device-integrity

SKILL.md

Device Integrity

Verify that requests to your server come from a genuine Apple device running a
legitimate instance of your 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 sensitive
requests.

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. Treat each token as
single-use: generate a new token for each server operation instead of caching or
reusing one. 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.

Use https://api.development.devicecheck.apple.com only while testing; use
https://api.devicecheck.apple.com for production.

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
}

For app extensions, App Attest is supported only in Action, extensible SSO, and
watchOS extensions. Treat other extension types as unsupported even if
isSupported returns true.

App Attest Key Generation

Generate one cryptographic key pair per user account on each device. The
private key stays in the Secure Enclave. The returned keyId is the only
identifier your app can later use to access the key, so record and reuse the
account/device-scoped keyId; do not share one key across users. Avoid
unnecessary regeneration because each new key affects App Attest key-count risk
metrics. Only treat the keyId as usable after your server verifies
attestation. If server verification fails, discard the keyId and generate a
new key before retrying.

import DeviceCheck

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

    /// Generate and record 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-\(currentAccountID)",
            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-\(currentAccountID)",
            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 per user account on a device, persist that
account/device keyId, and keep the key count low. Generating unnecessary keys
pollutes App Attest risk metrics.

App Attest Attestation Flow

Attestation proves that the key was generated on a genuine Apple device running
a legitimate instance of your app. You perform attestation once per key, then
store the verified public key and receipt on your server. The app stores the
keyId for future assertions after the server accepts the attestation.

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, checks Apple's nonce calculation, and
stores the verified public key and receipt for future assertion verification.
The attestation nonce is not SHA256(challenge) alone; it is
SHA256(authData || SHA256(challenge)) and is compared with the credential
certificate extension 1.2.840.113635.100.8.2. See
references/device-integrity-patterns.md
for the full server verification flow.

App Attest Assertion Flow

After attestation, use assertions to sign sensitive requests. Each assertion
proves the request came from the attested app instance and includes a
server-issued, one-time challenge to prevent replay.

Client-Side Assertion

import DeviceCheck
import CryptoKit

extension AppAttestManager {
    /// Generate an assertion for encoded client data.
    /// Client data should include a one-time server challenge and request context.
    func generateAssertion(for clientData: Data) async throws -> Data {
        guard let keyId else {
            throw DeviceIntegrityError.keyNotGenerated
        }

        let clientDataHash = Data(SHA256.hash(data: clientData))

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

Using Assertions in Network Requests

struct AppAttestClientData: Encodable {
    let challenge: String
    let method: String
    let path: String
    let bodySHA256: String
}

extension AppAttestManager {
    /// Perform an attested API request.
    func makeAttestedRequest(
        to url: URL,
        method: String = "POST",
        body: Data
    ) async throws -> (Data, URLResponse) {
        let challenge = try await fetchAssertionChallenge()
        let bodyHash = Data(SHA256.hash(data: body)).base64EncodedString()
        let clientData = try JSONEncoder().encode(
            AppAttestClientData(
                challenge: challenge,
                method: method,
                path: url.path,
                bodySHA256: bodyHash
            )
        )
        let assertion = try await generateAssertion(for: clientData)

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

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

    private func fetchAssertionChallenge() async throws -> String {
        let url = serverURL.appending(path: "assert/challenge")
        let (data, _) = try await URLSession.shared.data(from: url)
        return String(decoding: data, as: UTF8.self)
    }
}

Server-Side Assertion Verification

Your server decodes the assertion (CBOR), verifies the authenticator data and
counter, recomputes clientDataHash from the submitted client data, verifies
the signature over SHA256(authenticatorData || clientDataHash) with the
stored public key, and confirms the embedded challenge and request context. 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.

Security Boundaries

App Attest proves app-instance integrity for selected requests. It does not
replace user authentication, OAuth/JWT/session handling, API token design,
entitlement or subscription authorization, TLS, certificate pinning, or general
networking security. Treat those as handoffs to authentication, networking, or
broader security guidance, and still enforce normal authentication and
authorization after App Attest passes.

Error Handling

Handle DCError codes from DeviceCheck operations. Key cases:

.serverUnavailable — retry with exponential backoff

.invalidKey — the key was already attested, assertion used an unattested key, or the service rejected the key

.featureUnsupported — fall back to DCDevice tokens

.invalidInput — malformed clientDataHash or keyId

For attestKey, retry .serverUnavailable later with the same keyId and the
same clientDataHash. For other attestation errors, discard the key identifier
and create a new key before retrying. See
references/device-integrity-patterns.md
for full error handling code, retry strategy, and rejected-key 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 omitted during development, the app uses the App Attest
sandbox by default. After distribution through TestFlight, the App Store, or the
Apple Developer Enterprise Program, the app ignores the entitlement value and
uses production.

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 per user account on a device, persist the keyId, and keep key counts low.

Reusing DCDevice tokens. Treat generated tokens as single-use. Generate a new token for each server operation.

Skipping the fallback for unsupported devices or extensions. Not all devices and extension types support App Attest. Use DCDevice tokens or other risk assessment as fallback.

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

Signing only the raw request body. Assertion client data must include a one-time server challenge and enough request context for the server to bind the assertion to the request.

Verifying the wrong attestation nonce. Compare the certificate extension with SHA256(authData || SHA256(challenge)), not SHA256(challenge) alone.

Not implementing replay protection. The server must validate one-time challenges and track the assertion counter.

Mixing development and production environments. Sandbox keys and receipts do not work in production, and production keys and receipts do not work in sandbox.

Not handling DCError.invalidKey. Check for repeated attestation, unattested assertion keys, or service rejection; regenerate only after the state is known bad.

Review Checklist

 DCDevice tokens generated per server operation and never cached for reuse

 DCAppAttestService.isSupported checked before use; unsupported devices and extension types have a fallback

 Key generated once per user account on each device and keyId persisted only for that app account/device

 Attestation performed once per key; server stores verified public key and receipt

 Server validates attestation certificate chain, App ID hash, environment aaguid, credential ID, and nonce SHA256(authData || SHA256(challenge))

 Assertions include one-time challenge plus request context; server verifies signature, RP ID, counter, challenge, and request binding

 Protected endpoints still enforce normal user authentication and entitlement authorization after App Attest passes

 DCError cases handled: .serverUnavailable retries attestation with the same key/hash; bad keys are discarded and regenerated

 App Attest environment entitlement and sandbox/production server routing are consistent

 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

Validating apps that connect to your server

Attestation Object Validation Guide

App Attest Environment