optimize OTP seed (check password =) remove legacy code

app/src/main/java/sh/sar/basedbank/LockActivity.kt

@@ -18,8 +18,6 @@ import kotlinx.coroutines.withContext
 import sh.sar.basedbank.databinding.ActivityLockBinding
 import sh.sar.basedbank.ui.home.HomeActivity
 import sh.sar.basedbank.util.CredentialStore
-import java.security.MessageDigest
-import java.security.SecureRandom
 import javax.crypto.SecretKeyFactory
 import javax.crypto.spec.PBEKeySpec
 
@@ -31,7 +29,6 @@ class LockActivity : AppCompatActivity() {
     private lateinit var salt: String
     private lateinit var storedHash: String
     private var biometricsEnabled = false
-    private var isLegacyFormat = false
     private var isVerifying = false
 
     private val lockPrefs get() = getSharedPreferences("lock_attempts", MODE_PRIVATE)
@@ -51,17 +48,9 @@ class LockActivity : AppCompatActivity() {
         method = prefs.getString("security_method", "pin") ?: "pin"
         biometricsEnabled = prefs.getBoolean("biometrics_enabled", false)
 
-        // Try new encrypted format first; fall back to legacy SHA-256
-        val stored = CredentialStore(this).loadSecurityHash()
-        if (stored != null) {
-            salt = stored.first
-            storedHash = stored.second
-            isLegacyFormat = false
-        } else {
-            salt = prefs.getString("security_salt", "") ?: ""
-            storedHash = prefs.getString("security_hash", "") ?: ""
-            isLegacyFormat = true
-        }
+        val stored = CredentialStore(this).loadSecurityHash() ?: run { finish(); return }
+        salt = stored.first
+        storedHash = stored.second
 
         if (method == "pin") {
             binding.viewPin.visibility = View.VISIBLE
@@ -150,7 +139,6 @@ class LockActivity : AppCompatActivity() {
             val ok = withContext(Dispatchers.Default) { verify(entered) }
             isVerifying = false
             if (ok) {
-                migrateIfNeeded(entered)
                 resetFailures()
                 proceed()
             } else {
@@ -175,7 +163,6 @@ class LockActivity : AppCompatActivity() {
             val ok = withContext(Dispatchers.Default) { verify(entered) }
             isVerifying = false
             if (ok) {
-                migrateIfNeeded(entered)
                 resetFailures()
                 proceed()
             } else {
@@ -217,30 +204,8 @@ class LockActivity : AppCompatActivity() {
 
     private fun verify(input: String): Boolean {
         if (storedHash.isBlank()) return false
-        return if (isLegacyFormat) {
-            sha256Legacy(salt + input) == storedHash
-        } else {
-            val saltBytes = Base64.decode(salt, Base64.NO_WRAP)
-            pbkdf2(input, saltBytes) == storedHash
-        }
-    }
-
-    /**
-     * On the first successful unlock after legacy SHA-256 format is detected,
-     * transparently migrate to PBKDF2 + CredentialStore.
-     */
-    private fun migrateIfNeeded(input: String) {
-        if (!isLegacyFormat) return
-        try {
-            val newSalt = ByteArray(16).also { SecureRandom().nextBytes(it) }
-            val newHash = pbkdf2(input, newSalt)
-            val saltB64 = Base64.encodeToString(newSalt, Base64.NO_WRAP)
-            CredentialStore(this).saveSecurityHash(saltB64, newHash)
-            // Remove legacy plaintext fields
-            getSharedPreferences("prefs", MODE_PRIVATE).edit()
-                .remove("security_salt").remove("security_hash").apply()
-            isLegacyFormat = false
-        } catch (_: Exception) { /* migration will retry next unlock */ }
+        val saltBytes = Base64.decode(salt, Base64.NO_WRAP)
+        return pbkdf2(input, saltBytes) == storedHash
     }
 
     private fun triggerBiometric() {
@@ -313,9 +278,4 @@ class LockActivity : AppCompatActivity() {
         }
     }
 
-    /** Legacy: raw SHA-256(salt + input) — only used for migration path. */
-    private fun sha256Legacy(input: String) = MessageDigest.getInstance("SHA-256")
-        .digest(input.toByteArray()).joinToString("") { "%02x".format(it) }
-
-
 }