Swift Security Pro

Protect user data and credentials. Default to the most secure option.

When to use

• Storing tokens, passwords, or sensitive data.
• Reviewing networking, secrets handling, or auth.
• Adding biometric authentication.

Trigger: /swift-security-pro.

Core principles

• Secrets go in the Keychain, never UserDefaults or plist.
• Never hard-code API keys/secrets in source.
• Keep App Transport Security on; require TLS.
• Use biometrics for gating access, not for storing the secret itself.

Storing secrets

❌ UserDefaults — plaintext, backed up, readable

UserDefaults.standard.set(token, forKey: "authToken")

✅ Keychain

let query: [String: Any] = [
    kSecClass as String: kSecClassGenericPassword,
    kSecAttrAccount as String: "authToken",
    kSecValueData as String: Data(token.utf8),
    kSecAttrAccessible as String: kSecAttrAccessibleWhenUnlockedThisDeviceOnly
]
SecItemDelete(query as CFDictionary)
SecItemAdd(query as CFDictionary, nil)

Use ...ThisDeviceOnly accessibility so secrets don't migrate via backup.

No hard-coded secrets

❌

let apiKey = "sk_live_abc123"   // shipped in the binary, easily extracted

✅

• Inject at build time (xcconfig / CI secret) or fetch from your backend.
• Never commit keys; add config files to .gitignore.
• Treat anything in the app bundle as public.

Transport security

• Keep ATS enabled. Don't add NSAllowsArbitraryLoads.
• Use HTTPS everywhere; consider certificate pinning for high-value APIs via

URLSessionDelegate urlSession(_:didReceive:completionHandler:).

❌ Info.plist

<key>NSAppTransportSecurity</key><dict>
  <key>NSAllowsArbitraryLoads</key><true/>
</dict>

✅ Leave ATS on; scope rare exceptions to a specific domain only.

Data Protection

Mark sensitive files so they're encrypted at rest while locked:

try data.write(to: url, options: .completeFileProtection)

Biometric auth

import LocalAuthentication
let ctx = LAContext()
var error: NSError?
if ctx.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: &error) {
    let ok = try await ctx.evaluatePolicy(
        .deviceOwnerAuthenticationWithBiometrics,
        localizedReason: "Unlock your vault")
}

Biometrics gate access; the actual secret still lives in the Keychain (optionally with SecAccessControl requiring biometry). Always provide a passcode fallback.

Common mistakes checklist

• [ ] Tokens/passwords in UserDefaults or a plist.
• [ ] Hard-coded API keys/secrets in source or the bundle.
• [ ] NSAllowsArbitraryLoads / disabled ATS.
• [ ] Keychain items without ...ThisDeviceOnly for non-syncable secrets.
• [ ] Logging tokens / PII to the console.
• [ ] Treating biometric success as the secret instead of gating Keychain access.

Output format (when reviewing)

Per issue: file:line, the exposure, before/after fix. Lead with credential leaks and plaintext storage.