+    @Test(expected = ServiceUnavailable::class)
+    fun when_get_data_version_with_server_error_should_raise_exception() = runBlocking {
+        server("/db/ABC/version") {
+            respondError(HttpStatusCode.InternalServerError)
+        }.apply {
+            getDataVersion("ABC")
+        }
+        return@runBlocking
+    }
+
+    @Test(expected = KeyNotFoundException::class)
+    fun when_get_data_version_with_invalid_key_should_raise_exception() = runBlocking {
+        server("/db/ABC/version") {
+            respondError(HttpStatusCode.NotFound)
+        }.apply {
+            getDataVersion("ABC")
+        }
+        return@runBlocking
+    }
+
+    @Test
+    fun when_get_data_succeeds_should_return_data() = runBlocking {
+        server("/db/ABC") {
+            respondWithJson(data)
+        }.apply {
+            assertEquals(data, getData("ABC"))
+        }
+        return@runBlocking
+    }
+
+    @Test(expected = KeyNotFoundException::class)
+    fun when_get_data_with_invalid_key_should_raise_exception() = runBlocking {
+        server("/db/ABC") {
+            respondError(HttpStatusCode.NotFound)
+        }.apply {
+            getData("ABC")
+        }
+        return@runBlocking
+    }
+
+    @Test
+    fun when_put_succeeds_should_not_raise_exceptions() = runBlocking {
+        server("/db/ABC") {
+            respondOk()
+        }.apply {
+            put("ABC", data)
+        }
+        return@runBlocking
+    }
+
+    private fun server(
+        expectedPath: String,
+        action: MockRequestHandleScope.(HttpRequestData) -> HttpResponseData
+    ): AbstractSyncServer {
+        return RemoteSyncServer(
+            httpClient = HttpClient(MockEngine) {
+                install(JsonFeature)
+                engine {
+                    addHandler { request ->
+                        when (request.url.fullPath) {
+                            expectedPath -> action(request)
+                            else -> error("unexpected call: ${request.url.fullPath}")
+                        }
+                    }
+                }
+            },
+            preferences = prefs
+        )
+    }
+
+    private fun MockRequestHandleScope.respondWithJson(content: Any) =
+        respond(
+            mapper.writeValueAsBytes(content),
+            headers = headersOf("Content-Type" to listOf("application/json"))
+        )
+}

uhabits-android/src/main/AndroidManifest.xml

@@ -22,6 +22,8 @@
 
     <uses-permission android:name="android.permission.VIBRATE" />
     <uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED" />
+    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
+    <uses-permission android:name="android.permission.INTERNET" />
 
     <application
         android:name=".HabitsApplication"
@@ -40,6 +42,14 @@
                 android:value=".activities.habits.list.ListHabitsActivity" />
         </activity>
 
+        <activity
+            android:name=".activities.sync.SyncActivity"
+            android:exported="true">
+            <meta-data
+                android:name="android.support.PARENT_ACTIVITY"
+                android:value=".activities.settings.SettingsActivity" />
+        </activity>
+
         <meta-data
             android:name="com.google.android.backup.api_key"
             android:value="AEdPqrEAAAAI6aeWncbnMNo8E5GWeZ44dlc5cQ7tCROwFhOtiw" />
@@ -49,6 +59,16 @@
             android:exported="true"
             android:label="@string/main_activity_title"
             android:launchMode="singleTop">
+            <tools:validation testUrl="https://loophabits.org/sync/123" />
+            <intent-filter android:autoVerify="true">
+                <action android:name="android.intent.action.VIEW" />
+                <category android:name="android.intent.category.DEFAULT" />
+                <category android:name="android.intent.category.BROWSABLE" />
+                <data
+                    android:scheme="https"
+                    android:host="loophabits.org"
+                    android:pathPrefix="/sync" />
+            </intent-filter>
         </activity>
 
         <activity-alias

uhabits-android/src/main/java/org/isoron/uhabits/activities/common/dialogs/ConfirmSyncKeyDialog.kt

@@ -0,0 +1,45 @@
+/*
+ * Copyright (C) 2016-2021 Álinson Santos Xavier <git@axavier.org>
+ *
+ * This file is part of Loop Habit Tracker.
+ *
+ * Loop Habit Tracker is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by the
+ * Free Software Foundation, either version 3 of the License, or (at your
+ * option) any later version.
+ *
+ * Loop Habit Tracker 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 General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+package org.isoron.uhabits.activities.common.dialogs
+
+import android.content.Context
+import android.content.DialogInterface
+import androidx.appcompat.app.AlertDialog
+import org.isoron.uhabits.R
+import org.isoron.uhabits.core.ui.callbacks.OnConfirmedCallback
+import org.isoron.uhabits.inject.ActivityContext
+
+class ConfirmSyncKeyDialog(
+    @ActivityContext context: Context,
+    callback: OnConfirmedCallback
+) : AlertDialog(context) {
+    init {
+        setTitle(R.string.device_sync)
+        val res = context.resources
+        setMessage(res.getString(R.string.sync_confirm))
+        setButton(
+            BUTTON_POSITIVE,
+            res.getString(R.string.yes)
+        ) { dialog: DialogInterface?, which: Int -> callback.onConfirmed() }
+        setButton(
+            BUTTON_NEGATIVE,
+            res.getString(R.string.no)
+        ) { dialog: DialogInterface?, which: Int -> }
+    }
+}

uhabits-android/src/main/java/org/isoron/uhabits/activities/habits/list/ListHabitsActivity.kt

@@ -26,10 +26,12 @@ import android.view.MenuItem
 import androidx.appcompat.app.AppCompatActivity
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.launch
 import org.isoron.uhabits.BaseExceptionHandler
 import org.isoron.uhabits.HabitsApplication
 import org.isoron.uhabits.activities.habits.list.views.HabitCardListAdapter
 import org.isoron.uhabits.core.preferences.Preferences
+import org.isoron.uhabits.core.sync.SyncManager
 import org.isoron.uhabits.core.tasks.TaskRunner
 import org.isoron.uhabits.core.ui.ThemeSwitcher.Companion.THEME_DARK
 import org.isoron.uhabits.core.utils.MidnightTimer
@@ -47,6 +49,7 @@ class ListHabitsActivity : AppCompatActivity(), Preferences.Listener {
     lateinit var screen: ListHabitsScreen
     lateinit var prefs: Preferences
     lateinit var midnightTimer: MidnightTimer
+    lateinit var syncManager: SyncManager
     private val scope = CoroutineScope(Dispatchers.Main)
 
     private lateinit var menu: ListHabitsMenu
@@ -68,6 +71,7 @@ class ListHabitsActivity : AppCompatActivity(), Preferences.Listener {
 
         prefs = appComponent.preferences
         prefs.addListener(this)
+        syncManager = appComponent.syncManager
         pureBlack = prefs.isPureBlackEnabled
         midnightTimer = appComponent.midnightTimer
         rootView = component.listHabitsRootView
@@ -84,6 +88,9 @@ class ListHabitsActivity : AppCompatActivity(), Preferences.Listener {
         midnightTimer.onPause()
         screen.onDettached()
         adapter.cancelRefresh()
+        scope.launch {
+            syncManager.onPause()
+        }
         super.onPause()
     }
 
@@ -92,6 +99,9 @@ class ListHabitsActivity : AppCompatActivity(), Preferences.Listener {
         screen.onAttached()
         rootView.postInvalidate()
         midnightTimer.onResume()
+        scope.launch {
+            syncManager.onResume()
+        }
         taskRunner.run {
             AutoBackup(this@ListHabitsActivity).run()
         }

uhabits-android/src/main/java/org/isoron/uhabits/activities/habits/list/ListHabitsScreen.kt

@@ -22,11 +22,13 @@ package org.isoron.uhabits.activities.habits.list
 import android.app.Activity
 import android.content.Context
 import android.content.Intent
+import android.util.Log
 import androidx.appcompat.app.AppCompatActivity
 import dagger.Lazy
 import org.isoron.uhabits.R
 import org.isoron.uhabits.activities.common.dialogs.ColorPickerDialogFactory
 import org.isoron.uhabits.activities.common.dialogs.ConfirmDeleteDialog
+import org.isoron.uhabits.activities.common.dialogs.ConfirmSyncKeyDialog
 import org.isoron.uhabits.activities.common.dialogs.NumberPickerFactory
 import org.isoron.uhabits.activities.habits.edit.HabitTypeDialog
 import org.isoron.uhabits.activities.habits.list.views.HabitCardListAdapter
@@ -51,6 +53,8 @@ import org.isoron.uhabits.core.ui.screens.habits.list.ListHabitsBehavior.Message
 import org.isoron.uhabits.core.ui.screens.habits.list.ListHabitsBehavior.Message.FILE_NOT_RECOGNIZED
 import org.isoron.uhabits.core.ui.screens.habits.list.ListHabitsBehavior.Message.IMPORT_FAILED
 import org.isoron.uhabits.core.ui.screens.habits.list.ListHabitsBehavior.Message.IMPORT_SUCCESSFUL
+import org.isoron.uhabits.core.ui.screens.habits.list.ListHabitsBehavior.Message.SYNC_ENABLED
+import org.isoron.uhabits.core.ui.screens.habits.list.ListHabitsBehavior.Message.SYNC_KEY_ALREADY_INSTALLED
 import org.isoron.uhabits.core.ui.screens.habits.list.ListHabitsMenuBehavior
 import org.isoron.uhabits.core.ui.screens.habits.list.ListHabitsSelectionMenuBehavior
 import org.isoron.uhabits.inject.ActivityContext
@@ -99,6 +103,14 @@ class ListHabitsScreen
 
     fun onAttached() {
         commandRunner.addListener(this)
+        if (activity.intent.action == "android.intent.action.VIEW") {
+            val uri = activity.intent.data!!.toString()
+            val parts = uri.replace(Regex("^.*sync/"), "").split("#")
+            val syncKey = parts[0]
+            val encKey = parts[1]
+            Log.i("ListHabitsScreen", "sync: $syncKey enc: $encKey")
+            behavior.get().onSyncKeyOffer(syncKey, encKey)
+        }
     }
 
     fun onDettached() {
@@ -196,6 +208,8 @@ class ListHabitsScreen
                     DATABASE_REPAIRED -> R.string.database_repaired
                     COULD_NOT_GENERATE_BUG_REPORT -> R.string.bug_report_failed
                     FILE_NOT_RECOGNIZED -> R.string.file_not_recognized
+                    SYNC_ENABLED -> R.string.sync_enabled
+                    SYNC_KEY_ALREADY_INSTALLED -> R.string.sync_key_already_installed
                 }
             )
         )
@@ -230,6 +244,10 @@ class ListHabitsScreen
         numberPickerFactory.create(value, unit, callback).show()
     }
 
+    override fun showConfirmInstallSyncKey(callback: OnConfirmedCallback) {
+        ConfirmSyncKeyDialog(activity, callback).show()
+    }
+
     private fun getExecuteString(command: Command): String? {
         when (command) {
             is ArchiveHabitsCommand -> {

uhabits-android/src/main/java/org/isoron/uhabits/activities/settings/SettingsFragment.kt

@@ -19,6 +19,7 @@
 package org.isoron.uhabits.activities.settings
 
 import android.app.backup.BackupManager
+import android.content.Context
 import android.content.Intent
 import android.content.SharedPreferences
 import android.content.SharedPreferences.OnSharedPreferenceChangeListener
@@ -27,6 +28,7 @@ import android.os.Build.VERSION
 import android.os.Bundle
 import android.provider.Settings
 import android.util.Log
+import androidx.preference.CheckBoxPreference
 import androidx.preference.ListPreference
 import androidx.preference.Preference
 import androidx.preference.PreferenceCategory
@@ -41,6 +43,7 @@ import org.isoron.uhabits.activities.habits.list.RESULT_REPAIR_DB
 import org.isoron.uhabits.core.preferences.Preferences
 import org.isoron.uhabits.core.ui.NotificationTray
 import org.isoron.uhabits.core.utils.DateUtils.Companion.getLongWeekdayNames
+import org.isoron.uhabits.intents.IntentFactory
 import org.isoron.uhabits.notifications.AndroidNotificationTray.Companion.createAndroidNotificationChannel
 import org.isoron.uhabits.notifications.RingtoneManager
 import org.isoron.uhabits.widgets.WidgetUpdater
@@ -97,6 +100,13 @@ class SettingsFragment : PreferenceFragmentCompat(), OnSharedPreferenceChangeLis
             intent.putExtra(Settings.EXTRA_CHANNEL_ID, NotificationTray.REMINDERS_CHANNEL_ID)
             startActivity(intent)
             return true
+        } else if (key == "pref_sync_enabled_dummy") {
+            if (prefs.isSyncEnabled) {
+                prefs.disableSync()
+            } else {
+                val context: Context? = activity
+                context!!.startActivity(IntentFactory().startSyncActivity(context))
+            }
         }
         return super.onPreferenceTreeClick(preference)
     }
@@ -111,6 +121,7 @@ class SettingsFragment : PreferenceFragmentCompat(), OnSharedPreferenceChangeLis
             devCategory.isVisible = false
         }
         updateWeekdayPreference()
+        updateSyncPreferences()
 
         if (VERSION.SDK_INT < Build.VERSION_CODES.O)
             findPreference("reminderCustomize").isVisible = false
@@ -119,6 +130,12 @@ class SettingsFragment : PreferenceFragmentCompat(), OnSharedPreferenceChangeLis
         }
     }
 
+    private fun updateSyncPreferences() {
+        findPreference("pref_sync_display").isVisible = prefs.isSyncEnabled
+        (findPreference("pref_sync_enabled_dummy") as CheckBoxPreference).isChecked =
+            prefs.isSyncEnabled
+    }
+
     private fun updateWeekdayPreference() {
         val weekdayPref = findPreference("pref_first_weekday") as ListPreference
         val currentFirstWeekday = prefs.firstWeekday.daysSinceSunday + 1
@@ -140,6 +157,7 @@ class SettingsFragment : PreferenceFragmentCompat(), OnSharedPreferenceChangeLis
         }
         BackupManager.dataChanged("org.isoron.uhabits")
         updateWeekdayPreference()
+        updateSyncPreferences()
     }
 
     private fun setResultOnPreferenceClick(key: String, result: Int) {

uhabits-android/src/main/java/org/isoron/uhabits/activities/sync/SyncActivity.kt

@@ -0,0 +1,132 @@
+/*
+ * Copyright (C) 2016-2021 Álinson Santos Xavier <git@axavier.org>
+ *
+ * This file is part of Loop Habit Tracker.
+ *
+ * Loop Habit Tracker is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by the
+ * Free Software Foundation, either version 3 of the License, or (at your
+ * option) any later version.
+ *
+ * Loop Habit Tracker 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 General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package org.isoron.uhabits.activities.sync
+
+import android.content.ClipData
+import android.content.ClipboardManager
+import android.content.Context
+import android.graphics.Bitmap
+import android.graphics.Color
+import android.os.Bundle
+import android.text.Html
+import android.view.View
+import androidx.appcompat.app.AppCompatActivity
+import com.google.zxing.BarcodeFormat
+import com.google.zxing.qrcode.QRCodeWriter
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.invoke
+import kotlinx.coroutines.launch
+import org.isoron.uhabits.HabitsApplication
+import org.isoron.uhabits.R
+import org.isoron.uhabits.activities.AndroidThemeSwitcher
+import org.isoron.uhabits.core.models.PaletteColor
+import org.isoron.uhabits.core.ui.screens.sync.SyncBehavior
+import org.isoron.uhabits.databinding.ActivitySyncBinding
+import org.isoron.uhabits.sync.RemoteSyncServer
+import org.isoron.uhabits.utils.InterfaceUtils.getFontAwesome
+import org.isoron.uhabits.utils.setupToolbar
+import org.isoron.uhabits.utils.showMessage
+
+class SyncActivity : AppCompatActivity(), SyncBehavior.Screen {
+
+    private lateinit var binding: ActivitySyncBinding
+    private lateinit var behavior: SyncBehavior
+
+    private val scope = CoroutineScope(Dispatchers.Main)
+
+    override fun onCreate(savedInstance: Bundle?) {
+        super.onCreate(savedInstance)
+        val component = (application as HabitsApplication).component
+        val preferences = component.preferences
+        val server = RemoteSyncServer(preferences = preferences)
+        val themeSwitcher = AndroidThemeSwitcher(this, component.preferences)
+        themeSwitcher.apply()
+
+        behavior = SyncBehavior(this, preferences, server, component.logging)
+        binding = ActivitySyncBinding.inflate(layoutInflater)
+        binding.errorIcon.typeface = getFontAwesome(this)
+        binding.root.setupToolbar(
+            toolbar = binding.toolbar,
+            color = PaletteColor(11),
+            title = resources.getString(R.string.device_sync),
+            theme = themeSwitcher.currentTheme,
+        )
+        binding.syncLink.setOnClickListener { copyToClipboard() }
+        binding.instructions.text = Html.fromHtml(resources.getString(R.string.sync_instructions))
+        setContentView(binding.root)
+    }
+
+    override fun onResume() {
+        super.onResume()
+        scope.launch {
+            behavior.onResume()
+        }
+    }
+
+    private fun copyToClipboard() {
+        val clipboard = getSystemService(Context.CLIPBOARD_SERVICE) as ClipboardManager
+        clipboard.setPrimaryClip(ClipData.newPlainText("Loop Sync Link", binding.syncLink.text))
+        showMessage(resources.getString(R.string.copied_to_the_clipboard))
+    }
+
+    suspend fun generateQR(msg: String): Bitmap = Dispatchers.IO {
+        val writer = QRCodeWriter()
+        val matrix = writer.encode(msg, BarcodeFormat.QR_CODE, 1024, 1024)
+        val height = matrix.height
+        val width = matrix.width
+        val bitmap = Bitmap.createBitmap(width, height, Bitmap.Config.RGB_565)
+        val bgColor = Color.WHITE
+        val fgColor = Color.BLACK
+        for (x in 0 until width) {
+            for (y in 0 until height) {
+                val color = if (matrix.get(x, y)) fgColor else bgColor
+                bitmap.setPixel(x, y, color)
+            }
+        }
+        return@IO bitmap
+    }
+
+    suspend fun showQR(msg: String) {
+        binding.progress.visibility = View.GONE
+        binding.qrCode.visibility = View.VISIBLE
+        binding.qrCode.setImageBitmap(generateQR(msg))
+    }
+
+    override suspend fun showLoadingScreen() {
+        binding.qrCode.visibility = View.GONE
+        binding.progress.visibility = View.VISIBLE
+        binding.errorPanel.visibility = View.GONE
+    }
+
+    override suspend fun showErrorScreen() {
+        binding.qrCode.visibility = View.GONE
+        binding.progress.visibility = View.GONE
+        binding.errorPanel.visibility = View.VISIBLE
+    }
+
+    override suspend fun showLink(link: String) {
+        binding.qrCode.visibility = View.GONE
+        binding.progress.visibility = View.VISIBLE
+        binding.errorPanel.visibility = View.GONE
+        binding.syncLink.text = link
+        showQR(link)
+    }
+}

uhabits-android/src/main/java/org/isoron/uhabits/inject/HabitsApplicationComponent.kt

@@ -29,6 +29,7 @@ import org.isoron.uhabits.core.models.ModelFactory
 import org.isoron.uhabits.core.preferences.Preferences
 import org.isoron.uhabits.core.preferences.WidgetPreferences
 import org.isoron.uhabits.core.reminders.ReminderScheduler
+import org.isoron.uhabits.core.sync.SyncManager
 import org.isoron.uhabits.core.tasks.TaskRunner
 import org.isoron.uhabits.core.ui.NotificationTray
 import org.isoron.uhabits.core.ui.screens.habits.list.HabitCardListCache
@@ -63,4 +64,5 @@ interface HabitsApplicationComponent {
     val taskRunner: TaskRunner
     val widgetPreferences: WidgetPreferences
     val widgetUpdater: WidgetUpdater
+    val syncManager: SyncManager
 }

uhabits-android/src/main/java/org/isoron/uhabits/inject/HabitsModule.kt

@@ -19,6 +19,7 @@
 
 package org.isoron.uhabits.inject
 
+import android.content.Context
 import dagger.Module
 import dagger.Provides
 import org.isoron.uhabits.core.AppScope
@@ -33,6 +34,8 @@ import org.isoron.uhabits.core.models.sqlite.SQLiteHabitList
 import org.isoron.uhabits.core.preferences.Preferences
 import org.isoron.uhabits.core.preferences.WidgetPreferences
 import org.isoron.uhabits.core.reminders.ReminderScheduler
+import org.isoron.uhabits.core.sync.AbstractSyncServer
+import org.isoron.uhabits.core.sync.NetworkManager
 import org.isoron.uhabits.core.tasks.TaskRunner
 import org.isoron.uhabits.core.ui.NotificationTray
 import org.isoron.uhabits.database.AndroidDatabase
@@ -41,6 +44,8 @@ import org.isoron.uhabits.intents.IntentScheduler
 import org.isoron.uhabits.io.AndroidLogging
 import org.isoron.uhabits.notifications.AndroidNotificationTray
 import org.isoron.uhabits.preferences.SharedPreferencesStorage
+import org.isoron.uhabits.sync.AndroidNetworkManager
+import org.isoron.uhabits.sync.RemoteSyncServer
 import org.isoron.uhabits.utils.DatabaseUtils
 import java.io.File
 
@@ -109,6 +114,18 @@ class HabitsModule(dbFile: File) {
         return AndroidLogging()
     }
 
+    @Provides
+    @AppScope
+    fun getNetworkManager(@AppContext context: Context): NetworkManager {
+        return AndroidNetworkManager(context)
+    }
+
+    @Provides
+    @AppScope
+    fun getSyncServer(preferences: Preferences): AbstractSyncServer {
+        return RemoteSyncServer(preferences)
+    }
+
     @Provides
     @AppScope
     fun getDatabase(): Database {

uhabits-android/src/main/java/org/isoron/uhabits/intents/IntentFactory.kt

@@ -28,6 +28,7 @@ import org.isoron.uhabits.activities.habits.edit.EditHabitActivity
 import org.isoron.uhabits.activities.habits.show.ShowHabitActivity
 import org.isoron.uhabits.activities.intro.IntroActivity
 import org.isoron.uhabits.activities.settings.SettingsActivity
+import org.isoron.uhabits.activities.sync.SyncActivity
 import org.isoron.uhabits.core.models.Habit
 import javax.inject.Inject
 
@@ -100,4 +101,8 @@ class IntentFactory
         intent.putExtra("habitType", habitType)
         return intent
     }
+
+    fun startSyncActivity(context: Context): Intent {
+        return Intent(context, SyncActivity::class.java)
+    }
 }

uhabits-android/src/main/java/org/isoron/uhabits/sync/AndroidNetworkManager.kt

@@ -0,0 +1,59 @@
+/*
+ * Copyright (C) 2016-2021 Álinson Santos Xavier <git@axavier.org>
+ *
+ * This file is part of Loop Habit Tracker.
+ *
+ * Loop Habit Tracker is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by the
+ * Free Software Foundation, either version 3 of the License, or (at your
+ * option) any later version.
+ *
+ * Loop Habit Tracker 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 General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package org.isoron.uhabits.sync
+
+import android.content.Context
+import android.net.ConnectivityManager
+import android.net.Network
+import android.net.NetworkRequest
+import org.isoron.uhabits.core.sync.NetworkManager
+
+class AndroidNetworkManager(
+    val context: Context,
+) : NetworkManager, ConnectivityManager.NetworkCallback() {
+
+    val listeners = mutableListOf<NetworkManager.Listener>()
+    var connected = false
+
+    init {
+        val cm = context.getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager
+        cm.registerNetworkCallback(NetworkRequest.Builder().build(), this)
+    }
+
+    override fun addListener(listener: NetworkManager.Listener) {
+        if (connected) listener.onNetworkAvailable()
+        else listener.onNetworkLost()
+        listeners.add(listener)
+    }
+
+    override fun remoteListener(listener: NetworkManager.Listener) {
+        listeners.remove(listener)
+    }
+
+    override fun onAvailable(network: Network) {
+        connected = true
+        for (l in listeners) l.onNetworkAvailable()
+    }
+
+    override fun onLost(network: Network) {
+        connected = false
+        for (l in listeners) l.onNetworkLost()
+    }
+}

uhabits-android/src/main/java/org/isoron/uhabits/sync/RemoteSyncServer.kt

@@ -0,0 +1,105 @@
+/*
+ * Copyright (C) 2016-2021 Álinson Santos Xavier <git@axavier.org>
+ *
+ * This file is part of Loop Habit Tracker.
+ *
+ * Loop Habit Tracker is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by the
+ * Free Software Foundation, either version 3 of the License, or (at your
+ * option) any later version.
+ *
+ * Loop Habit Tracker 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 General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package org.isoron.uhabits.sync
+
+import android.util.Log
+import io.ktor.client.HttpClient
+import io.ktor.client.engine.android.Android
+import io.ktor.client.features.ClientRequestException
+import io.ktor.client.features.ServerResponseException
+import io.ktor.client.features.json.JsonFeature
+import io.ktor.client.request.get
+import io.ktor.client.request.header
+import io.ktor.client.request.post
+import io.ktor.client.request.put
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.invoke
+import org.isoron.uhabits.core.preferences.Preferences
+import org.isoron.uhabits.core.sync.AbstractSyncServer
+import org.isoron.uhabits.core.sync.EditConflictException
+import org.isoron.uhabits.core.sync.GetDataVersionResponse
+import org.isoron.uhabits.core.sync.KeyNotFoundException
+import org.isoron.uhabits.core.sync.RegisterReponse
+import org.isoron.uhabits.core.sync.ServiceUnavailable
+import org.isoron.uhabits.core.sync.SyncData
+
+class RemoteSyncServer(
+    private val preferences: Preferences,
+    private val httpClient: HttpClient = HttpClient(Android) {
+        install(JsonFeature)
+    }
+) : AbstractSyncServer {
+
+    override suspend fun register(): String = Dispatchers.IO {
+        try {
+            val url = "${preferences.syncBaseURL}/register"
+            Log.i("RemoteSyncServer", "POST $url")
+            val response: RegisterReponse = httpClient.post(url)
+            return@IO response.key
+        } catch (e: ServerResponseException) {
+            throw ServiceUnavailable()
+        }
+    }
+
+    override suspend fun put(key: String, newData: SyncData) = Dispatchers.IO {
+        try {
+            val url = "${preferences.syncBaseURL}/db/$key"
+            Log.i("RemoteSyncServer", "PUT $url")
+            val response: String = httpClient.put(url) {
+                header("Content-Type", "application/json")
+                body = newData
+            }
+        } catch (e: ServerResponseException) {
+            throw ServiceUnavailable()
+        } catch (e: ClientRequestException) {
+            Log.w("RemoteSyncServer", "ClientRequestException", e)
+            if (e.message!!.contains("409")) throw EditConflictException()
+            if (e.message!!.contains("404")) throw KeyNotFoundException()
+            throw e
+        }
+    }
+
+    override suspend fun getData(key: String): SyncData = Dispatchers.IO {
+        try {
+            val url = "${preferences.syncBaseURL}/db/$key"
+            Log.i("RemoteSyncServer", "GET $url")
+            return@IO httpClient.get<SyncData>(url)
+        } catch (e: ServerResponseException) {
+            throw ServiceUnavailable()
+        } catch (e: ClientRequestException) {
+            Log.w("RemoteSyncServer", "ClientRequestException", e)
+            throw KeyNotFoundException()
+        }
+    }
+
+    override suspend fun getDataVersion(key: String): Long = Dispatchers.IO {
+        try {
+            val url = "${preferences.syncBaseURL}/db/$key/version"
+            Log.i("RemoteSyncServer", "GET $url")
+            val response: GetDataVersionResponse = httpClient.get(url)
+            return@IO response.version
+        } catch (e: ServerResponseException) {
+            throw ServiceUnavailable()
+        } catch (e: ClientRequestException) {
+            Log.w("RemoteSyncServer", "ClientRequestException", e)
+            throw KeyNotFoundException()
+        }
+    }
+}

uhabits-android/src/main/res/layout/activity_sync.xml

@@ -0,0 +1,147 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+  ~ Copyright (C) 2016-2021 Álinson Santos Xavier <git@axavier.org>
+  ~
+  ~ This file is part of Loop Habit Tracker.
+  ~
+  ~ Loop Habit Tracker is free software: you can redistribute it and/or modify
+  ~ it under the terms of the GNU General Public License as published by the
+  ~ Free Software Foundation, either version 3 of the License, or (at your
+  ~ option) any later version.
+  ~
+  ~ Loop Habit Tracker 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 General Public License for
+  ~ more details.
+  ~
+  ~ You should have received a copy of the GNU General Public License along
+  ~ with this program. If not, see <http://www.gnu.org/licenses/>.
+  -->
+
+<androidx.coordinatorlayout.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:app="http://schemas.android.com/apk/res-auto"
+    xmlns:tools="http://schemas.android.com/tools"
+    android:layout_width="match_parent"
+    android:layout_height="match_parent"
+    android:background="?attr/contrast0"
+    android:fitsSystemWindows="true"
+    android:orientation="vertical"
+    tools:context=".activities.habits.edit.EditHabitActivity">
+
+    <com.google.android.material.appbar.AppBarLayout
+        android:layout_width="match_parent"
+        android:layout_height="wrap_content">
+
+        <androidx.appcompat.widget.Toolbar
+            android:id="@+id/toolbar"
+            android:layout_width="match_parent"
+            android:layout_height="wrap_content"
+            android:background="?attr/colorPrimary"
+            android:elevation="2dp"
+            android:gravity="end"
+            android:minHeight="?attr/actionBarSize"
+            android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"
+            app:title="@string/device_sync"
+            app:titleTextColor="@color/white">
+
+        </androidx.appcompat.widget.Toolbar>
+
+    </com.google.android.material.appbar.AppBarLayout>
+
+    <ScrollView
+        android:layout_width="match_parent"
+        android:layout_height="wrap_content"
+        app:layout_behavior="@string/appbar_scrolling_view_behavior">
+
+        <LinearLayout
+            android:layout_width="match_parent"
+            android:layout_height="wrap_content"
+            android:orientation="vertical"
+            android:paddingLeft="4dp"
+            android:paddingTop="8dp"
+            android:paddingRight="4dp">
+
+            <TextView
+                android:layout_width="match_parent"
+                android:layout_height="wrap_content"
+                android:lineSpacingExtra="4sp"
+                android:padding="8dp"
+                android:paddingBottom="16dp"
+                android:textSize="@dimen/regularTextSize"
+                android:id="@+id/instructions"
+                />
+
+            <!-- Sync Link (QR) -->
+            <FrameLayout style="@style/FormOuterBox">
+
+                <LinearLayout
+                    style="@style/FormInnerBox">
+                    <TextView
+                        style="@style/FormLabel"
+                        android:translationZ="0.01dp"
+                        android:text="@string/sync_link_qr" />
+
+                    <LinearLayout
+                        android:id="@+id/errorPanel"
+                        android:layout_width="match_parent"
+                        android:layout_height="200dp"
+                        android:orientation="vertical"
+                        android:gravity="center">
+
+                        <TextView
+                            android:id="@+id/errorIcon"
+                            android:layout_width="wrap_content"
+                            android:layout_height="wrap_content"
+                            android:textSize="40dp"
+                            android:layout_margin="40dp"
+                            android:text="@string/fa_exclamation_circle" />
+
+                        <TextView
+                            android:layout_width="wrap_content"
+                            android:layout_height="wrap_content"
+                            android:text="Error generating code. Please try again later."
+                            />
+                    </LinearLayout>
+
+                    <ProgressBar
+                        android:id="@+id/progress"
+                        android:layout_width="50dp"
+                        android:layout_height="200dp"
+                        android:layout_gravity="center"
+                        android:indeterminate="true"
+                        />
+
+                    <ImageView
+                        android:id="@+id/qrCode"
+                        android:layout_margin="10dp"
+                        android:layout_width="200dp"
+                        android:layout_height="200dp"
+                        android:layout_gravity="center"
+                        />
+                </LinearLayout>
+            </FrameLayout>
+
+            <!-- Sync Link -->
+            <FrameLayout style="@style/FormOuterBox">
+
+                <LinearLayout style="@style/FormInnerBox">
+
+                    <TextView
+                        style="@style/FormLabel"
+                        android:text="@string/sync_link" />
+
+                    <TextView
+                        android:id="@+id/syncLink"
+                        style="@style/FormInput"
+                        android:singleLine="true"
+                        android:text=""
+                        android:background="@drawable/ripple"
+                        android:textSize="@dimen/smallTextSize"
+                        android:layout_margin="8dp"
+                        />
+                </LinearLayout>
+            </FrameLayout>
+        </LinearLayout>
+    </ScrollView>
+
+</androidx.coordinatorlayout.widget.CoordinatorLayout>

uhabits-android/src/main/res/values/strings.xml

@@ -222,6 +222,18 @@
     <string name="decrement">Decrement</string>
     <string name="pref_skip_title">Enable skip days</string>
     <string name="pref_skip_description">Toggle twice to add a skip instead of a checkmark. Skips keep your score unchanged and don\'t break your streak.</string>
+    <string name="device_sync">Device sync</string>
+    <string name="pref_sync_summary">When enabled, an encrypted copy of your data will be uploaded to our servers. See privacy policy.</string>
+    <string name="pref_sync_title">Sync data across devices</string>
+    <string name="display_sync_code">Show device sync instructions</string>
+    <string name="sync_instructions"><![CDATA[<b>Instructions:</b><br/>1. Install Loop in your second device.<br/>2. Open the link below in your second device.<br/><b>Important:</b> Do not not make this information public. It gives anyone access to your data.]]></string>
+    <string name="sync_link">Sync link</string>
+    <string name="sync_link_qr">Sync link (QR code)</string>
+    <string name="password">Password</string>
+    <string name="copied_to_the_clipboard">Copied to the clipboard</string>
+    <string name="sync_confirm"><![CDATA[Are you trying to enable device sync?\n\nThis feature allows you to sync your data across multiple devices. When enabled, an encrypted copy of your data will be uploaded to our servers.]]></string>
+    <string name="sync_enabled">Device sync enabled</string>
+    <string name="sync_key_already_installed">Sync key already installed</string>
     <string name="pref_unknown_title">Show question marks for missing data</string>
     <string name="pref_unknown_description">Differentiate days without data from actual lapses. To enter a lapse, toggle twice.</string>
     <string name="you_are_now_a_developer">You are now a developer</string>

uhabits-android/src/main/res/xml/preferences.xml

@@ -114,6 +114,30 @@
 
     </PreferenceCategory>
 
+    <PreferenceCategory
+        android:key="deviceSync"
+        android:title="@string/device_sync" >
+
+        <CheckBoxPreference
+            android:defaultValue="false"
+            android:key="pref_sync_enabled_dummy"
+            android:summary="@string/pref_sync_summary"
+            android:title="@string/pref_sync_title"
+            app:iconSpaceReserved="false" />
+
+        <Preference
+            android:key="pref_sync_display"
+            android:title="@string/display_sync_code"
+            app:iconSpaceReserved="false">
+            <intent
+                android:action="android.intent.action.VIEW"
+                android:targetClass="org.isoron.uhabits.activities.sync.SyncActivity"
+                android:targetPackage="org.isoron.uhabits" />
+        </Preference>
+
+    </PreferenceCategory>
+
+
     <PreferenceCategory
             android:key="databaseCategory"
             android:title="@string/database">

uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/preferences/Preferences.kt

@@ -204,6 +204,27 @@ open class Preferences(private val storage: Storage) {
         set(value) {
             storage.putBoolean("pref_skip_enabled", value)
         }
+    val syncBaseURL: String
+        get() = storage.getString("pref_sync_base_url", "")
+    val syncKey: String
+        get() = storage.getString("pref_sync_key", "")
+    val encryptionKey: String
+        get() = storage.getString("pref_encryption_key", "")
+    val isSyncEnabled: Boolean
+        get() = storage.getBoolean("pref_sync_enabled", false)
+
+    fun enableSync(syncKey: String, encKey: String) {
+        storage.putBoolean("pref_sync_enabled", true)
+        storage.putString("pref_sync_key", syncKey)
+        storage.putString("pref_encryption_key", encKey)
+        for (l in listeners) l.onSyncEnabled()
+    }
+
+    fun disableSync() {
+        storage.putBoolean("pref_sync_enabled", false)
+        storage.putString("pref_sync_key", "")
+        storage.putString("pref_encryption_key", "")
+    }
 
     var areQuestionMarksEnabled: Boolean
         get() = storage.getBoolean("pref_unknown_enabled", false)
@@ -244,6 +265,7 @@ open class Preferences(private val storage: Storage) {
         fun onCheckmarkSequenceChanged() {}
         fun onNotificationsChanged() {}
         fun onQuestionMarksChanged() {}
+        fun onSyncEnabled() {}
     }
 
     interface Storage {

uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/sync/AbstractSyncServer.kt

@@ -0,0 +1,60 @@
+/*
+ * Copyright (C) 2016-2021 Álinson Santos Xavier <git@axavier.org>
+ *
+ * This file is part of Loop Habit Tracker.
+ *
+ * Loop Habit Tracker is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by the
+ * Free Software Foundation, either version 3 of the License, or (at your
+ * option) any later version.
+ *
+ * Loop Habit Tracker 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 General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package org.isoron.uhabits.core.sync
+
+interface AbstractSyncServer {
+    /**
+     * Generates and returns a new sync key, which can be used to store and retrive
+     * data.
+     *
+     * @throws ServiceUnavailable If key cannot be generated at this time, for example,
+     *      due to insufficient server resources, temporary server maintenance or network problems.
+     */
+    suspend fun register(): String
+
+    /**
+     * Replaces data for a given sync key.
+     *
+     * @throws KeyNotFoundException If key is not found
+     * @throws EditConflictException If the version of the data provided is not
+     *      exactly the current data version plus one.
+     * @throws ServiceUnavailable If data cannot be put at this time, for example, due
+     *      to insufficient server resources or network problems.
+     */
+    suspend fun put(key: String, newData: SyncData)
+
+    /**
+     * Returns data for a given sync key.
+     *
+     * @throws KeyNotFoundException If key is not found
+     * @throws ServiceUnavailable If data cannot be retrieved at this time, for example, due
+     *      to insufficient server resources or network problems.
+     */
+    suspend fun getData(key: String): SyncData
+
+    /**
+     * Returns the current data version for the given key
+     *
+     * @throws KeyNotFoundException If key is not found
+     * @throws ServiceUnavailable If data cannot be retrieved at this time, for example, due
+     *      to insufficient server resources or network problems.
+     */
+    suspend fun getDataVersion(key: String): Long
+}

uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/sync/EncryptionExt.kt

@@ -0,0 +1,142 @@
+/*
+ * Copyright (C) 2016-2021 Álinson Santos Xavier <git@axavier.org>
+ *
+ * This file is part of Loop Habit Tracker.
+ *
+ * Loop Habit Tracker is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by the
+ * Free Software Foundation, either version 3 of the License, or (at your
+ * option) any later version.
+ *
+ * Loop Habit Tracker 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 General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+@file:Suppress("UnstableApiUsage")
+
+package org.isoron.uhabits.core.sync
+
+import com.google.common.io.ByteStreams
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.invoke
+import org.apache.commons.codec.binary.Base64
+import java.io.ByteArrayInputStream
+import java.io.ByteArrayOutputStream
+import java.io.File
+import java.io.FileInputStream
+import java.io.FileOutputStream
+import java.nio.ByteBuffer
+import java.util.zip.GZIPInputStream
+import java.util.zip.GZIPOutputStream
+import javax.crypto.Cipher
+import javax.crypto.KeyGenerator
+import javax.crypto.SecretKey
+import javax.crypto.spec.IvParameterSpec
+import javax.crypto.spec.SecretKeySpec
+
+/**
+ * Encryption key which can be used with [File.encryptToString], [String.decryptToFile],
+ * [ByteArray.encrypt] and [ByteArray.decrypt].
+ *
+ * To randomly generate a new key, use [EncryptionKey.generate]. To load a key from a
+ * Base64-encoded string, use [EncryptionKey.fromBase64].
+ */
+class EncryptionKey private constructor(
+    val base64: String,
+    val secretKey: SecretKey
+) {
+    companion object {
+
+        fun fromBase64(base64: String): EncryptionKey {
+            val keySpec = SecretKeySpec(base64.decodeBase64(), "AES")
+            return EncryptionKey(base64, keySpec)
+        }
+
+        private fun fromSecretKey(spec: SecretKey): EncryptionKey {
+            val base64 = spec.encoded.encodeBase64().trim()
+            return EncryptionKey(base64, spec)
+        }
+
+        suspend fun generate(): EncryptionKey = Dispatchers.IO {
+            try {
+                val generator = KeyGenerator.getInstance("AES").apply { init(256) }
+                return@IO fromSecretKey(generator.generateKey())
+            } catch (e: Exception) {
+                throw RuntimeException(e)
+            }
+        }
+    }
+}
+
+/**
+ * Encrypts the byte stream using the provided symmetric encryption key.
+ *
+ * The initialization vector (16 bytes) is prepended to the cipher text. To decrypt the result, use
+ * [ByteArray.decrypt], providing the same key.
+ */
+fun ByteArray.encrypt(key: EncryptionKey): ByteArray {
+    val cipher = Cipher.getInstance("AES/CBC/PKCS5PADDING")
+    cipher.init(Cipher.ENCRYPT_MODE, key.secretKey)
+    val encrypted = cipher.doFinal(this)
+    return ByteBuffer
+        .allocate(16 + encrypted.size)
+        .put(cipher.iv)
+        .put(encrypted)
+        .array()
+}
+
+/**
+ * Decrypts a byte stream generated by [ByteArray.encrypt].
+ */
+fun ByteArray.decrypt(key: EncryptionKey): ByteArray {
+    val buffer = ByteBuffer.wrap(this)
+    val iv = ByteArray(16)
+    buffer.get(iv)
+    val encrypted = ByteArray(buffer.remaining())
+    buffer.get(encrypted)
+    val cipher = Cipher.getInstance("AES/CBC/PKCS5PADDING")
+    cipher.init(Cipher.DECRYPT_MODE, key.secretKey, IvParameterSpec(iv))
+    return cipher.doFinal(encrypted)
+}
+
+/**
+ * Takes a string produced by [File.encryptToString], decodes it with Base64, decompresses it with
+ * gzip, decrypts it with the provided key, then writes the output to the specified file.
+ */
+fun String.decryptToFile(key: EncryptionKey, output: File) {
+    val bytes = this.decodeBase64().decrypt(key)
+    ByteArrayInputStream(bytes).use { bytesInputStream ->
+        GZIPInputStream(bytesInputStream).use { gzipInputStream ->
+            FileOutputStream(output).use { fileOutputStream ->
+                ByteStreams.copy(gzipInputStream, fileOutputStream)
+            }
+        }
+    }
+}
+
+/**
+ * Compresses the file with gzip, encrypts it using the the provided key, then returns a string
+ * containing the Base64-encoded cipher bytes.
+ *
+ * To decrypt and decompress the cipher text back into a file, use [String.decryptToFile].
+ */
+fun File.encryptToString(key: EncryptionKey): String {
+    ByteArrayOutputStream().use { bytesOutputStream ->
+        FileInputStream(this).use { inputStream ->
+            GZIPOutputStream(bytesOutputStream).use { gzipOutputStream ->
+                ByteStreams.copy(inputStream, gzipOutputStream)
+                gzipOutputStream.close()
+                val bytes = bytesOutputStream.toByteArray()
+                return bytes.encrypt(key).encodeBase64()
+            }
+        }
+    }
+}
+
+fun ByteArray.encodeBase64(): String = Base64.encodeBase64(this).decodeToString()
+fun String.decodeBase64(): ByteArray = Base64.decodeBase64(this.toByteArray())

uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/sync/NetworkManager.kt

@@ -0,0 +1,29 @@
+/*
+ * Copyright (C) 2016-2021 Álinson Santos Xavier <git@axavier.org>
+ *
+ * This file is part of Loop Habit Tracker.
+ *
+ * Loop Habit Tracker is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by the
+ * Free Software Foundation, either version 3 of the License, or (at your
+ * option) any later version.
+ *
+ * Loop Habit Tracker 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 General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package org.isoron.uhabits.core.sync
+
+interface NetworkManager {
+    fun addListener(listener: Listener)
+    fun remoteListener(listener: Listener)
+    interface Listener {
+        fun onNetworkAvailable()
+        fun onNetworkLost()
+    }
+}

uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/sync/SyncData.kt

@@ -0,0 +1,29 @@
+/*
+ * Copyright (C) 2016-2021 Álinson Santos Xavier <git@axavier.org>
+ *
+ * This file is part of Loop Habit Tracker.
+ *
+ * Loop Habit Tracker is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by the
+ * Free Software Foundation, either version 3 of the License, or (at your
+ * option) any later version.
+ *
+ * Loop Habit Tracker 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 General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package org.isoron.uhabits.core.sync
+
+data class SyncData(
+    val version: Long,
+    val content: String
+)
+
+data class RegisterReponse(val key: String)
+
+data class GetDataVersionResponse(val version: Long)

uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/sync/SyncException.kt

@@ -0,0 +1,28 @@
+/*
+ * Copyright (C) 2016-2021 Álinson Santos Xavier <git@axavier.org>
+ *
+ * This file is part of Loop Habit Tracker.
+ *
+ * Loop Habit Tracker is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by the
+ * Free Software Foundation, either version 3 of the License, or (at your
+ * option) any later version.
+ *
+ * Loop Habit Tracker 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 General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package org.isoron.uhabits.core.sync
+
+open class SyncException : RuntimeException()
+
+class KeyNotFoundException : SyncException()
+
+class ServiceUnavailable : SyncException()
+
+class EditConflictException : SyncException()

uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/sync/SyncManager.kt

@@ -0,0 +1,183 @@
+/*
+ * Copyright (C) 2016-2021 Álinson Santos Xavier <git@axavier.org>
+ *
+ * This file is part of Loop Habit Tracker.
+ *
+ * Loop Habit Tracker is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by the
+ * Free Software Foundation, either version 3 of the License, or (at your
+ * option) any later version.
+ *
+ * Loop Habit Tracker 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 General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package org.isoron.uhabits.core.sync
+
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.invoke
+import kotlinx.coroutines.launch
+import org.isoron.uhabits.core.AppScope
+import org.isoron.uhabits.core.commands.Command
+import org.isoron.uhabits.core.commands.CommandRunner
+import org.isoron.uhabits.core.database.Database
+import org.isoron.uhabits.core.io.Logging
+import org.isoron.uhabits.core.io.LoopDBImporter
+import org.isoron.uhabits.core.preferences.Preferences
+import java.io.File
+import javax.inject.Inject
+
+@AppScope
+class SyncManager @Inject constructor(
+    val preferences: Preferences,
+    val commandRunner: CommandRunner,
+    val logging: Logging,
+    val networkManager: NetworkManager,
+    val server: AbstractSyncServer,
+    val db: Database,
+    val dbImporter: LoopDBImporter,
+) : Preferences.Listener, CommandRunner.Listener, NetworkManager.Listener {
+
+    private var logger = logging.getLogger("SyncManager")
+    private var connected = false
+    private val tmpFile = File.createTempFile("import", "")
+    private var currVersion = 1L
+    private var dirty = true
+    private lateinit var encryptionKey: EncryptionKey
+    private lateinit var syncKey: String
+
+    init {
+        preferences.addListener(this)
+        commandRunner.addListener(this)
+        networkManager.addListener(this)
+    }
+
+    fun sync() = CoroutineScope(Dispatchers.Main).launch {
+        if (!preferences.isSyncEnabled) {
+            logger.info("Device sync is disabled. Skipping sync.")
+            return@launch
+        }
+
+        encryptionKey = EncryptionKey.fromBase64(preferences.encryptionKey)
+        syncKey = preferences.syncKey
+        logger.info("Starting sync (key: $syncKey)")
+
+        try {
+            pull()
+            push()
+            logger.info("Sync finished successfully.")
+        } catch (e: ConnectionLostException) {
+            logger.info("Network unavailable. Aborting sync.")
+        } catch (e: ServiceUnavailable) {
+            logger.info("Sync service unavailable. Aborting sync.")
+        } catch (e: Exception) {
+            logger.error("Unexpected sync exception. Disabling sync.")
+            logger.error(e)
+            preferences.disableSync()
+        }
+    }
+
+    private suspend fun push(depth: Int = 0) {
+        if (depth >= 5) {
+            throw RuntimeException()
+        }
+
+        if (!dirty) {
+            logger.info("Local database not modified. Skipping push.")
+            return
+        }
+
+        logger.info("Encrypting local database...")
+        val encryptedDB = db.file!!.encryptToString(encryptionKey)
+        val size = encryptedDB.length / 1024
+
+        try {
+            logger.info("Pushing local database (version $currVersion, $size KB)")
+            assertConnected()
+            server.put(preferences.syncKey, SyncData(currVersion, encryptedDB))
+            dirty = false
+        } catch (e: EditConflictException) {
+            logger.info("Sync conflict detected while pushing.")
+            setCurrentVersion(0)
+            pull()
+            push(depth = depth + 1)
+        }
+    }
+
+    private suspend fun pull() = Dispatchers.IO {
+        logger.info("Querying remote database version...")
+        assertConnected()
+        val remoteVersion = server.getDataVersion(syncKey)
+        logger.info("Remote database version: $remoteVersion")
+
+        if (remoteVersion <= currVersion) {
+            logger.info("Local database is up-to-date. Skipping merge.")
+        } else {
+            logger.info("Pulling remote database...")
+            assertConnected()
+            val data = server.getData(syncKey)
+            val size = data.content.length / 1024
+            logger.info("Pulled remote database (version ${data.version}, $size KB)")
+            logger.info("Decrypting remote database and merging with local changes...")
+            data.content.decryptToFile(encryptionKey, tmpFile)
+
+            try {
+                db.beginTransaction()
+                dbImporter.importHabitsFromFile(tmpFile)
+                db.setTransactionSuccessful()
+            } catch (e: Exception) {
+                logger.error("Failed to import database")
+                logger.error(e)
+            } finally {
+                db.endTransaction()
+            }
+
+            dirty = true
+            setCurrentVersion(data.version + 1)
+        }
+    }
+
+    fun onResume() = sync()
+
+    fun onPause() = sync()
+
+    override fun onSyncEnabled() {
+        logger.info("Sync enabled.")
+        setCurrentVersion(1)
+        dirty = true
+        sync()
+    }
+
+    override fun onNetworkAvailable() {
+        logger.info("Network available.")
+        connected = true
+        sync()
+    }
+
+    override fun onNetworkLost() {
+        logger.info("Network unavailable.")
+        connected = false
+    }
+
+    override fun onCommandFinished(command: Command) {
+        if (!dirty) setCurrentVersion(currVersion + 1)
+        dirty = true
+    }
+
+    private fun assertConnected() {
+        if (!connected) throw ConnectionLostException()
+    }
+
+    private fun setCurrentVersion(v: Long) {
+        currVersion = v
+        logger.info("Setting local database version: $currVersion")
+    }
+}
+
+class ConnectionLostException : RuntimeException()

uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/ui/screens/habits/list/ListHabitsBehavior.kt

@@ -26,6 +26,7 @@ import org.isoron.uhabits.core.models.Timestamp
 import org.isoron.uhabits.core.preferences.Preferences
 import org.isoron.uhabits.core.tasks.ExportCSVTask
 import org.isoron.uhabits.core.tasks.TaskRunner
+import org.isoron.uhabits.core.ui.callbacks.OnConfirmedCallback
 import org.isoron.uhabits.core.utils.DateUtils.Companion.getToday
 import java.io.File
 import java.io.IOException
@@ -110,6 +111,17 @@ open class ListHabitsBehavior @Inject constructor(
         )
     }
 
+    fun onSyncKeyOffer(syncKey: String, encryptionKey: String) {
+        if (prefs.syncKey == syncKey) {
+            screen.showMessage(Message.SYNC_KEY_ALREADY_INSTALLED)
+            return
+        }
+        screen.showConfirmInstallSyncKey {
+            prefs.enableSync(syncKey, encryptionKey)
+            screen.showMessage(Message.SYNC_ENABLED)
+        }
+    }
+
     enum class Message {
         COULD_NOT_EXPORT,
         IMPORT_SUCCESSFUL,
@@ -117,6 +129,8 @@ open class ListHabitsBehavior @Inject constructor(
         DATABASE_REPAIRED,
         COULD_NOT_GENERATE_BUG_REPORT,
         FILE_NOT_RECOGNIZED,
+        SYNC_ENABLED,
+        SYNC_KEY_ALREADY_INSTALLED
     }
 
     interface BugReporter {
@@ -147,5 +161,6 @@ open class ListHabitsBehavior @Inject constructor(
 
         fun showSendBugReportToDeveloperScreen(log: String)
         fun showSendFileScreen(filename: String)
+        fun showConfirmInstallSyncKey(callback: OnConfirmedCallback)
     }
 }

uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/ui/screens/sync/SyncBehavior.kt

@@ -0,0 +1,66 @@
+/*
+ * Copyright (C) 2016-2021 Álinson Santos Xavier <git@axavier.org>
+ *
+ * This file is part of Loop Habit Tracker.
+ *
+ * Loop Habit Tracker is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by the
+ * Free Software Foundation, either version 3 of the License, or (at your
+ * option) any later version.
+ *
+ * Loop Habit Tracker 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 General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package org.isoron.uhabits.core.ui.screens.sync
+
+import org.isoron.uhabits.core.io.Logging
+import org.isoron.uhabits.core.preferences.Preferences
+import org.isoron.uhabits.core.sync.AbstractSyncServer
+import org.isoron.uhabits.core.sync.EncryptionKey
+
+class SyncBehavior(
+    val screen: Screen,
+    val preferences: Preferences,
+    val server: AbstractSyncServer,
+    val logging: Logging,
+) {
+    val logger = logging.getLogger("SyncBehavior")
+
+    suspend fun onResume() {
+        if (preferences.syncKey.isBlank()) {
+            register()
+        } else {
+            displayCurrentKey()
+        }
+    }
+
+    suspend fun displayCurrentKey() {
+        screen.showLink("https://loophabits.org/sync/${preferences.syncKey}#${preferences.encryptionKey}")
+    }
+
+    suspend fun register() {
+        screen.showLoadingScreen()
+        try {
+            val syncKey = server.register()
+            val encKey = EncryptionKey.generate()
+            preferences.enableSync(syncKey, encKey.base64)
+            displayCurrentKey()
+        } catch (e: Exception) {
+            logger.error("Unexpected exception")
+            logger.error(e)
+            screen.showErrorScreen()
+        }
+    }
+
+    interface Screen {
+        suspend fun showLoadingScreen()
+        suspend fun showErrorScreen()
+        suspend fun showLink(link: String)
+    }
+}

uhabits-core/src/jvmTest/java/org/isoron/uhabits/core/sync/EncryptionExtTest.kt

@@ -0,0 +1,70 @@
+/*
+ * Copyright (C) 2016-2021 Álinson Santos Xavier <git@axavier.org>
+ *
+ * This file is part of Loop Habit Tracker.
+ *
+ * Loop Habit Tracker is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by the
+ * Free Software Foundation, either version 3 of the License, or (at your
+ * option) any later version.
+ *
+ * Loop Habit Tracker 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 General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+package org.isoron.uhabits.core.sync
+
+import kotlinx.coroutines.runBlocking
+import org.hamcrest.CoreMatchers.equalTo
+import org.hamcrest.Matchers.greaterThan
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertThat
+import org.junit.Test
+import java.io.File
+import java.io.PrintWriter
+import java.util.Random
+
+class EncryptionExtTest {
+
+    @Test
+    fun test_encode_decode() {
+        val original = ByteArray(5000)
+        Random().nextBytes(original)
+        val encoded = original.encodeBase64()
+        val decoded = encoded.decodeBase64()
+        assertThat(decoded, equalTo(original))
+    }
+
+    @Test
+    fun test_encrypt_decrypt_bytes() = runBlocking {
+        val original = ByteArray(5000)
+        Random().nextBytes(original)
+        val key = EncryptionKey.generate()
+        val encrypted = original.encrypt(key)
+        val decrypted = encrypted.decrypt(key)
+        assertThat(decrypted, equalTo(original))
+    }
+
+    @Test
+    fun test_encrypt_decrypt_file() = runBlocking {
+        val original = File.createTempFile("file", ".txt")
+        val writer = PrintWriter(original.outputStream())
+        writer.println("hello world")
+        writer.println("encryption test")
+        writer.close()
+        assertThat(original.length(), equalTo(28L))
+
+        val key = EncryptionKey.generate()
+        val encrypted = original.encryptToString(key)
+        assertThat(encrypted.length, greaterThan(10))
+
+        val decrypted = File.createTempFile("file", ".txt")
+        encrypted.decryptToFile(key, decrypted)
+        assertThat(decrypted.length(), equalTo(28L))
+        assertEquals("hello world\nencryption test\n", decrypted.readText())
+    }
+}

uhabits-server/build.gradle.kts

@@ -48,6 +48,8 @@ dependencies {
     implementation("io.prometheus:simpleclient_hotspot:0.11.0")
     testImplementation("io.ktor:ktor-server-tests:$ktorVersion")
     testImplementation("com.nhaarman.mockitokotlin2:mockito-kotlin:2.2.0")
+    testImplementation(kotlin("test"))
+    testImplementation(kotlin("test-junit"))
 }
 
 tasks.withType<ShadowJar> {