Phase 2: AES-256-GCM vault backed by Android Keystore

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

@@ -0,0 +1,54 @@
+package io.github.abrarul.modularai.security
+
+import android.security.keystore.KeyGenParameterSpec
+import android.security.keystore.KeyProperties
+import android.util.Base64
+import java.security.KeyStore
+import javax.crypto.Cipher
+import javax.crypto.KeyGenerator
+import javax.crypto.SecretKey
+import javax.crypto.spec.GCMParameterSpec
+
+/** AES-256-GCM encryption; the key never leaves the Android Keystore. */
+object Vault {
+    private const val ALIAS = "modularai_vault_key"
+    private const val PROVIDER = "AndroidKeyStore"
+    private const val TRANSFORM = "AES/GCM/NoPadding"
+
+    private fun key(): SecretKey {
+        val ks = KeyStore.getInstance(PROVIDER).apply { load(null) }
+        (ks.getKey(ALIAS, null) as? SecretKey)?.let { return it }
+        val gen = KeyGenerator.getInstance(KeyProperties.KEY_ALGORITHM_AES, PROVIDER)
+        gen.init(
+            KeyGenParameterSpec.Builder(
+                ALIAS, KeyProperties.PURPOSE_ENCRYPT or KeyProperties.PURPOSE_DECRYPT
+            )
+                .setKeySize(256)
+                .setBlockModes(KeyProperties.BLOCK_MODE_GCM)
+                .setEncryptionPaddings(KeyProperties.ENCRYPTION_PADDING_NONE)
+                .build()
+        )
+        return gen.generateKey()
+    }
+
+    /** Returns base64(iv || ciphertext+tag). */
+    fun encrypt(plain: String): String {
+        val c = Cipher.getInstance(TRANSFORM)
+        c.init(Cipher.ENCRYPT_MODE, key())
+        val out = c.iv + c.doFinal(plain.toByteArray(Charsets.UTF_8))
+        return Base64.encodeToString(out, Base64.NO_WRAP)
+    }
+
+    fun decrypt(blob: String): String {
+        val raw = Base64.decode(blob, Base64.NO_WRAP)
+        val iv = raw.copyOfRange(0, 12)
+        val c = Cipher.getInstance(TRANSFORM)
+        c.init(Cipher.DECRYPT_MODE, key(), GCMParameterSpec(128, iv))
+        return String(c.doFinal(raw, 12, raw.size - 12), Charsets.UTF_8)
+    }
+
+    /** Deletes the key: every encrypted record becomes permanently unreadable. */
+    fun destroy() {
+        KeyStore.getInstance(PROVIDER).apply { load(null) }.deleteEntry(ALIAS)
+    }
+}