Phase 2: passphrase-encrypted backup and restore (PBKDF2 + AES-256-GCM)

app/src/main/java/io/github/abrarul/modularai/security/Backup.kt

@@ -0,0 +1,63 @@
+package io.github.abrarul.modularai.security
+
+import android.content.Context
+import io.github.abrarul.modularai.data.AppDb
+import io.github.abrarul.modularai.data.Capture
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.withContext
+import org.json.JSONArray
+import org.json.JSONObject
+import java.io.File
+import java.security.SecureRandom
+import javax.crypto.AEADBadTagException
+import javax.crypto.Cipher
+import javax.crypto.SecretKeyFactory
+import javax.crypto.spec.GCMParameterSpec
+import javax.crypto.spec.PBEKeySpec
+import javax.crypto.spec.SecretKeySpec
+
+/** File layout: magic "MAB1" | salt(16) | iv(12) | AES-256-GCM(JSON). Key = PBKDF2-HMAC-SHA256(passphrase). */
+object Backup {
+    private val MAGIC = "MAB1".toByteArray()
+    private const val ITER = 210_000
+
+    private fun key(pass: String, salt: ByteArray): SecretKeySpec {
+        val f = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA256")
+        return SecretKeySpec(f.generateSecret(PBEKeySpec(pass.toCharArray(), salt, ITER, 256)).encoded, "AES")
+    }
+
+    suspend fun export(ctx: Context, pass: String): File = withContext(Dispatchers.IO) {
+        require(pass.length >= 8) { "Passphrase must be at least 8 characters" }
+        val arr = JSONArray()
+        AppDb.get(ctx).captures().list().forEach {
+            arr.put(JSONObject().put("t", it.createdAt).put("text", Vault.decrypt(it.cipher)))
+        }
+        val salt = ByteArray(16).also { SecureRandom().nextBytes(it) }
+        val c = Cipher.getInstance("AES/GCM/NoPadding")
+        c.init(Cipher.ENCRYPT_MODE, key(pass, salt))
+        val body = c.doFinal(arr.toString().toByteArray(Charsets.UTF_8))
+        val dir = ctx.getExternalFilesDir(null) ?: ctx.filesDir
+        File(dir, "backup-${System.currentTimeMillis()}.mab").also { it.writeBytes(MAGIC + salt + c.iv + body) }
+    }
+
+    suspend fun restore(ctx: Context, file: File, pass: String): Int = withContext(Dispatchers.IO) {
+        val raw = file.readBytes()
+        require(raw.size > 4 + 16 + 12 + 16 && raw.copyOfRange(0, 4).contentEquals(MAGIC)) { "Not a ModularAI backup file" }
+        val salt = raw.copyOfRange(4, 20)
+        val iv = raw.copyOfRange(20, 32)
+        val c = Cipher.getInstance("AES/GCM/NoPadding")
+        c.init(Cipher.DECRYPT_MODE, key(pass, salt), GCMParameterSpec(128, iv))
+        val json = try {
+            String(c.doFinal(raw, 32, raw.size - 32), Charsets.UTF_8)
+        } catch (_: AEADBadTagException) {
+            error("Wrong passphrase or damaged file")
+        }
+        val arr = JSONArray(json)
+        val dao = AppDb.get(ctx).captures()
+        for (i in 0 until arr.length()) {
+            val o = arr.getJSONObject(i)
+            dao.insert(Capture(createdAt = o.getLong("t"), cipher = Vault.encrypt(o.getString("text"))))
+        }
+        arr.length()
+    }
+}