Axiom App Attest

App Attest

Device-backed app integrity verification for fraud prevention. Proves three things to your server: the request came from a genuine Apple device, running your genuine app, with an untampered payload.

When to Use This Skill

Use when you need to:

• Verify requests come from legitimate app instances (not modified/cloned apps)
• Prevent fraud in purchases, promotions, or competitive features
• Implement DCAppAttestService attestation or assertion flows
• Handle DeviceCheck 2-bit per-device state for promotional abuse
• Build server-side validation for attestation objects or assertion signatures
• Plan a gradual App Attest rollout for a large install base

Example Prompts

"How do I verify my app hasn't been tampered with?" "DCAppAttestService attestKey keeps failing with serverUnavailable" "How do I prevent users from claiming a free trial multiple times?" "What's the difference between attestation and assertion?" "How do I validate an attestation object on my server?" "isSupported returns false — should I block the user?" "We have 2M DAU, how do I roll out App Attest safely?" "How do I detect if someone is creating fake app instances?"

Red Flags

Signs you're headed for trouble:

• Validating app integrity on-device — Modified apps control the runtime. Any local check can be patched out. Verification MUST happen server-side.
• Not guarding with isSupported — DCAppAttestService crashes on unsupported devices. Always check before calling any API.
• Blocking users when isSupported returns false — Some legitimate devices return false. Treat as risk signal, not hard block.
• Reusing keys across multiple users on same device — One key per user per device. Shared keys break account-level trust association.
• Enabling App Attest for all users at once — attestKey calls Apple's servers. At scale, rate limiting causes failures. Gradual rollout required (WWDC 2021-10244).
• Using assertions for every API call — Cryptographic cost per call. Reserve for sensitive operations (purchases, account changes), not routine fetches.
• Discarding key on serverUnavailable error — Transient Apple server issue. Retry with same key. Only discard on other errors.
• Skipping counter validation on server — Counter must be ever-increasing. Without this, replay attacks succeed.

Three Properties Verified

App Attest proves three things about each request:

Privacy design: Anonymous. No hardware identifiers. Keys don't survive reinstall/migration/restore. Apple can't correlate across apps or users.

Key Generation

import DeviceCheck

func generateAppAttestKey(for userId: String) async throws -> String {
    let service = DCAppAttestService.shared

    guard service.isSupported else {
        // NOT an error — use as risk signal, not blocker
        reportUnattestedDevice()
        throw AppAttestError.unsupported
    }

    let keyId = try await service.generateKey()
    // Cache persistently — one key per user per device
    UserDefaults.standard.set(keyId, forKey: "appAttestKeyId_\(userId)")
    return keyId
}

Key lifecycle: One key per user per device. Cache keyId persistently (Keychain or UserDefaults). Keys don't survive reinstall, migration, or restore. App Clips share identity with full app. Generate new key on sign-out.

Attestation Flow

Attestation registers the key with Apple and your server. Happens once per key.

digraph attestation {
    "Server issues\nchallenge" [shape=ellipse];
    "SHA256 hash\nchallenge" [shape=box];
    "attestKey API\n(Apple servers)" [shape=box];
    "Send attestation\nto your server" [shape=box];
    "Server validates\ncertificate chain" [shape=box];
    "Store public key\n+ key association" [shape=doublecircle];

    "Error?" [shape=diamond];
    "serverUnavailable?" [shape=diamond];
    "Retry same key" [shape=box];
    "Discard key\ngenerate new" [shape=box];

    "Server issues\nchallenge" -> "SHA256 hash\nchallenge";
    "SHA256 hash\nchallenge" -> "attestKey API\n(Apple servers)";
    "attestKey API\n(Apple servers)" -> "Error?" ;
    "Error?" -> "Send attestation\nto your server" [label="success"];
    "Error?" -> "serverUnavailable?" [label="error"];
    "serverUnavailable?" -> "Retry same key" [label="yes"];
    "serverUnavailable?" -> "Discard key\ngenerate new" [label="no"];
    "Send attestation\nto your server" -> "Server validates\ncertificate chain";
    "Server validates\ncertificate chain" -> "Store public key\n+ key association";
}

func attestKey(userId: String) async throws {
    guard let keyId = storedKeyId(for: userId) else {
        throw AppAttestError.noKey
    }

    // 1. Get one-time challenge from YOUR server (minimum 16 bytes)
    let challenge = try await server.fetchAttestationChallenge()

    // 2. Hash the challenge
    let hash = Data(SHA256.hash(data: challenge))

    // 3. Request attestation from Apple
    do {
        let attestation = try await service.attestKey(keyId, clientDataHash: hash)
        // 4. Send attestation object to YOUR server for validation
        try await server.verifyAttestation(attestation, keyId: keyId, challenge: challenge)
    } catch DCError.serverUnavailable {
        // Transient — retry with SAME key later
        scheduleAttestationRetry(keyId: keyId, userId: userId)
    } catch {
        // Other error — key is compromised or invalid
        // Discard and generate a new key
        clearStoredKey(for: userId)
        try await generateAndAttestNewKey(userId: userId)
    }
}

Challenge requirements: Server-generated, single-use, minimum 16 bytes, short-lived (expire after minutes, not hours).

Assertion Flow

Assertions prove ongoing request integrity. No Apple server involvement — on-device only.

func assertRequest(payload: Data, userId: String) async throws -> Data {
    guard let keyId = storedKeyId(for: userId) else {
        throw AppAttestError.noKey
    }

    // Hash the payload you want to protect
    let hash = Data(SHA256.hash(data: payload))

    // Generate assertion (on-device, no network)
    let assertion = try await service.generateAssertion(keyId, clientDataHash: hash)

    // Send assertion + original payload to server
    // Server verifies signature and checks counter
    return assertion
}

When to assert: Reserve for moments that cost you money or trust if faked.

Performance: Secure Enclave operations. Fast enough for individual actions, expensive on every request.

Server-Side Validation

Your server does the actual trust verification. The app only generates cryptographic material.

Attestation Validation (once per key)

1. Certificate chain — Verify roots to Apple's App Attest root CA (Apple Private PKI)
2. Nonce — Recompute SHA256(challenge || clientDataHash), match against credential certificate
3. App identity hash — SHA256(teamId + "." + bundleId) must match your app
4. Counter — Store initial value (assertions increment from here)
5. Key association — Extract and store public key, associate with user account

Assertion Validation (per sensitive request)

1. Signature — Verify using stored public key from attestation
2. App identity hash — Must match attestation's hash (prevents cross-app replay)
3. Counter — Must be strictly greater than last seen value (replay protection)
4. Client data hash — Recompute from request payload, must match what was signed

Counter is critical: Without strictly-increasing counter validation, replay attacks succeed indefinitely.

Rollout Strategy

From WWDC 2021-10244: attestKey makes a network call to Apple's servers. Apple rate-limits these calls per app.

Gradual Enablement Pattern

func shouldEnableAppAttest(userId: String) -> Bool {
    guard DCAppAttestService.shared.isSupported else { return false }
    // Server controls rollout percentage — start at 1%, ramp daily
    return server.isAppAttestEnabled(for: userId)
}

Rollout process: Start at 1%. Monitor attestation success rate. If above 95%, double daily. If rate limiting errors spike, pause. Treat unattested requests as lower-trust during rollout (additional fraud signals), not blocked.

DeviceCheck Integration

DeviceCheck stores 2 bits of state per device on Apple's servers. Different purpose from App Attest.

Promotional Fraud Prevention

import DeviceCheck

func checkTrialEligibility() async throws -> Bool {
    guard DCDevice.current.isSupported else { return true }

    let token = try await DCDevice.current.generateToken()
    // Server calls Apple: POST https://api.devicecheck.apple.com/v1/query_two_bits
    let state = try await server.queryDeviceState(token: token)
    return !state.bit0  // bit0 = has claimed trial
}

func markTrialClaimed() async throws {
    let token = try await DCDevice.current.generateToken()
    // Server calls Apple: POST https://api.devicecheck.apple.com/v1/update_two_bits
    try await server.updateDeviceState(token: token, bit0: true)
}

2 bits, your rules: Apple stores bits + timestamp. Semantics are yours (e.g., bit0=trial claimed, bit1=abuse flagged). Reset on your schedule. Shared across all apps from the same developer team — coordinate meaning across your portfolio.

Risk Metric Service

After attestation, redeem the receipt with Apple to get risk metrics:

Server-side: POST receipt to https://data.appattest.apple.com/v1/attestationData (use data-development.appattest.apple.com for sandbox). Response includes approximate key count for the device.

How to use: Most devices have 1-3 keys. High key counts signal an attacker creating many fake identities. Redeem periodically (Apple rate-limits), establish a baseline for your app, and combine with other fraud signals (velocity, behavioral analysis).

Anti-Rationalization Table

Pressure Scenarios

Scenario 1: "Block users who fail attestation"

Pressure: "If they can't attest, they're probably running a modified app. Block them."

Reality: isSupported returns false on legitimate devices (older hardware, enterprise MDM, simulator). During rollout, most users simply haven't been enrolled yet. Blocking = blocking real customers.

Correct action: Trust tiers on server. Attested = high trust. Unattested = lower trust with additional fraud signals. Never hard-block on attestation failure alone.

Push-back template: "Some legitimate devices return isSupported=false. Let's use attestation as one signal in a risk score — high trust for attested, additional checks for unattested."

Scenario 2: "Enable App Attest for everyone at once"

Pressure: "We've been building this for weeks. Ship it to everyone."

Reality: attestKey calls Apple's servers. Apple rate-limits per app. At 5M DAU, flipping the switch causes a thundering herd — mass failures, error floods, confused users. WWDC 2021-10244 explicitly recommends gradual rollout.

Correct action: Server-controlled rollout starting at 1%. At 5M DAU, expect ~1 week to full rollout.

Push-back template: "Apple rate-limits attestKey calls — their WWDC session recommends gradual rollout. I'll set up server-side percentage control starting at 1%, ramping to 100% over about a week."

Checklist

Before shipping App Attest:

Key Generation:

• isSupported checked before any DCAppAttestService call
• Graceful handling when isSupported returns false (risk signal, not block)
• Key ID cached persistently per user
• One key per user per device (not shared)

Attestation:

• Challenge from server is single-use, minimum 16 bytes, short-lived
• serverUnavailable retries with same key
• Other errors discard key and generate new
• Attestation object sent to server for validation (not validated on-device)

Assertion:

• Used only for sensitive operations (not every API call)
• Payload hash covers the actual request data being protected
• Server validates signature with stored public key
• Server validates counter is strictly increasing

Server:

• Certificate chain validated against Apple's App Attest root CA
• App identity hash (teamId + bundleId) verified
• Counter stored and checked for strict increase
• Public key associated with user account

Rollout:

• Server-controlled percentage (not client-side)
• Gradual ramp with monitoring
• Unattested users handled gracefully (lower trust, not blocked)
• Rollback plan if attestation success rate drops

Resources

WWDC: 2021-10244

Docs: /devicecheck, /devicecheck/establishing-your-app-s-integrity, /devicecheck/validating-apps-that-connect-to-your-server

Skills: axiom-cryptokit