iOS Security Skill

Comprehensive guide to iOS security covering secure storage, authentication, network security, and compliance with Apple's security requirements.

Keychain Services

Secure Credential Storage

import Security

enum KeychainError: Error {
    case duplicateItem
    case itemNotFound
    case unexpectedStatus(OSStatus)
}

final class KeychainManager {
    static let shared = KeychainManager()
    private init() {}

    func save(password: String, for account: String, service: String) throws {
        let passwordData = password.data(using: .utf8)!

        let query: [String: Any] = [
            kSecClass as String: kSecClassGenericPassword,
            kSecAttrAccount as String: account,
            kSecAttrService as String: service,
            kSecValueData as String: passwordData,
            kSecAttrAccessible as String: kSecAttrAccessibleWhenUnlockedThisDeviceOnly
        ]

        let status = SecItemAdd(query as CFDictionary, nil)

        if status == errSecDuplicateItem {
            // Update existing item
            let updateQuery: [String: Any] = [
                kSecClass as String: kSecClassGenericPassword,
                kSecAttrAccount as String: account,
                kSecAttrService as String: service
            ]
            let updates: [String: Any] = [
                kSecValueData as String: passwordData
            ]
            let updateStatus = SecItemUpdate(updateQuery as CFDictionary, updates as CFDictionary)
            guard updateStatus == errSecSuccess else {
                throw KeychainError.unexpectedStatus(updateStatus)
            }
        } else if status != errSecSuccess {
            throw KeychainError.unexpectedStatus(status)
        }
    }

    func retrievePassword(for account: String, service: String) throws -> 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 else {
            if status == errSecItemNotFound {
                throw KeychainError.itemNotFound
            }
            throw KeychainError.unexpectedStatus(status)
        }

        guard let data = result as? Data,
              let password = String(data: data, encoding: .utf8) else {
            throw KeychainError.unexpectedStatus(errSecDecode)
        }

        return password
    }

    func delete(account: String, service: String) throws {
        let query: [String: Any] = [
            kSecClass as String: kSecClassGenericPassword,
            kSecAttrAccount as String: account,
            kSecAttrService as String: service
        ]

        let status = SecItemDelete(query as CFDictionary)
        guard status == errSecSuccess || status == errSecItemNotFound else {
            throw KeychainError.unexpectedStatus(status)
        }
    }
}

Keychain Access Levels

Attribute	Description
kSecAttrAccessibleWhenUnlocked	Accessible when device unlocked
kSecAttrAccessibleWhenUnlockedThisDeviceOnly	Same, but not backed up
kSecAttrAccessibleAfterFirstUnlock	Accessible after first unlock
kSecAttrAccessibleAfterFirstUnlockThisDeviceOnly	Same, but not backed up
kSecAttrAccessibleWhenPasscodeSetThisDeviceOnly	Requires passcode, not backed up

Biometric Authentication

Face ID / Touch ID

import LocalAuthentication

final class BiometricAuthManager {
    enum BiometricType {
        case none
        case touchID
        case faceID
        case opticID  // Vision Pro
    }

    var biometricType: BiometricType {
        let context = LAContext()
        guard context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: nil) else {
            return .none
        }

        switch context.biometryType {
        case .touchID: return .touchID
        case .faceID: return .faceID
        case .opticID: return .opticID
        case .none: return .none
        @unknown default: return .none
        }
    }

    func authenticate(reason: String) async throws -> Bool {
        let context = LAContext()
        context.localizedFallbackTitle = "Use Passcode"

        var error: NSError?
        guard context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: &error) else {
            if let error {
                throw error
            }
            return false
        }

        return try await context.evaluatePolicy(
            .deviceOwnerAuthenticationWithBiometrics,
            localizedReason: reason
        )
    }

    func authenticateWithFallback(reason: String) async throws -> Bool {
        let context = LAContext()

        // Use deviceOwnerAuthentication to allow passcode fallback
        return try await context.evaluatePolicy(
            .deviceOwnerAuthentication,
            localizedReason: reason
        )
    }
}

Keychain with Biometric Protection

func saveBiometricProtectedItem(data: Data, account: String) throws {
    let access = SecAccessControlCreateWithFlags(
        nil,
        kSecAttrAccessibleWhenPasscodeSetThisDeviceOnly,
        .biometryCurrentSet,  // Invalidates if biometrics change
        nil
    )!

    let query: [String: Any] = [
        kSecClass as String: kSecClassGenericPassword,
        kSecAttrAccount as String: account,
        kSecValueData as String: data,
        kSecAttrAccessControl as String: access,
        kSecUseAuthenticationContext as String: LAContext()
    ]

    let status = SecItemAdd(query as CFDictionary, nil)
    guard status == errSecSuccess else {
        throw KeychainError.unexpectedStatus(status)
    }
}

Data Protection

File Protection

// Set protection level when writing files
func writeSecureFile(data: Data, to url: URL) throws {
    try data.write(to: url, options: .completeFileProtection)
}

// Set protection on existing file
func setProtection(for url: URL) throws {
    try FileManager.default.setAttributes(
        [.protectionKey: FileProtectionType.complete],
        ofItemAtPath: url.path
    )
}

Protection Levels

Level	Description
.complete	Only accessible when device unlocked
.completeUnlessOpen	Accessible if file was open when locked
.completeUntilFirstUserAuthentication	Accessible after first unlock
.none	No protection (avoid for sensitive data)

Cryptography

CryptoKit (Preferred)

import CryptoKit

// Symmetric Encryption (AES-GCM)
func encrypt(data: Data, using key: SymmetricKey) throws -> Data {
    let sealedBox = try AES.GCM.seal(data, using: key)
    return sealedBox.combined!
}

func decrypt(data: Data, using key: SymmetricKey) throws -> Data {
    let sealedBox = try AES.GCM.SealedBox(combined: data)
    return try AES.GCM.open(sealedBox, using: key)
}

// Key Derivation
func deriveKey(from password: String, salt: Data) -> SymmetricKey {
    let passwordData = Data(password.utf8)
    let key = HKDF<SHA256>.deriveKey(
        inputKeyMaterial: SymmetricKey(data: passwordData),
        salt: salt,
        info: Data("encryption".utf8),
        outputByteCount: 32
    )
    return key
}

// Hashing
func hash(data: Data) -> String {
    let digest = SHA256.hash(data: data)
    return digest.map { String(format: "%02x", $0) }.joined()
}

// HMAC
func authenticate(data: Data, key: SymmetricKey) -> Data {
    let authCode = HMAC<SHA256>.authenticationCode(for: data, using: key)
    return Data(authCode)
}

Network Security

App Transport Security (ATS)

<!-- Info.plist - Secure Configuration -->
<key>NSAppTransportSecurity</key>
<dict>
    <!-- ATS enabled by default, don't add anything for HTTPS-only -->
</dict>

<!-- If you MUST allow exceptions (document why!) -->
<key>NSAppTransportSecurity</key>
<dict>
    <key>NSExceptionDomains</key>
    <dict>
        <key>legacy-api.example.com</key>
        <dict>
            <key>NSExceptionAllowsInsecureHTTPLoads</key>
            <true/>
            <key>NSExceptionMinimumTLSVersion</key>
            <string>TLSv1.2</string>
        </dict>
    </dict>
</dict>

Certificate Pinning

import CryptoKit

final class CertificatePinningDelegate: NSObject, URLSessionDelegate {
    private let pinnedCertificateHashes: Set<String>

    init(pinnedHashes: [String]) {
        self.pinnedCertificateHashes = Set(pinnedHashes)
    }

    func urlSession(
        _ session: URLSession,
        didReceive challenge: URLAuthenticationChallenge,
        completionHandler: @escaping (URLSession.AuthChallengeDisposition, URLCredential?) -> Void
    ) {
        guard challenge.protectionSpace.authenticationMethod == NSURLAuthenticationMethodServerTrust,
              let serverTrust = challenge.protectionSpace.serverTrust else {
            completionHandler(.cancelAuthenticationChallenge, nil)
            return
        }

        // Get server certificate
        guard let certificate = SecTrustCopyCertificateChain(serverTrust) as? [SecCertificate],
              let serverCert = certificate.first else {
            completionHandler(.cancelAuthenticationChallenge, nil)
            return
        }

        // Hash the public key
        let serverCertData = SecCertificateCopyData(serverCert) as Data
        let hash = SHA256.hash(data: serverCertData)
        let hashString = hash.map { String(format: "%02x", $0) }.joined()

        if pinnedCertificateHashes.contains(hashString) {
            completionHandler(.useCredential, URLCredential(trust: serverTrust))
        } else {
            completionHandler(.cancelAuthenticationChallenge, nil)
        }
    }
}

App Attest (iOS 14+)

Device Integrity Verification

import DeviceCheck

final class AppAttestManager {
    private let service = DCAppAttestService.shared

    var isSupported: Bool {
        service.isSupported
    }

    func generateKey() async throws -> String {
        try await service.generateKey()
    }

    func attestKey(_ keyId: String, clientDataHash: Data) async throws -> Data {
        try await service.attestKey(keyId, clientDataHash: clientDataHash)
    }

    func generateAssertion(_ keyId: String, clientDataHash: Data) async throws -> Data {
        try await service.generateAssertion(keyId, clientDataHash: clientDataHash)
    }
}

// Usage in authentication flow
final class SecureAuthService {
    private let attestManager = AppAttestManager()
    private var keyId: String?

    func setupAttestation() async throws {
        guard attestManager.isSupported else {
            throw SecurityError.attestationNotSupported
        }

        keyId = try await attestManager.generateKey()

        // Get challenge from server
        let challenge = try await fetchChallenge()
        let clientDataHash = SHA256.hash(data: challenge)

        // Attest the key
        let attestation = try await attestManager.attestKey(
            keyId!,
            clientDataHash: Data(clientDataHash)
        )

        // Send attestation to server for verification
        try await verifyAttestation(attestation)
    }
}

Secure Coding Practices

Input Validation

// Validate and sanitize all external input
struct InputValidator {
    static func validateEmail(_ email: String) -> Bool {
        let pattern = "^[A-Z0-9a-z._%+-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,}$"
        return email.range(of: pattern, options: .regularExpression) != nil
    }

    static func sanitizeForDisplay(_ input: String) -> String {
        // Prevent potential XSS in WebViews
        input
            .replacingOccurrences(of: "<", with: "&lt;")
            .replacingOccurrences(of: ">", with: "&gt;")
    }
}

Secure Memory

// Clear sensitive data when done
extension Data {
    mutating func secureZero() {
        guard count > 0 else { return }
        withUnsafeMutableBytes { ptr in
            memset_s(ptr.baseAddress, ptr.count, 0, ptr.count)
        }
    }
}

// Use for temporary sensitive data
class SecureString {
    private var data: Data

    init(_ string: String) {
        data = Data(string.utf8)
    }

    var value: String {
        String(data: data, encoding: .utf8) ?? ""
    }

    deinit {
        data.secureZero()
    }
}

References

• See references/owasp-mobile-top10.md for OWASP Mobile security guidelines
• See references/privacy-manifest.md for privacy manifest requirements