Use Android's hardware-backed keystore to store backup key

This commit also disables the old UI as it does not work with the new key

app/src/main/AndroidManifest.xml

@@ -35,12 +35,7 @@
 
         <activity
             android:name="com.stevesoltys.backup.activity.MainActivity"
-            android:label="@string/app_name">
-            <intent-filter>
-                <action android:name="android.intent.action.MAIN" />
-                <category android:name="android.intent.category.LAUNCHER" />
-            </intent-filter>
-        </activity>
+            android:label="@string/app_name"/>
 
         <activity
             android:name="com.stevesoltys.backup.activity.backup.CreateBackupActivity"

app/src/main/java/com/stevesoltys/backup/security/KeyManager.kt

@@ -0,0 +1,48 @@
+package com.stevesoltys.backup.security
+
+import android.os.Build.VERSION.SDK_INT
+import android.security.keystore.KeyProperties.*
+import android.security.keystore.KeyProtection
+import java.security.KeyStore
+import java.security.KeyStore.SecretKeyEntry
+import javax.crypto.SecretKey
+import javax.crypto.spec.SecretKeySpec
+
+private const val KEY_SIZE = 256
+private const val KEY_ALIAS = "com.stevesoltys.backup"
+private const val ANDROID_KEY_STORE = "AndroidKeyStore"
+
+object KeyManager {
+
+    private val keyStore by lazy {
+        KeyStore.getInstance(ANDROID_KEY_STORE).apply {
+            load(null)
+        }
+    }
+
+    fun storeBackupKey(seed: ByteArray) {
+        if (seed.size < KEY_SIZE / 8) throw IllegalArgumentException()
+        // TODO check if using first 256 of 512 bytes produced by PBKDF2WithHmacSHA512 is safe!
+        val secretKeySpec = SecretKeySpec(seed, 0, KEY_SIZE / 8, "AES")
+        val ksEntry = SecretKeyEntry(secretKeySpec)
+        keyStore.setEntry(KEY_ALIAS, ksEntry, getKeyProtection())
+    }
+
+    fun hasBackupKey() = keyStore.containsAlias(KEY_ALIAS) &&
+            keyStore.entryInstanceOf(KEY_ALIAS, SecretKeyEntry::class.java)
+
+    fun getBackupKey(): SecretKey {
+        val ksEntry = keyStore.getEntry(KEY_ALIAS, null) as SecretKeyEntry
+        return ksEntry.secretKey
+    }
+
+    private fun getKeyProtection(): KeyProtection {
+        val builder = KeyProtection.Builder(PURPOSE_ENCRYPT or PURPOSE_DECRYPT)
+                .setBlockModes(BLOCK_MODE_GCM)
+                .setEncryptionPaddings(ENCRYPTION_PADDING_NONE)
+                .setRandomizedEncryptionRequired(true)
+        if (SDK_INT >= 28) builder.setUnlockedDeviceRequired(true)
+        return builder.build()
+    }
+
+}

app/src/main/java/com/stevesoltys/backup/settings/RecoveryCodeViewModel.kt

@@ -1,10 +1,10 @@
 package com.stevesoltys.backup.settings
 
 import android.app.Application
-import android.util.ByteStringUtils
 import androidx.lifecycle.AndroidViewModel
 import com.stevesoltys.backup.LiveEvent
 import com.stevesoltys.backup.MutableLiveEvent
+import com.stevesoltys.backup.security.KeyManager
 import io.github.novacrypto.bip39.*
 import io.github.novacrypto.bip39.Validation.InvalidChecksumException
 import io.github.novacrypto.bip39.Validation.InvalidWordCountException
@@ -46,8 +46,7 @@ class RecoveryCodeViewModel(application: Application) : AndroidViewModel(applica
         }
         val mnemonic = input.joinToString(" ")
         val seed = SeedCalculator(JavaxPBKDF2WithHmacSHA512.INSTANCE).calculateSeed(mnemonic, "")
-        // TODO use KeyManager to store secret
-        setBackupPassword(getApplication(), ByteStringUtils.toHexString(seed))
+        KeyManager.storeBackupKey(seed)
         recoveryCodeSaved.setEvent(true)
     }
 

app/src/main/java/com/stevesoltys/backup/settings/SettingsManager.kt

@@ -25,6 +25,7 @@ fun getBackupFolderUri(context: Context): Uri? {
  * This is insecure and not supposed to be part of a release,
  * but rather an intermediate step towards a generated passphrase.
  */
+@Deprecated("Replaced by KeyManager#storeBackupKey()")
 fun setBackupPassword(context: Context, password: String) {
     getDefaultSharedPreferences(context)
             .edit()
@@ -32,6 +33,7 @@ fun setBackupPassword(context: Context, password: String) {
             .apply()
 }
 
+@Deprecated("Replaced by KeyManager#getBackupKey()")
 fun getBackupPassword(context: Context): String? {
     return getDefaultSharedPreferences(context).getString(PREF_KEY_BACKUP_PASSWORD, null)
 }

app/src/main/java/com/stevesoltys/backup/settings/SettingsViewModel.kt

@@ -5,12 +5,13 @@ import android.content.Intent
 import android.content.Intent.FLAG_GRANT_READ_URI_PERMISSION
 import android.content.Intent.FLAG_GRANT_WRITE_URI_PERMISSION
 import androidx.lifecycle.AndroidViewModel
+import com.stevesoltys.backup.security.KeyManager
 
 class SettingsViewModel(application: Application) : AndroidViewModel(application) {
 
     private val app = application
 
-    fun recoveryCodeIsSet() = getBackupPassword(getApplication()) != null
+    fun recoveryCodeIsSet() = KeyManager.hasBackupKey()
     fun locationIsSet() = getBackupFolderUri(getApplication()) != null
 
     fun handleChooseFolderResult(result: Intent?) {