package uk.orllewin.sianel.data

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 backed by a non-exportable AndroidKeyStore key.
 *
 * Replaces the deprecated Jetpack Security (`EncryptedSharedPreferences`) library:
 * we keep the key in the Keystore and store only `base64(iv || ciphertext)` in a
 * plain SharedPreferences. No Tink dependency, no R8 keep-rule fragility.
 */
object Crypto {
    private const val KEYSTORE = "AndroidKeyStore"
    private const val KEY_ALIAS = "sianel_prefs_key"
    private const val TRANSFORMATION = "AES/GCM/NoPadding"
    private const val IV_LENGTH = 12          // GCM standard nonce size
    private const val TAG_LENGTH_BITS = 128

    private fun secretKey(): SecretKey {
        val ks = KeyStore.getInstance(KEYSTORE).apply { load(null) }
        (ks.getEntry(KEY_ALIAS, null) as? KeyStore.SecretKeyEntry)?.let { return it.secretKey }
        return KeyGenerator.getInstance(KeyProperties.KEY_ALGORITHM_AES, KEYSTORE).apply {
            init(
                KeyGenParameterSpec.Builder(
                    KEY_ALIAS,
                    KeyProperties.PURPOSE_ENCRYPT or KeyProperties.PURPOSE_DECRYPT,
                )
                    .setBlockModes(KeyProperties.BLOCK_MODE_GCM)
                    .setEncryptionPaddings(KeyProperties.ENCRYPTION_PADDING_NONE)
                    .setKeySize(256)
                    .build()
            )
        }.generateKey()
    }

    fun encrypt(plaintext: String): String {
        val cipher = Cipher.getInstance(TRANSFORMATION).apply {
            init(Cipher.ENCRYPT_MODE, secretKey())
        }
        val iv = cipher.iv
        val ciphertext = cipher.doFinal(plaintext.toByteArray(Charsets.UTF_8))
        return Base64.encodeToString(iv + ciphertext, Base64.NO_WRAP)
    }

    /** Returns null if the blob is corrupt or was encrypted under a since-invalidated key. */
    fun decrypt(stored: String): String? = runCatching {
        val combined = Base64.decode(stored, Base64.NO_WRAP)
        val iv = combined.copyOfRange(0, IV_LENGTH)
        val ciphertext = combined.copyOfRange(IV_LENGTH, combined.size)
        val cipher = Cipher.getInstance(TRANSFORMATION).apply {
            init(Cipher.DECRYPT_MODE, secretKey(), GCMParameterSpec(TAG_LENGTH_BITS, iv))
        }
        String(cipher.doFinal(ciphertext), Charsets.UTF_8)
    }.getOrNull()
}