Device Integrity

Device Integrity

Verify that requests to your server come from a genuine Apple device running your unmodified app. DeviceCheck provides per-device bits for simple flags (e.g., "claimed promo offer"). App Attest uses Secure Enclave keys and Apple attestation to cryptographically prove app legitimacy on each request.

Contents

• DCDevice (DeviceCheck Tokens)
• DCAppAttestService (App Attest)
• App Attest Key Generation
• App Attest Attestation Flow
• App Attest Assertion Flow
• Server Verification Guidance
• Error Handling
• Common Patterns
• Common Mistakes
• Review Checklist

DCDevice (DeviceCheck Tokens)

DCDevice generates a unique, ephemeral token that identifies a device. The token is sent to your server, which then communicates with Apple's servers to read or set two per-device bits. Available on iOS 11+.

Token Generation

import DeviceCheck

func generateDeviceToken() async throws -> Data {
    guard DCDevice.current.isSupported else {
        throw DeviceIntegrityError.deviceCheckUnsupported
    }

    return try await DCDevice.current.generateToken()
}

Sending the Token to Your Server

func sendTokenToServer(_ token: Data) async throws {
    let tokenString = token.base64EncodedString()

    var request = URLRequest(url: serverURL.appending(path: "verify-device"))
    request.httpMethod = "POST"
    request.setValue("application/json", forHTTPHeaderField: "Content-Type")
    request.httpBody = try JSONEncoder().encode(["device_token": tokenString])

    let (_, response) = try await URLSession.shared.data(for: request)
    guard let httpResponse = response as? HTTPURLResponse,
          httpResponse.statusCode == 200 else {
        throw DeviceIntegrityError.serverVerificationFailed
    }
}

Server-Side Overview

Your server uses the device token to call Apple's DeviceCheck API endpoints:

The server authenticates with a DeviceCheck private key from the Apple Developer portal, creating a signed JWT for each request.

What the Two Bits Are For

Apple stores two Boolean values per device per developer team. You decide what they mean. Common uses:

• Bit 0: Device has claimed a promotional offer.
• Bit 1: Device has been flagged for fraud.

Bits persist across app reinstall; device reset does not clear them. You control when to reset them via the server API.

DCAppAttestService (App Attest)

DCAppAttestService validates that a specific instance of your app on a specific device is legitimate. It uses a hardware-backed key in the Secure Enclave to create cryptographic attestations and assertions. Available on iOS 14+.

The flow has three phases:

1. Key generation -- create a key pair in the Secure Enclave.
2. Attestation -- Apple certifies the key belongs to a genuine Apple device running your app.
3. Assertion -- sign server requests with the attested key to prove ongoing legitimacy.

Checking Support

import DeviceCheck

let attestService = DCAppAttestService.shared

guard attestService.isSupported else {
    // Fall back to DCDevice token or other risk assessment.
    // App Attest is not available on simulators or all device models.
    return
}

App Attest Key Generation

Generate a cryptographic key pair stored in the Secure Enclave. The returned keyId is a string identifier you persist (e.g., in Keychain) for later attestation and assertion calls.

import DeviceCheck

actor AppAttestManager {
    private let service = DCAppAttestService.shared
    private var keyId: String?

    /// Generate and persist a key pair for App Attest.
    func generateKeyIfNeeded() async throws -> String {
        if let existingKeyId = loadKeyIdFromKeychain() {
            self.keyId = existingKeyId
            return existingKeyId
        }

        let newKeyId = try await service.generateKey()
        saveKeyIdToKeychain(newKeyId)
        self.keyId = newKeyId
        return newKeyId
    }

    // MARK: - Keychain helpers (simplified)

    private func saveKeyIdToKeychain(_ keyId: String) {
        let data = Data(keyId.utf8)
        let query: [String: Any] = [
            kSecClass as String: kSecClassGenericPassword,
            kSecAttrAccount as String: "app-attest-key-id",
            kSecAttrService as String: Bundle.main.bundleIdentifier ?? "",
            kSecValueData as String: data,
            kSecAttrAccessible as String: kSecAttrAccessibleAfterFirstUnlockThisDeviceOnly
        ]
        SecItemDelete(query as CFDictionary) // Remove old if exists
        SecItemAdd(query as CFDictionary, nil)
    }

    private func loadKeyIdFromKeychain() -> String? {
        let query: [String: Any] = [
            kSecClass as String: kSecClassGenericPassword,
            kSecAttrAccount as String: "app-attest-key-id",
            kSecAttrService as String: Bundle.main.bundleIdentifier ?? "",
            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)
    }
}

Important: Generate the key once and persist the keyId. Generating a new key invalidates any previous attestation.

App Attest Attestation Flow

Attestation proves that the key was generated on a genuine Apple device running your unmodified app. You perform attestation once per key, then store the attestation object on your server.

Client-Side Attestation

import DeviceCheck
import CryptoKit

extension AppAttestManager {
    /// Attest the key with Apple. Send the attestation object to your server.
    func attestKey() async throws -> Data {
        guard let keyId else {
            throw DeviceIntegrityError.keyNotGenerated
        }

        // 1. Request a one-time challenge from your server
        let challenge = try await fetchServerChallenge()

        // 2. Hash the challenge (Apple requires a SHA-256 hash)
        let challengeHash = Data(SHA256.hash(data: challenge))

        // 3. Ask Apple to attest the key
        let attestation = try await service.attestKey(keyId, clientDataHash: challengeHash)

        // 4. Send the attestation object to your server for verification
        try await sendAttestationToServer(
            keyId: keyId,
            attestation: attestation,
            challenge: challenge
        )

        return attestation
    }

    private func fetchServerChallenge() async throws -> Data {
        let url = serverURL.appending(path: "attest/challenge")
        let (data, _) = try await URLSession.shared.data(from: url)
        return data
    }

    private func sendAttestationToServer(
        keyId: String,
        attestation: Data,
        challenge: Data
    ) async throws {
        var request = URLRequest(url: serverURL.appending(path: "attest/verify"))
        request.httpMethod = "POST"
        request.setValue("application/json", forHTTPHeaderField: "Content-Type")

        let payload: [String: String] = [
            "key_id": keyId,
            "attestation": attestation.base64EncodedString(),
            "challenge": challenge.base64EncodedString()
        ]
        request.httpBody = try JSONEncoder().encode(payload)

        let (_, response) = try await URLSession.shared.data(for: request)
        guard let httpResponse = response as? HTTPURLResponse,
              httpResponse.statusCode == 200 else {
            throw DeviceIntegrityError.attestationVerificationFailed
        }
    }
}

Server-Side Attestation Verification

Your server must:

1. Verify the attestation object is a valid CBOR-encoded structure.
2. Extract the certificate chain and validate it against Apple's App Attest root CA.
3. Verify the nonce in the attestation matches SHA256(challenge).
4. Extract and store the public key and receipt for future assertion verification.

See Validating apps that connect to your server for the full server verification algorithm.

App Attest Assertion Flow

After attestation, use assertions to sign individual requests. Each assertion proves the request came from the attested app instance.

Client-Side Assertion

import DeviceCheck
import CryptoKit

extension AppAttestManager {
    /// Generate an assertion to accompany a server request.
    /// - Parameter requestData: The request payload to sign (e.g., JSON body).
    /// - Returns: The assertion data to include with the request.
    func generateAssertion(for requestData: Data) async throws -> Data {
        guard let keyId else {
            throw DeviceIntegrityError.keyNotGenerated
        }

        // Hash the request data -- the server will verify this matches
        let clientDataHash = Data(SHA256.hash(data: requestData))

        return try await service.generateAssertion(keyId, clientDataHash: clientDataHash)
    }
}

Using Assertions in Network Requests

extension AppAttestManager {
    /// Perform an attested API request.
    func makeAttestedRequest(
        to url: URL,
        method: String = "POST",
        body: Data
    ) async throws -> (Data, URLResponse) {
        let assertion = try await generateAssertion(for: body)

        var request = URLRequest(url: url)
        request.httpMethod = method
        request.setValue("application/json", forHTTPHeaderField: "Content-Type")
        request.setValue(assertion.base64EncodedString(), forHTTPHeaderField: "X-App-Assertion")
        request.httpBody = body

        return try await URLSession.shared.data(for: request)
    }
}

Server-Side Assertion Verification

Your server must:

1. Decode the assertion (CBOR).
2. Verify the authenticator data, including the counter (must be greater than the stored counter).
3. Verify the signature using the stored public key from attestation.
4. Confirm the clientDataHash matches the SHA-256 of the received request body.
5. Update the stored counter to prevent replay attacks.

Server Verification Guidance

Attestation vs. Assertion

Recommended Server Architecture

1. Challenge endpoint -- generate a random nonce, store it server-side with a short TTL (e.g., 5 minutes).
2. Attestation verification endpoint -- validate the attestation object, store the public key and receipt keyed by keyId.
3. Assertion verification middleware -- verify assertions on sensitive endpoints (purchases, account changes).

Risk Assessment

Combine App Attest with fraud risk assessment for defense in depth. App Attest alone does not guarantee the user is not abusing the app -- it confirms the app is genuine.

Error Handling

DCError Codes

import DeviceCheck

func handleAttestError(_ error: Error) {
    if let dcError = error as? DCError {
        switch dcError.code {
        case .unknownSystemFailure:
            // Transient system error -- retry with exponential backoff
            break
        case .featureUnsupported:
            // Device or OS does not support this feature
            // Fall back to alternative verification
            break
        case .invalidKey:
            // Key is corrupted or was invalidated
            // Generate a new key and re-attest
            break
        case .invalidInput:
            // The clientDataHash or keyId was malformed
            break
        case .serverUnavailable:
            // Apple's attestation server is unreachable -- retry later
            break
        @unknown default:
            break
        }
    }
}

Retry Strategy

extension AppAttestManager {
    func attestKeyWithRetry(maxAttempts: Int = 3) async throws -> Data {
        var lastError: Error?

        for attempt in 0..<maxAttempts {
            do {
                return try await attestKey()
            } catch let error as DCError where error.code == .serverUnavailable {
                lastError = error
                let delay = UInt64(pow(2.0, Double(attempt))) * 1_000_000_000
                try await Task.sleep(nanoseconds: delay)
            } catch {
                throw error // Non-retryable errors propagate immediately
            }
        }

        throw lastError ?? DeviceIntegrityError.attestationFailed
    }
}

Handling Invalidated Keys

If attestKey returns DCError.invalidKey, the Secure Enclave key has been invalidated (e.g., OS update, Secure Enclave reset). Delete the stored keyId from Keychain and generate a new key:

extension AppAttestManager {
    func handleInvalidKey() async throws -> String {
        deleteKeyIdFromKeychain()
        keyId = nil
        return try await generateKeyIfNeeded()
    }

    private func deleteKeyIdFromKeychain() {
        let query: [String: Any] = [
            kSecClass as String: kSecClassGenericPassword,
            kSecAttrAccount as String: "app-attest-key-id",
            kSecAttrService as String: Bundle.main.bundleIdentifier ?? ""
        ]
        SecItemDelete(query as CFDictionary)
    }
}

Common Patterns

Full Integration Manager

Combine the patterns above into a single actor that manages the full lifecycle:

1. Check isSupported and fall back to DCDevice tokens on unsupported devices.
2. Call generateKeyIfNeeded() on launch to create or load the persisted key.
3. Call attestKeyWithRetry() once after key generation.
4. Use generateAssertion(for:) on each sensitive server request.
5. Handle DCError.invalidKey by regenerating and re-attesting.

Gradual Rollout

Apple recommends a gradual rollout. Gate App Attest behind a remote feature flag and fall back to DCDevice tokens on unsupported devices.

Environment Entitlement

Set the App Attest environment in your entitlements file. Use development during testing and production for App Store builds:

<key>com.apple.developer.devicecheck.appattest-environment</key>
<string>production</string>

When the entitlement is missing, the system uses development in debug builds and production for App Store and TestFlight builds.

Error Type

enum DeviceIntegrityError: Error {
    case deviceCheckUnsupported
    case keyNotGenerated
    case attestationFailed
    case attestationVerificationFailed
    case assertionFailed
    case serverVerificationFailed
}

Common Mistakes

1. Generating a new key on every launch. Generate once, persist the keyId in Keychain.
2. Skipping the fallback for unsupported devices. Not all devices support App Attest. Use DCDevice tokens as fallback.
3. Trusting attestation client-side. All verification must happen on your server.
4. Not implementing replay protection. The server must track and increment the assertion counter.
5. Missing the environment entitlement. Without it, debug builds use development and App Store uses production. Mismatches cause attestation failures.
6. Not handling DCError.invalidKey. Keys can be invalidated by OS updates. Detect and regenerate.

Review Checklist

• DCAppAttestService.isSupported checked before use; fallback to DCDevice when unsupported
• Key generated once and keyId persisted in Keychain
• Attestation performed once per key; attestation object sent to server
• Server validates attestation against Apple's App Attest root CA
• Assertions generated for each sensitive request; server verifies signature and counter
• DCError cases handled: .serverUnavailable with retry, .invalidKey with key regeneration
• App Attest environment entitlement set correctly for debug vs. production
• Gradual rollout considered; feature flag in place for enabling/disabling