ios-auth-keychain-storage

iOS Auth & Keychain Storage

RULES — Follow these with no exceptions

1. Auth tokens go in Keychain. Never UserDefaults. UserDefaults is unencrypted plist-on-disk — a device compromise exposes every token. This rule is enforced by the blocking userdefaults-token hook.
2. Use kSecClassGenericPassword with a fixed kSecAttrAccount value (e.g. "auth_token"). One token per account name.
3. Set kSecAttrAccessible: kSecAttrAccessibleAfterFirstUnlock — the token is readable when the app is backgrounded, but not before first unlock after reboot. Required for background refresh to work.
4. Provide exactly three public methods: save(token:), getToken() -> String?, deleteToken(). No variants, no convenience overloads.
5. Treat all Keychain errors as "no token present" — if SecItemCopyMatching returns non-zero, return nil from getToken(). Don't throw — the only sane response to a broken Keychain is to force re-login.
6. Never log tokens. Not in print, not in error messages, not in analytics. Even in debug builds.
7. The APIClient calls KeychainService.shared.getToken() on every request. Feature code should never read the token directly.

---

When to invoke

• Creating Services/KeychainService.swift.
• Adding login / logout / token-refresh flows.
• Debugging an auth loop (usually a sign the token isn't persisting across launches).

Why not UserDefaults

UserDefaults writes to ~/Library/Preferences/<bundle-id>.plist as unencrypted XML. Anyone with filesystem access (jailbroken device, backup extraction, or xcrun simctl on simulator) reads tokens in plain text.

Keychain:

• Encrypted at rest with the device's Secure Enclave.
• Scoped per-app by default.
• Survives app reinstall if you opt in (don't — you want a fresh login after reinstall).
• Accessible from extensions if you share the Keychain access group.

The canonical API

final class KeychainService {
    static let shared = KeychainService()
    private let account = "auth_token"
    private let service = Bundle.main.bundleIdentifier ?? "com.yourcompany.yourapp"

    func save(token: String) {
        let data = Data(token.utf8)
        let query: [String: Any] = [
            kSecClass as String: kSecClassGenericPassword,
            kSecAttrAccount as String: account,
            kSecAttrService as String: service,
        ]
        SecItemDelete(query as CFDictionary)  // remove any existing
        var attrs = query
        attrs[kSecValueData as String] = data
        attrs[kSecAttrAccessible as String] = kSecAttrAccessibleAfterFirstUnlock
        SecItemAdd(attrs as CFDictionary, nil)
    }

    func getToken() -> String? {
        let query: [String: Any] = [
            kSecClass as String: kSecClassGenericPassword,
            kSecAttrAccount as String: account,
            kSecAttrService as String: service,
            kSecReturnData as String: true,
            kSecMatchLimit as String: kSecMatchLimitOne,
        ]
        var result: AnyObject?
        let status = SecItemCopyMatching(query as CFDictionary, &result)
        guard status == errSecSuccess, let data = result as? Data else { return nil }
        return String(data: data, encoding: .utf8)
    }

    func deleteToken() {
        let query: [String: Any] = [
            kSecClass as String: kSecClassGenericPassword,
            kSecAttrAccount as String: account,
            kSecAttrService as String: service,
        ]
        SecItemDelete(query as CFDictionary)
    }
}

See <plugin-root>/templates/KeychainService.swift for the canonical drop-in implementation.

Typical login flow

func login(email: String, password: String) async throws {
    let response: DataResponse<LoginResponse> = try await APIClient.shared.post(
        path: "/auth/login",
        body: LoginRequest(email: email, password: password)
    )
    KeychainService.shared.save(token: response.data.token)
    AppState.shared.isAuthenticated = true
}

func logout() {
    KeychainService.shared.deleteToken()
    AppState.shared.isAuthenticated = false
}

Common pitfalls

Using UserDefaults "just to get things working"

Anti-pattern (blocked by the userdefaults-token hook):

// ❌ NEVER
UserDefaults.standard.set(token, forKey: "auth_token")

Fix: KeychainService.shared.save(token:). There is no scenario where UserDefaults for a token is correct. The userdefaults-token hook rejects any Write/Edit that stores token, password, secret, apiKey, bearer, or credential values in UserDefaults.

Using kSecAttrAccessibleAlways or WhenUnlocked

• kSecAttrAccessibleAlways is deprecated and downgrades security.
• kSecAttrAccessibleWhenUnlocked breaks background requests because the token is unreadable while the device is locked.

Fix: kSecAttrAccessibleAfterFirstUnlock.

Leaving stale entries on login

Symptom: Logging in with a new account returns the old account's token.

Cause: SecItemAdd fails silently with errSecDuplicateItem if the account already exists.

Fix: Always SecItemDelete before SecItemAdd (as shown above), or use SecItemUpdate.

Logging the token in error messages

// ❌
print("Auth failed with token: \(token)")

Fix: Log token presence (token == nil), not the token itself.

Template reference

See <plugin-root>/templates/KeychainService.swift for the canonical implementation. Copy into Services/KeychainService.swift on day 1 — before writing any auth flow.