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DeviceCheck iOS research

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# Reverse Engineering Apple's DeviceCheck Token Generation

This document presents an end-to-end reverse engineering of Apple's DeviceCheck token generation flow on iOS, starting from the public `DCDevice` API and following execution through `devicecheckd`, `DeviceCheckInternal.framework`, and the underlying cryptographic primitives.

The goal is to provide a reproducible, engineering-grade description useful for security engineering, red teaming, and low-level reverse engineering work.

## Chapter 1 – DeviceCheck.framework entry point

When creating a token from `DCDevice`, the method `-[DCDevice generateTokenWithCompletionHandler:]` is used – inside it calls `DCDeviceMetadataDaemonConnection`. This method creates a connection to the iPhone’s `devicecheckd` daemon:
```objc
NSXPCConnection *xpc_connection = (NSXPCConnection *)objc_msgSend(
                              objc_alloc((Class)&OBJC_CLASS___NSXPCConnection),
                              "initWithMachServiceName:options:",
                              CFSTR("com.apple.devicecheckd"),
                              0LL);
```

`devicecheckd`

After connecting to the devicecheckd daemon via XPC, the following chain of calls starts. When a connection is received, the daemon invokes `-[DCXPCListener listener:shouldAcceptNewConnection:]` -> `-[DCClientHandler initWithConnection:]`, and after the client makes an RPC call -> `-[DCClientHandler fetchOpaqueBlobWithCompletion]`. First of all, `-[DCClientHandler fetchOpaqueBlobWithCompletion]` calls:
```objc
if ( -[DCClientHandler _isSupported](self, "_isSupported") )
The value of this variable is hard-coded in DeviceIdentityIsSupported from DeviceIdentity.framework:
__int64 DeviceIdentityIsSupported_1()
{
  return 1LL;
}
```
I assume that if DCDevice is unavailable on the device, then there will be a different build of the framework where it is hard-coded to 0. After checking support, it calls `-[DCClientHandler _generateAppIDFromCurrentConnection]` – this method obtains <TeamID>.<BundleIdentifier> (format: ABCDE12345.com.example.myApp) using entitlements (`-[NSXPCConnection valueForEntitlement:]`), and if that fails, it uses `SecTaskCopyTeamIdentifier` and `SecTaskCopySigningIdentifier`. If team_id is valid and not "0000000000", the method combines the team_id and bundle_id with a dot; otherwise it uses only the bundle_id. It returns an appID (<TeamID>.<BundleIdentifier> or <BundleIdentifier>):
`return [appID length] ? appID : nil;`
Returning to `-[DCClientHandler fetchOpaqueBlobWithCompletion]` (important note: I will not consider the fallback cases where the code goes into the else branch for error handling):
```objc
if ( app_id )
{
  DCContext_class = objc_alloc_init((Class)&OBJC_CLASS___DCContext);
  objc_msgSend(DCContext_class, "setClientAppID:", app_id);  // we set our "<TeamID>.<BundleIdentifier>" in the class

// allocate DCDDeviceMetadata and initialize DCCryptoProxyImpl from DeviceCheckInternal.framework
  DCDDeviceMetadata = objc_alloc((Class)&OBJC_CLASS___DCDDeviceMetadata);
  DCCryptoProxyImpl = objc_alloc_init((Class)&OBJC_CLASS___DCCryptoProxyImpl);

// initialize DCDDeviceMetadata from DeviceCheckInternal.framework
  init_DCDDeviceMetadata = objc_msgSend(
                             DCDDeviceMetadata,
                             "initWithContext:cryptoProxy:",
                             DCContext_class,
                             DCCryptoProxyImpl);

objc_msgSend(init_DCDDeviceMetadata, "generateEncryptedBlobWithCompletion:", v4);
}
```
In summary, the `devicecheckd` daemon returns an encrypted token (opaque blob) to `DeviceCheck.framework` via XPC in the completion handler passed to `-[DCDDeviceMetadata generateEncryptedBlobWithCompletion:]`. This blob is later exposed to callers as the token returned by `-[DCDevice generateTokenWithCompletionHandler:]`.

### DCContext, DCDDeviceMetadata, and DCCryptoProxyImpl
The behavior described below is implemented in `DeviceCheckInternal.framework`.

The first thing that happens in `-[DCClientHandler fetchOpaqueBlobWithCompletion]` is the initialization of DCContext and assignment of our <TeamID>.<BundleIdentifier> or <BundleIdentifier> to it:
```objc
DCContext_class = objc_alloc_init((Class)&OBJC_CLASS___DCContext);
objc_msgSend(DCContext_class, "setClientAppID:", app_id);
Since DCContext has no -init method of its own, it simply inherits the implementation from NSObject. Memory is allocated for the object and a pointer to the DCContext class is set. Fields (for example _clientAppID) are zero-initialized. Then the -init selector is sent to the DCContext object. Because DCContext does not override this method, the standard -[NSObject init] is called, which simply returns self without additional logic. After that, the method -[DCContext setClientAppID:] is called, which sets self->_clientAppID to our <TeamID>.<BundleIdentifier> or <BundleIdentifier>:
id __cdecl __noreturn -[DCContext clientAppID](DCContext *self, SEL a2)
{
  return objc_getProperty_33(self, a2, 8LL, 1);
}
```
Skipping allocation of DCDDeviceMetadata, we look at:
```objc
DCCryptoProxyImpl = objc_alloc_init((Class)&OBJC_CLASS___DCCryptoProxyImpl);
Similarly, memory is allocated for the object, a pointer to the DCCryptoProxyImpl class is set, and then -[NSObject init] is called. Now our DCDDeviceMetadata (which was just allocated) is initialized:
init_DCDDeviceMetadata = objc_msgSend(
                           DCDDeviceMetadata,
                           "initWithContext:cryptoProxy:",
                           DCContext_class, // our class holding <TeamID>.<BundleIdentifier> or <BundleIdentifier>
                           DCCryptoProxyImpl_class_arg);
```
What happens during the initialization of DCDDeviceMetadata:
```objc
// we are actually working with the instance's memory area, not with a C-structure per se
00000000 struct DCDDeviceMetadata // sizeof=0x18
00000000 {
00000000     unsigned __int8 superclass_opaque[8];
00000008     DCCryptoProxy *_cryptoProxy;
00000010     DCContext *_context;
00000018 };

id -[DCDDeviceMetadata initWithContext:cryptoProxy:](DCDDeviceMetadata *self, SEL a2, id DCContext_class_arg, id DCCryptoProxyImpl_class_arg)
{
  DCDDeviceMetadata *dc_device_metadata = -[DCDDeviceMetadata init](self, "init"); // -[NSObject init]

if ( dc_device_metadata )
  {
    // set up fields in DCDDeviceMetadata
    j__objc_storeStrong((id *)&dc_device_metadata->_cryptoProxy, DCCryptoProxyImpl_class_arg);
    j__objc_storeStrong((id *)&dc_device_metadata->_context, DCContext_class_arg);  // our context with our <TeamID>.<BundleIdentifier> or <BundleIdentifier>
  }

return (id *)dc_device_metadata;
}
```
Great, all classes have been initialized and now the devicecheckd daemon calls objc_msgSend(init_DCDDeviceMetadata, "generateEncryptedBlobWithCompletion:", v4); – a method that will begin generating the token.

# Chapter 2
`DeviceCheckInternal.framework`

Important note: almost all methods are rewritten by me to make them easier to read and understand. Our daemon invoked the method `objc_msgSend(init_DCDDeviceMetadata, "generateEncryptedBlobWithCompletion:", v4);` – this is the start of token creation.
```objc
void __cdecl -[DCDDeviceMetadata generateEncryptedBlobWithCompletion:](DCDDeviceMetadata *self, SEL a2, id completion_arg)
{
  id v4 = objc_retain(completion_arg); // retain the completion argument

// exactly what was set up in -[DCDDeviceMetadata initWithContext:cryptoProxy:]
  DCCryptoProxy *cryptoProxy = self->_cryptoProxy;
  DCContext *context = self->_context; // our context with our <TeamID>.<BundleIdentifier> or <BundleIdentifier>

[cryptoProxy fetchOpaqueBlobWithContext:context
                              completion:^(NSData *blob, NSError *error) {
      // this block corresponds to __57__DCDDeviceMetadata_generateEncryptedBlobWithCompletion___block_invoke
      // take pointer to the original XPC completion block saved at the time of the call.
      // if argument a2 (data) is non-zero, call completion(data, nil).
      // if a2 is zero, create an NSError with code 0 and call completion(nil, error).
  }];
}
```

At this point, the method `-[DCCryptoProxy fetchOpaqueBlobWithContext:completion:]` is called with our context (containing <TeamID>.<BundleIdentifier> or <BundleIdentifier>) and the completion handler. IDA decompiles the code poorly, so the snippet below is a cleaned-up equivalent for readability:
```objc
void __cdecl -[DCCryptoProxyImpl fetchOpaqueBlobWithContext:completion:](
        DCCryptoProxyImpl *self,
        SEL a2,
        id DCContext_argDCContext_arg,
        id completion_arg)
{
    // hold onto context (<TeamID>.<BundleIdentifier> or <BundleIdentifier>) and completion
    id retainedContext = [DCContext_argDCContext_arg retain];
    void (^copiedCompletion)(NSData *, NSError *) = [completion_arg copy];
  
    if (os_log_type_enabled(self.logger, OS_LOG_TYPE_DEFAULT))
    {
      os_log(self.logger, "Generating certificate...");
    }
  
    __block id blockContext = retainedContext;
    __block void (^blockCompletion)(NSData *, NSError *) = copiedCompletion;

[self _fetchPublicKey:^(NSData *publicKey) {
          DCCertificateGenerator *generator = [[DCCertificateGenerator alloc]
              initWithContext:blockContext // our context with our <TeamID>.<BundleIdentifier> or <BundleIdentifier>
                     publicKey:publicKey]; // our key
  
          [generator generateEncryptedCertificateChainWithCompletion:
                                    ^(NSData *encryptedChain, NSError *error)
            {
                blockCompletion(encryptedChain, error);
                [blockCompletion release];
                [blockContext release];
                [generator release];
            }
          ];
      }
    ];
}
```

### Getting the publicKey
First, the publicKey is obtained, and then it is passed on. How do we get the public key:
```objc
void __cdecl -[DCCryptoProxyImpl _fetchPublicKey:](DCCryptoProxyImpl *self, SEL a2, id completion)
{
  /*
  void *v4; // x20
  id v5; // x19
  _QWORD v6[4]; // [xsp+8h] [xbp-38h] BYREF
  id v7; // [xsp+28h] [xbp-18h]

v4 = (void *)objc_claimAutoreleasedReturnValue_5(+[DCAssetFetcher sharedFetcher](&OBJC_CLASS___DCAssetFetcher, "sharedFetcher"));
  v6[0] = _NSConcreteStackBlock_ptr;
  v6[1] = 3221225472LL;
  v6[2] = __37__DCCryptoProxyImpl__fetchPublicKey___block_invoke;
  v6[3] = &unk_20A9B2838;
  objc_msgSend(v4, "fetchPublicKeyAssetWithCompletion:", v6);
  */

DCAssetFetcher *fetcher = [DCAssetFetcher sharedFetcher];
  [fetcher fetchPublicKeyAssetWithCompletion:^(NSData *publicKey) {
      completion(publicKey);
  }];
}
```

This method calls `-[DCAssetFetcher fetchPublicKeyAssetWithCompletion:]`.
```objc
void __cdecl -[DCAssetFetcher fetchPublicKeyAssetWithCompletion:](DCAssetFetcher *self, SEL a2, id publicKeyCompletion)
{
  /*
  DCAssetFetcherContext *v5 = objc_alloc_init(&OBJC_CLASS___DCAssetFetcherContext);
  id v4 = objc_retain(publicKeyCompletion);

-[DCAssetFetcherContext setAllowCatalogRefresh:](v5, "setAllowCatalogRefresh:", 0LL);
  -[DCAssetFetcher _fetchAssetWithContext:completionHandler:](self, "_fetchAssetWithContext:completionHandler:", v5, v4);
  objc_release(v4);
  objc_release(v5);
  */

DCAssetFetcherContext *context = [[DCAssetFetcherContext alloc] init];
  void (^completionBlock)(NSData *) = [publicKeyCompletion retain];

[context setAllowCatalogRefresh:NO]; // self->_allowCatalogRefresh = NO;
  [self _fetchAssetWithContext:context
        completionHandler:completionBlock];
}
```

We now continue following the call chain:
```objc
void __cdecl -[DCAssetFetcher _fetchAssetWithContext:completionHandler:](
        DCAssetFetcher *self,
        SEL a2,
        (DCAssetFetcherContext *)context, // another context, without team or bundle ID
        id (void (^)(NSData *assetData, NSError *error))completion)
{
    DCAssetFetcherContext *retainedContext = [context retain];
    void (^completionBlock)(NSData *, NSError *) = [completion retain];

if (os_log_type_enabled(self.logger, OS_LOG_TYPE_DEFAULT)) {
        os_log(self.logger, "Querying...");
    }

[self _queryMetadataWithContext:retainedContext
                         completion:completionBlock];

[completionBlock release];
    [retainedContext release];
}
```
The most interesting part here is `_queryMetadataWithContext:`:

```objc
void __cdecl __noreturn -[DCAssetFetcher _queryMetadataWithContext:completion:](
        DCAssetFetcher *self,
        SEL a2,
        (DCAssetFetcherContext *)context, // another context, without team or bundle ID
        completion:(void (^)(NSData *assetData, NSError *error))completion)
{
    DCAssetFetcherContext *retainedContext = [context retain];
    void (^completionBlock)(NSData *, NSError *) = [completion retain];

if (os_log_type_enabled(self.logger, OS_LOG_TYPE_DEFAULT))
    {
        os_log(self.logger,
               "Starting to fetch asset with context: %@",
               retainedContext);
    }

id assetQuery = [[self _assetQuery] retain]; // claimAutoreleasedReturnValue
    NSUInteger resultCode = [assetQuery queryMetaDataSync]; // this is all from libobjc.A

// Branch: skip cache or absent (ignoreCachedMetadata || resultCode == 2)
    if ([retainedContext ignoreCachedMetadata] || resultCode == 2)
    {
        [self _handleMissingMetadataWithContext:retainedContext
                                   completion:completionBlock];
    } else {
      if (resultCode != 0)
      {
          // Error: generate NSError and return immediately
          NSError *error = [NSError errorWithDomain:@"com.apple.twobit.fetcherror"
                                               code:0xFFFF_FFFF_FFFF_F448
                                           userInfo:nil];
          completionBlock(nil, error);
          [assetQuery release];
          return;
      }

// Success: pass data upward
      [self _handleSuccessForQuery:assetQuery
                         completion:completionBlock];
    }

[assetQuery release];
    [completionBlock release];
    [retainedContext release];
}
```
.........
```objc
- (void)_handleMissingMetadataWithContext:(DCAssetFetchContext *)context
                               completion:(void (^)(DCAsset *asset, NSError *error))completion {
    NSLog(@"[DCAssetFetcher] Query sync result indicated missing asset catalog");

if (context.allowCatalogRefresh) {
        context.allowCatalogRefresh = NO;
        context.ignoreCachedMetadata = NO;

// trigger asset catalog refresh and retry the request after completion
        [self->_assetQuery refreshCatalogWithCompletion:^{
            [self _queryMetadataWithContext:context completion:completion];
        }];
        return;
    }

NSError *error = [NSError errorWithDomain:@"com.apple.twobit.fetcherror"
                                         code:-3101
                                     userInfo:nil];
    completion(nil, error);
}
```

....

```objc
- (void)_handleSuccessForQuery:(DCAssetQuery *)query
                    completion:(void (^)(DCAsset *asset, NSError *error))completion {
    NSArray *results = [query results];

if (results.count == 0) {
        NSError *error = [NSError errorWithDomain:@"com.apple.twobit.fetcherror"
                                             code:-3100
                                         userInfo:nil];
        completion(nil, error);
        return;
    }

if (results.count > 1) {
        NSLog(@"[DCAssetFetcher] Warning: more than one asset found, using first one");
    }

NSDictionary *mobileAsset = results.firstObject;

NSError *validationError = nil;
    DCAsset *asset = [self _validateAsset:mobileAsset error:&validationError];

if (!asset) {
        completion(nil, validationError);
        return;
    }

[[DCXPCActivityController sharedInstance] updateActivityScheduleWithAsset:asset];
    completion(asset, nil);
}
```

......

```objc
- (DCAsset *)_validateAsset:(NSDictionary *)mobileAsset error:(NSError **)error {
    DCAsset *asset = [DCAsset assetWithMobileAsset:mobileAsset];
    if (!asset) {
        if (error) {
            *error = [NSError errorWithDomain:@"com.apple.twobit.fetcherror"
                                         code:-3200
                                     userInfo:nil];
        }
        return nil;
    }
    return asset;
}
```

......
```objc
+ (DCAsset *)assetWithMobileAsset:(NSDictionary *)mobileAsset {
    NSNumber *version = mobileAsset[@"com.apple.MobileAsset.AssetVersion"];
    if (![version isKindOfClass:[NSNumber class]] || version.integerValue != 1) {
        NSLog(@"[DCAsset] Unknown asset version: %@", version);
        return nil;
    }

NSData *pubKeyData = mobileAsset[@"com.apple.devicecheck.pubvalue"]; // assetProperty:
    if (![pubKeyData isKindOfClass:[NSData class]] || pubKeyData.length == 0) {
        NSLog(@"[DCAsset] No public key found in asset");
        return nil;
    }

DCAsset *asset = [[DCAsset alloc] init];
    asset.version = 1;
    asset.publicKey = pubKeyData;

NSNumber *refreshInterval = mobileAsset[@"com.apple.devicecheck.refreshtimer"]; // assetProperty:
    if ([refreshInterval isKindOfClass:[NSNumber class]]) {
        asset.publicKeyRefreshInterval = refreshInterval.doubleValue;
    }

return asset;
}
```

....

```objc
- (void)updateActivityScheduleWithAsset:(DCAsset *)asset {
    if (asset.publicKeyRefreshInterval <= 0) return;

NSDictionary *criteria = @{
        XPC_ACTIVITY_INTERVAL : @(asset.publicKeyRefreshInterval),
        XPC_ACTIVITY_REPEATING : @YES,
        XPC_ACTIVITY_REQUIRE_NETWORK : @YES
    };

xpc_activity_register("com.apple.devicecheck.notify", criteria, ^(xpc_activity_t activity) {
        [[DCAssetFetcher sharedInstance] performMetadataRefreshForActivity];
    });
}
```

Interestingly, none of this metadata fetching is required for the key material itself: the effective public key is embedded in `__37__DCCryptoProxyImpl__fetchPublicKey___block_invoke`:
```+[NSData dataWithBytes:length:](&OBJC_CLASS___NSData, "dataWithBytes:length:", &fallback_server_pubkey, 65LL);```
It is hard-coded and the same in all versions of macOS, iOS, and iPadOS:
`0450d934fa67bcf6f2dfbf96629e0a7238e9205d75f28cfcd84f35a6592bbe058a9c0f8edbca2acb67efb774971ca45f7d856a694fb1b9c40b94fb2e7a5a9498b0`
This is 130 bytes of key – the key itself is 65 characters:
```
╭─    ~ ······························································································································· ✔  at 12:04:28 
╰─ unhex 0450d934fa67bcf6f2dfbf96629e0a7238e9205d75f28cfcd84f35a6592bbe058a9c0f8edbca2acb67efb774971ca45f7d856a694fb1b9c40b94fb2e7a5a9498b0
P�4�g���߿�b�
r8� ]u���O5�Y+������*�g�t��_}�jiO���
                                    ��.zZ���%
```
This value is the public key used by DeviceCheck.
# Chapter 3
`DeviceCheckInternal.framework`
Important note: almost all methods are rewritten by me to make them easier to read and understand. Returning to `fetchOpaqueBlobWithContext:`
```objc
void __cdecl -[DCCryptoProxyImpl fetchOpaqueBlobWithContext:completion:](
        DCCryptoProxyImpl *self,
        SEL a2,
        id DCContext_argDCContext_arg,
        id completion_arg)
{
    // hold onto context (<TeamID>.<BundleIdentifier> or <BundleIdentifier>) and completion
    id retainedContext = [DCContext_argDCContext_arg retain];
    void (^copiedCompletion)(NSData *, NSError *) = [completion_arg copy];
  
    if (os_log_type_enabled(self.logger, OS_LOG_TYPE_DEFAULT))
    {
      os_log(self.logger, "Generating certificate...");
    }
  
    __block id blockContext = retainedContext;
    __block void (^blockCompletion)(NSData *, NSError *) = copiedCompletion;

[self _fetchPublicKey:^(NSData *publicKey) // here we receive our key
      {
          DCCertificateGenerator *generator = [[DCCertificateGenerator alloc]
              initWithContext:blockContext // our context with our <TeamID>.<BundleIdentifier> or <BundleIdentifier>
                     publicKey:publicKey]; // our key
  
          [generator generateEncryptedCertificateChainWithCompletion:
                                    ^(NSData *encryptedChain, NSError *error)
            {
                blockCompletion(encryptedChain, error);
                [blockCompletion release];
                [blockContext release];
                [generator release];
            }
          ];
      }
    ];
}
```
Now let’s look at the initialization of generator:
```objc
00000000 struct DCCertificateGenerator // sizeof=0x18
00000000 {
00000000     unsigned __int8 superclass_opaque[8];
00000008     NSData *_publicKey;
00000010     DCContext *_context;
00000018 };

id __cdecl -[DCCertificateGenerator initWithContext:publicKey:](DCCertificateGenerator *self, SEL a2, id context_arg, id publicKey_arg)
{
  DCCertificateGenerator *v9 = -[DCCertificateGenerator init](self, "init"); // -[NSObject init]
  if ( v9 )
  {
    j__objc_storeStrong((id *)&v9->_publicKey, publicKey_arg); // our obtained public key
    j__objc_storeStrong((id *)&v9->_context, context_arg);     // our context with our <TeamID>.<BundleIdentifier> or <BundleIdentifier>
  }
  return (id *)v9;
}
```

After successful initialization, generateEncryptedCertificateChainWithCompletion is called – creation of the token:
```objc
void __cdecl -[DCCertificateGenerator generateEncryptedCertificateChainWithCompletion:](
        DCCertificateGenerator *self,
        SEL a2,
        id a3)
{
  id v3; // x20
  _QWORD v4[4]; // [xsp+0h] [xbp-40h] BYREF
  DCCertificateGenerator *v5; // [xsp+20h] [xbp-20h]
  id v6; // [xsp+28h] [xbp-18h]

v4[0] = _NSConcreteStackBlock_ptr;
  v4[1] = 3221225472LL;
  v4[2] = __74__DCCertificateGenerator_generateEncryptedCertificateChainWithCompletion___block_invoke;
  v4[3] = &unk_20A9B2860;
  v5 = self;
  v6 = objc_retain(a3);
  v3 = objc_retain(v6);
  -[DCCertificateGenerator _generateCertificateChainWithCompletion:](v5, "_generateCertificateChainWithCompletion:", v4);
  objc_release(v6);
  objc_release(v3);
}
```
Conceptually, `_generateCertificateChainWithCompletion` retrieves a small certificate chain from the keychain and passes it into `__74__DCCertificateGenerator_generateEncryptedCertificateChainWithCompletion___block_invoke`, which then hands it to `_encryptData`, where the final token is constructed. For our purposes, the important point is that two root certificates are fetched from the keychain in `__DeviceIdentityIssueClientCertificateWithCompletion_block_invoke`; representative examples are shown below:
```
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
```
or:
```
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
MIICIzCCAaigAwIBAgIIeNjhG9tnDGgwCgYIKoZIzj0EAwIwUzEnMCUGA1UEAwwe
QmFzaWMgQXR0ZXN0YXRpb24gVXNlciBSb290IENBMRMwEQYDVQQKDApBcHBsZSBJ
bmMuMRMwEQYDVQQIDApDYWxpZm9ybmlhMB4XDTE3MDQyMDA0MjAwMFoXDTMyMDMy
MDAwMDAwMFowUzEnMCUGA1UEAwweQmFzaWMgQXR0ZXN0YXRpb24gVXNlciBSb290
IENBMTETMBEGA1UECgwKQXBwbGUgSW5jLjETMBEGA1UECAwKQ2FsaWZvcm5pYTBZ
MBMGByqGSM49AgEGCCqGSM49AwEHA0gAMEUCIHLplLqirOgMmrPkMSQaDOpl/MAy
EYejw/otUrfGGITiAiEAuLs1MYHGWuUdhyofXfY0S45GsSYXA/g8ombHMkcU54A=
-----END CERTIFICATE-----
```
Rather than focusing on the X.509 details, the core of the token logic lives in `_encryptData`, which builds and encrypts the final payload. The following reconstructed implementation illustrates that function:
```objc
- (NSData *)_encryptData:(NSData *)data // certificates
        serverSyncedDate:(NSDate *)serverDate 
                    error:(NSError **)error {
    // Log start of encryption
    os_log_t log = os_log_create("com.apple.devicecheck", "DeviceCheck");
    if (os_log_type_enabled(log, OS_LOG_TYPE_DEFAULT)) {
        os_log(log, "Encrypting data...");
    }

// Get client App ID as UTF-8 data
    NSString *clientAppID = [self.context clientAppID];
    NSData *clientAppData = [clientAppID dataUsingEncoding:NSUTF8StringEncoding];
    NSUInteger clientAppLen = clientAppData.length;
    const void *clientAppBytes = clientAppData.bytes;

// Prepare input data
    NSUInteger inputLen = data.length;
    const void *inputBytes = data.bytes;

// Current timestamp from server-synced date
    NSTimeInterval timestamp = [serverDate timeIntervalSince1970];

// AES-GCM mode descriptor from CommonCrypto
    const struct ccmode_gcm *gcm = ccaes_gcm_encrypt_mode();

// Allocate buffers: output and plaintext payload
    size_t payloadLen = inputLen + clientAppLen + 81;
    size_t outputLen = inputLen + clientAppLen + 235;
    uint8_t *outBuf = calloc(1, outputLen);
    if (!outBuf) {
        if (error) *error = [NSError errorWithDomain:@"DeviceCheckError"
                                                code:-1 
                                            userInfo:nil];
        return nil;
    }
    uint8_t *payloadBuf = calloc(1, payloadLen);
    if (!payloadBuf) {
        free(outBuf);
        if (error) *error = [NSError errorWithDomain:@"DeviceCheckError"
                                                code:-1 
                                            userInfo:nil];
        return nil;
    }

// Write header/type (value 2) and payload length in outBuf
    *((uint32_t *)outBuf) = 2;
    *((uint32_t *)(outBuf + 150)) = (uint32_t)payloadLen;

// Copy device's static public key into outBuf at offset 5
    NSData *devicePubKeyData = self.publicKey;  // NSData containing the public key bytes
    memcpy(outBuf + 5, devicePubKeyData.bytes, devicePubKeyData.length);

// Assemble payload: [timestamp (8 bytes), inputLen (4 bytes), clientAppLen (4 bytes), inputBytes, clientAppBytes]
    *((uint32_t *)(payloadBuf + 73)) = (uint32_t)inputLen;
    memcpy(payloadBuf + 81, inputBytes, inputLen);
    *((uint32_t *)(payloadBuf + 77)) = (uint32_t)clientAppLen;
    memcpy(payloadBuf + 81 + inputLen, clientAppBytes, clientAppLen);
    *((uint64_t *)(payloadBuf + 65)) = (uint64_t)timestamp;

// Log system call (no-op with 0) as seen in disassembly
    DCLogSystem(0);

// Create ephemeral ECDH key using the keybag
    id keybag = [self keybagHandle];
    uint64_t refKey = 0;
    int aksErr = aks_ref_key_create((__int64)keybag, 11, 4, 0, 0, &refKey);
    if (aksErr != 0) {
        DCLogSystem(aksErr);
        free(outBuf);
        free(payloadBuf);
        if (error) *error = [NSError errorWithDomain:@"DeviceCheckError"
                                                code:aksErr 
                                            userInfo:nil];
        return nil;
    }

// Get ephemeral public key (should be 65 bytes) and copy it
    size_t ecdhPubLen = 0;
    const uint8_t *ecdhPub = (const uint8_t *)aks_ref_key_get_public_key(refKey, &ecdhPubLen);
    if (ecdhPubLen != 65) {
        DCLogSystem(ecdhPubLen);
        free(outBuf);
        free(payloadBuf);
        if (error) *error = [NSError errorWithDomain:@"DeviceCheckError"
                                                code:-2 
                                            userInfo:nil];
        return nil;
    }
    memcpy(outBuf + 85, ecdhPub, ecdhPubLen);

// Compute ECDH shared secret with device's static public key
    const uint8_t *devicePubBytes = devicePubKeyData.bytes;
    size_t devicePubLen = devicePubKeyData.length;
    int ecdhErr = aks_ref_key_compute_key(refKey, 0, 0, (__int64)devicePubBytes, devicePubLen);
    if (ecdhErr != 0) {
        DCLogSystem(ecdhErr);
        free(outBuf);
        free(payloadBuf);
        if (error) *error = [NSError errorWithDomain:@"DeviceCheckError"
                                                code:ecdhErr 
                                            userInfo:nil];
        return nil;
    }

// The shared secret is at refKey; skip first 2 bytes (per disassembly analysis)
    uint8_t *sharedSecret = (uint8_t *)refKey + 2;
    size_t sharedLen = devicePubLen - 2;

// Derive key material with HKDF-SHA256 (44 bytes: 32-byte key + 12-byte IV)
    uint8_t hkdfOut[44];
    cchkdf(ccsha256_di(), sharedLen, sharedSecret, 0, NULL, 0x2C /*44 bytes*/, hkdfOut);
    uint8_t *aesKey = hkdfOut;          // first 32 bytes
    uint8_t *aesIV = hkdfOut + 32;      // next 12 bytes

// Encrypt payloadBuf with AES-GCM; tag-> outBuf+1, ciphertext-> outBuf+154
    int gcmErr = ccgcm_one_shot(gcm,
                                32, aesKey,
                                12, aesIV,
                                0, NULL,
                                payloadLen, payloadBuf,
                                outBuf + 154,
                                16, outBuf + 1);
    if (gcmErr != 0) {
        DCLogSystem(gcmErr);
        free(outBuf);
        free(payloadBuf);
        if (error) *error = [NSError errorWithDomain:@"DeviceCheckError"
                                                code:gcmErr 
                                            userInfo:nil];
        return nil;
    }

// Build NSData for the encrypted payload
    NSData *encryptedData = [NSData dataWithBytes:outBuf length:outputLen];

// Log base64 payload if logging is enabled
    if (os_log_type_enabled(log, OS_LOG_TYPE_DEFAULT)) {
        NSData *b64 = [encryptedData base64EncodedDataWithOptions:0];
        NSString *payloadStr = [[NSString alloc] initWithData:b64
                                                     encoding:NSUTF8StringEncoding];
        os_log(log, "\nPayload (base64):\n%{public}s\n\n", payloadStr.UTF8String);
    }

free(outBuf);
    free(payloadBuf);
    return encryptedData;
}
```
# Chapter 4. Conclusion and security implications

This write-up traces the full DeviceCheck token generation path: from calling `DCDevice` in an app, through XPC into the `devicecheckd` daemon, into `DeviceCheckInternal.framework`, and finally to `_encryptData`, where the payload is constructed and encrypted. Key stages are covered – initializing the context with `TeamID.BundleID`, resolving the public key (including the hard-coded fallback), assembling the certificate chain, and forming the opaque blob returned to the caller.

From a security and red-team perspective, this analysis provides the primitives needed to reason about DeviceCheck’s trust model, to emulate or instrument the client in controlled environments, and to assess how robustly backends validate and interpret DeviceCheck tokens.

Work by [andrd3v](https://github.com/andrd3v). Thanks for the help, [whoeevee](https://github.com/whoeevee).

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