device-integrity

SKILL.md

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

References

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:

Endpoint
Purpose

https://api.devicecheck.apple.com/v1/query_two_bits
Read the two bits for a device

https://api.devicecheck.apple.com/v1/update_two_bits
Set the two bits for a device

https://api.devicecheck.apple.com/v1/validate_device_token
Validate a device token without reading bits

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. 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:

Key generation -- create a key pair in the Secure Enclave.

Attestation -- Apple certifies the key belongs to a genuine Apple device running your app.

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 validates the attestation object (CBOR), verifies the certificate chain against Apple's App Attest root CA, and stores the public key and receipt for future assertion verification. See references/device-integrity-patterns.md for the full server verification flow.

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 decodes the assertion (CBOR), verifies the authenticator data and counter, checks the signature against the stored public key, and confirms the clientDataHash. See references/device-integrity-patterns.md for step-by-step server verification.

Server Verification Guidance

See references/device-integrity-patterns.md for full server architecture guidance including attestation vs. assertion comparison, recommended endpoint design, and risk assessment.

Error Handling

Handle DCError codes from DeviceCheck operations. Key cases:

.serverUnavailable — retry with exponential backoff

.invalidKey — key invalidated (OS update, Secure Enclave reset); regenerate and re-attest

.featureUnsupported — fall back to DCDevice tokens

.invalidInput — malformed clientDataHash or keyId

See references/device-integrity-patterns.md for full error handling code, retry strategy, and key invalidation recovery.

Common Patterns

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.

See references/device-integrity-patterns.md for the full integration manager pattern, gradual rollout guidance, and error type definition.

Common Mistakes

Generating a new key on every launch. Generate once, persist the keyId in Keychain.

Skipping the fallback for unsupported devices. Not all devices support App Attest. Use DCDevice tokens as fallback.

Trusting attestation client-side. All verification must happen on your server.

Not implementing replay protection. The server must track and increment the assertion counter.

Missing the environment entitlement. Without it, debug builds use development and App Store uses production. Mismatches cause attestation failures.

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

References

Extended patterns: references/device-integrity-patterns.md

DeviceCheck framework

DCDevice

DCAppAttestService

Establishing your app's integrity