func saveToKeychain(account: String, data: Data, service: String) throws {
    let query: [String: Any] = [
        kSecClass as String: kSecClassGenericPassword,
        kSecAttrAccount as String: account,
        kSecAttrService as String: service,
        kSecValueData as String: data,
        kSecAttrAccessible as String: kSecAttrAccessibleAfterFirstUnlockThisDeviceOnly
    ]

    let status = SecItemAdd(query as CFDictionary, nil)

    if status == errSecDuplicateItem {
        let updateQuery: [String: Any] = [
            kSecClass as String: kSecClassGenericPassword,
            kSecAttrAccount as String: account,
            kSecAttrService as String: service
        ]
        let updates: [String: Any] = [kSecValueData as String: data]
        let updateStatus = SecItemUpdate(updateQuery as CFDictionary, updates as CFDictionary)
        guard updateStatus == errSecSuccess else {
            throw KeychainError.updateFailed(updateStatus)
        }
    } else if status != errSecSuccess {
        throw KeychainError.saveFailed(status)
    }
}

func readFromKeychain(account: String, service: String) throws -> Data {
    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 {
        throw KeychainError.readFailed(status)
    }
    return data
}

func deleteFromKeychain(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.deleteFailed(status)
    }
}

let query: [String: Any] = [
    kSecClass as String: kSecClassGenericPassword,
    kSecAttrAccount as String: "shared-token",
    kSecAttrAccessGroup as String: "TEAMID.com.company.shared"
]

// Non-sensitive preference -- @AppStorage is fine
@AppStorage("hasCompletedOnboarding") private var hasOnboarded = false

// Sensitive credential -- MUST use Keychain
// WRONG: @AppStorage("authToken") private var token = ""
// CORRECT: Use saveToKeychain(account:data:service:)

// Set file protection
try data.write(to: url, options: .completeFileProtection)

// Check protection level
let attributes = try FileManager.default.attributesOfItem(atPath: path)
let protection = attributes[.protectionKey] as? FileProtectionType

import CryptoKit

let key = SymmetricKey(size: .bits256)

func encrypt(_ data: Data, using key: SymmetricKey) throws -> Data {
    let sealed = try AES.GCM.seal(data, using: key)
    guard let combined = sealed.combined else {
        throw CryptoError.sealFailed
    }
    return combined
}

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

let hash = SHA256.hash(data: data)
let hashString = hash.compactMap { String(format: "%02x", $0) }.joined()

// Also available: SHA384, SHA512

let key = SymmetricKey(size: .bits256)

// Sign
let signature = HMAC<SHA256>.authenticationCode(for: data, using: key)

// Verify
let isValid = HMAC<SHA256>.isValidAuthenticationCode(signature, authenticating: data, using: key)

guard SecureEnclave.isAvailable else { return }

let accessControl = SecAccessControlCreateWithFlags(
    nil, kSecAttrAccessibleWhenPasscodeSetThisDeviceOnly,
    [.privateKeyUsage, .biometryCurrentSet], nil
)!
let privateKey = try SecureEnclave.P256.Signing.PrivateKey(accessControl: accessControl)

let signature = try privateKey.signature(for: data)  // May trigger biometric prompt
let isValid = privateKey.publicKey.isValidSignature(signature, for: data)

// Persist: store dataRepresentation in Keychain, restore with:
let restored = try SecureEnclave.P256.Signing.PrivateKey(
    dataRepresentation: privateKey.dataRepresentation
)

import LocalAuthentication

func authenticateWithBiometrics() async throws -> Bool {
    let context = LAContext()
    var error: NSError?

    guard context.canEvaluatePolicy(
        .deviceOwnerAuthenticationWithBiometrics, error: &error
    ) else {
        // Biometrics not available -- fall back to passcode
        if context.canEvaluatePolicy(.deviceOwnerAuthentication, error: &error) {
            return try await context.evaluatePolicy(
                .deviceOwnerAuthentication,
                localizedReason: "Authenticate to access your account"
            )
        }
        throw AuthError.biometricsUnavailable
    }

    return try await context.evaluatePolicy(
        .deviceOwnerAuthenticationWithBiometrics,
        localizedReason: "Authenticate to access your account"
    )
}

<key>NSFaceIDUsageDescription</key>
<string>Authenticate to access your secure data</string>

let context = LAContext()
context.localizedFallbackTitle = "Use Passcode"
context.touchIDAuthenticationAllowableReuseDuration = 30
let currentState = context.evaluatedPolicyDomainState // Compare to detect enrollment changes

let access = SecAccessControlCreateWithFlags(
    nil,
    kSecAttrAccessibleWhenPasscodeSetThisDeviceOnly,
    .biometryCurrentSet,
    nil
)!

let query: [String: Any] = [
    kSecClass as String: kSecClassGenericPassword,
    kSecAttrAccount as String: "auth-token",
    kSecValueData as String: tokenData,
    kSecAttrAccessControl as String: access,
    kSecUseAuthenticationContext as String: LAContext()
]

<!-- Only for legacy servers you cannot upgrade -->
<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>

import CryptoKit

class PinnedSessionDelegate: NSObject, URLSessionDelegate {
    // SHA-256 hash of the certificate's Subject Public Key Info
    private let pinnedHashes: Set<String> = [
        "base64EncodedSHA256HashOfSPKI=="
    ]

    func urlSession(
        _ session: URLSession,
        didReceive challenge: URLAuthenticationChallenge
    ) async -> (URLSession.AuthChallengeDisposition, URLCredential?) {
        guard let trust = challenge.protectionSpace.serverTrust,
              let chain = SecTrustCopyCertificateChain(trust) as? [SecCertificate],
              let certificate = chain.first else {
            return (.cancelAuthenticationChallenge, nil)
        }

        guard let publicKey = SecCertificateCopyKey(certificate),
              let publicKeyData = SecKeyCopyExternalRepresentation(
                  publicKey, nil
              ) as Data? else {
            return (.cancelAuthenticationChallenge, nil)
        }

        let hash = SHA256.hash(data: publicKeyData)
        let hashString = Data(hash).base64EncodedString()

        if pinnedHashes.contains(hashString) {
            return (.useCredential, URLCredential(trust: trust))
        }

        return (.cancelAuthenticationChallenge, nil)
    }
}

// WRONG
logger.debug("User logged in with token: \(token)")

// CORRECT
logger.debug("User logged in successfully")

var sensitiveData = Data(/* ... */)
defer {
    sensitiveData.resetBytes(in: 0..<sensitiveData.count)
}

guard let url = URL(string: input),
      ["https"].contains(url.scheme?.lowercased()) else {
    throw SecurityError.invalidURL
}
let resolved = url.standardized.path
guard resolved.hasPrefix(allowedDirectory.path) else {
    throw SecurityError.pathTraversal
}

// Forces a build error until the real key is configured
#error("Add your API key to Secrets.plist -- see README for setup")
private let apiKey = Secrets.value(for: "API_KEY")