Here's a walkthrough of how I solved this in my in-app purchase library RMStore. I will explain how to verify a transaction, which includes verifying the whole receipt.
At a glance
Get the receipt and verify the transaction. If it fails, refresh the receipt and try again. This makes the verification process asynchronous as refreshing the receipt is asynchronous.
From RMStoreAppReceiptVerifier:
RMAppReceipt *receipt = [RMAppReceipt bundleReceipt];
const BOOL verified = [self verifyTransaction:transaction inReceipt:receipt success:successBlock failure:nil]; // failureBlock is nil intentionally. See below.
if (verified) return;
// Apple recommends to refresh the receipt if validation fails on iOS
[[RMStore defaultStore] refreshReceiptOnSuccess:^{
RMAppReceipt *receipt = [RMAppReceipt bundleReceipt];
[self verifyTransaction:transaction inReceipt:receipt success:successBlock failure:failureBlock];
} failure:^(NSError *error) {
[self failWithBlock:failureBlock error:error];
}];
Getting the receipt data
The receipt is in [[NSBundle mainBundle] appStoreReceiptURL] and is actually a PCKS7 container. I suck at cryptography so I used OpenSSL to open this container. Others apparently have done it purely with system frameworks.
Adding OpenSSL to your project is not trivial. The RMStore wiki should help.
If you opt to use OpenSSL to open the PKCS7 container, your code could look like this. From RMAppReceipt:
+ (NSData*)dataFromPKCS7Path:(NSString*)path
{
const char *cpath = [[path stringByStandardizingPath] fileSystemRepresentation];
FILE *fp = fopen(cpath, "rb");
if (!fp) return nil;
PKCS7 *p7 = d2i_PKCS7_fp(fp, NULL);
fclose(fp);
if (!p7) return nil;
NSData *data;
NSURL *certificateURL = [[NSBundle mainBundle] URLForResource:@"AppleIncRootCertificate" withExtension:@"cer"];
NSData *certificateData = [NSData dataWithContentsOfURL:certificateURL];
if ([self verifyPKCS7:p7 withCertificateData:certificateData])
{
struct pkcs7_st *contents = p7->d.sign->contents;
if (PKCS7_type_is_data(contents))
{
ASN1_OCTET_STRING *octets = contents->d.data;
data = [NSData dataWithBytes:octets->data length:octets->length];
}
}
PKCS7_free(p7);
return data;
}
We'll get into the details of the verification later.
Getting the receipt fields
The receipt is expressed in ASN1 format. It contains general information, some fields for verification purposes (we'll come to that later) and specific information of each applicable in-app purchase.
Again, OpenSSL comes to the rescue when it comes to reading ASN1. From RMAppReceipt, using a few helper methods:
NSMutableArray *purchases = [NSMutableArray array];
[RMAppReceipt enumerateASN1Attributes:asn1Data.bytes length:asn1Data.length usingBlock:^(NSData *data, int type) {
const uint8_t *s = data.bytes;
const NSUInteger length = data.length;
switch (type)
{
case RMAppReceiptASN1TypeBundleIdentifier:
_bundleIdentifierData = data;
_bundleIdentifier = RMASN1ReadUTF8String(&s, length);
break;
case RMAppReceiptASN1TypeAppVersion:
_appVersion = RMASN1ReadUTF8String(&s, length);
break;
case RMAppReceiptASN1TypeOpaqueValue:
_opaqueValue = data;
break;
case RMAppReceiptASN1TypeHash:
_hash = data;
break;
case RMAppReceiptASN1TypeInAppPurchaseReceipt:
{
RMAppReceiptIAP *purchase = [[RMAppReceiptIAP alloc] initWithASN1Data:data];
[purchases addObject:purchase];
break;
}
case RMAppReceiptASN1TypeOriginalAppVersion:
_originalAppVersion = RMASN1ReadUTF8String(&s, length);
break;
case RMAppReceiptASN1TypeExpirationDate:
{
NSString *string = RMASN1ReadIA5SString(&s, length);
_expirationDate = [RMAppReceipt formatRFC3339String:string];
break;
}
}
}];
_inAppPurchases = purchases;
Getting the in-app purchases
Each in-app purchase is also in ASN1. Parsing it is very similar than parsing the general receipt information.
From RMAppReceipt, using the same helper methods:
[RMAppReceipt enumerateASN1Attributes:asn1Data.bytes length:asn1Data.length usingBlock:^(NSData *data, int type) {
const uint8_t *p = data.bytes;
const NSUInteger length = data.length;
switch (type)
{
case RMAppReceiptASN1TypeQuantity:
_quantity = RMASN1ReadInteger(&p, length);
break;
case RMAppReceiptASN1TypeProductIdentifier:
_productIdentifier = RMASN1ReadUTF8String(&p, length);
break;
case RMAppReceiptASN1TypeTransactionIdentifier:
_transactionIdentifier = RMASN1ReadUTF8String(&p, length);
break;
case RMAppReceiptASN1TypePurchaseDate:
{
NSString *string = RMASN1ReadIA5SString(&p, length);
_purchaseDate = [RMAppReceipt formatRFC3339String:string];
break;
}
case RMAppReceiptASN1TypeOriginalTransactionIdentifier:
_originalTransactionIdentifier = RMASN1ReadUTF8String(&p, length);
break;
case RMAppReceiptASN1TypeOriginalPurchaseDate:
{
NSString *string = RMASN1ReadIA5SString(&p, length);
_originalPurchaseDate = [RMAppReceipt formatRFC3339String:string];
break;
}
case RMAppReceiptASN1TypeSubscriptionExpirationDate:
{
NSString *string = RMASN1ReadIA5SString(&p, length);
_subscriptionExpirationDate = [RMAppReceipt formatRFC3339String:string];
break;
}
case RMAppReceiptASN1TypeWebOrderLineItemID:
_webOrderLineItemID = RMASN1ReadInteger(&p, length);
break;
case RMAppReceiptASN1TypeCancellationDate:
{
NSString *string = RMASN1ReadIA5SString(&p, length);
_cancellationDate = [RMAppReceipt formatRFC3339String:string];
break;
}
}
}];
It should be noted that certain in-app purchases, such as consumables and non-renewable subscriptions, will appear only once in the receipt. You should verify these right after the purchase (again, RMStore helps you with this).
Verification at a glance
Now we got all the fields from the receipt and all its in-app purchases. First we verify the receipt itself, and then we simply check if the receipt contains the product of the transaction.
Below is the method that we called back at the beginning. From RMStoreAppReceiptVerificator:
- (BOOL)verifyTransaction:(SKPaymentTransaction*)transaction
inReceipt:(RMAppReceipt*)receipt
success:(void (^)())successBlock
failure:(void (^)(NSError *error))failureBlock
{
const BOOL receiptVerified = [self verifyAppReceipt:receipt];
if (!receiptVerified)
{
[self failWithBlock:failureBlock message:NSLocalizedString(@"The app receipt failed verification", @"")];
return NO;
}
SKPayment *payment = transaction.payment;
const BOOL transactionVerified = [receipt containsInAppPurchaseOfProductIdentifier:payment.productIdentifier];
if (!transactionVerified)
{
[self failWithBlock:failureBlock message:NSLocalizedString(@"The app receipt doest not contain the given product", @"")];
return NO;
}
if (successBlock)
{
successBlock();
}
return YES;
}
Verifying the receipt
Verifying the receipt itself boils down to:
- Checking that the receipt is valid PKCS7 and ASN1. We have done this implicitly already.
- Verifying that the receipt is signed by Apple. This was done before parsing the receipt and will be detailed below.
- Checking that the bundle identifier included in the receipt corresponds to your bundle identifier. You should hardcode your bundle identifier, as it doesn't seem to be very difficult to modify your app bundle and use some other receipt.
- Checking that the app version included in the receipt corresponds to your app version identifier. You should hardcode the app version, for the same reasons indicated above.
- Check the receipt hash to make sure the receipt correspond to the current device.
The 5 steps in code at a high-level, from RMStoreAppReceiptVerificator:
- (BOOL)verifyAppReceipt:(RMAppReceipt*)receipt
{
// Steps 1 & 2 were done while parsing the receipt
if (!receipt) return NO;
// Step 3
if (![receipt.bundleIdentifier isEqualToString:self.bundleIdentifier]) return NO;
// Step 4
if (![receipt.appVersion isEqualToString:self.bundleVersion]) return NO;
// Step 5
if (![receipt verifyReceiptHash]) return NO;
return YES;
}
Let's drill-down into steps 2 and 5.
Verifying the receipt signature
Back when we extracted the data we glanced over the receipt signature verification. The receipt is signed with the Apple Inc. Root Certificate, which can be downloaded from Apple Root Certificate Authority. The following code takes the PKCS7 container and the root certificate as data and checks if they match:
+ (BOOL)verifyPKCS7:(PKCS7*)container withCertificateData:(NSData*)certificateData
{ // Based on: https://developer.apple.com/library/content/releasenotes/General/ValidateAppStoreReceipt/Chapters/ValidateLocally.html#//apple_ref/doc/uid/TP40010573-CH1-SW17
static int verified = 1;
int result = 0;
OpenSSL_add_all_digests(); // Required for PKCS7_verify to work
X509_STORE *store = X509_STORE_new();
if (store)
{
const uint8_t *certificateBytes = (uint8_t *)(certificateData.bytes);
X509 *certificate = d2i_X509(NULL, &certificateBytes, (long)certificateData.length);
if (certificate)
{
X509_STORE_add_cert(store, certificate);
BIO *payload = BIO_new(BIO_s_mem());
result = PKCS7_verify(container, NULL, store, NULL, payload, 0);
BIO_free(payload);
X509_free(certificate);
}
}
X509_STORE_free(store);
EVP_cleanup(); // Balances OpenSSL_add_all_digests (), per http://www.openssl.org/docs/crypto/OpenSSL_add_all_algorithms.html
return result == verified;
}
This was done back at the beginning, before the receipt was parsed.
Verifying the receipt hash
The hash included in the receipt is a SHA1 of the device id, some opaque value included in the receipt and the bundle id.
This is how you would verify the receipt hash on iOS. From RMAppReceipt:
- (BOOL)verifyReceiptHash
{
// TODO: Getting the uuid in Mac is different. See: https://developer.apple.com/library/content/releasenotes/General/ValidateAppStoreReceipt/Chapters/ValidateLocally.html#//apple_ref/doc/uid/TP40010573-CH1-SW5
NSUUID *uuid = [[UIDevice currentDevice] identifierForVendor];
unsigned char uuidBytes[16];
[uuid getUUIDBytes:uuidBytes];
// Order taken from: https://developer.apple.com/library/content/releasenotes/General/ValidateAppStoreReceipt/Chapters/ValidateLocally.html#//apple_ref/doc/uid/TP40010573-CH1-SW5
NSMutableData *data = [NSMutableData data];
[data appendBytes:uuidBytes length:sizeof(uuidBytes)];
[data appendData:self.opaqueValue];
[data appendData:self.bundleIdentifierData];
NSMutableData *expectedHash = [NSMutableData dataWithLength:SHA_DIGEST_LENGTH];
SHA1(data.bytes, data.length, expectedHash.mutableBytes);
return [expectedHash isEqualToData:self.hash];
}
And that's the gist of it. I might be missing something here or there, so I might come back to this post later. In any case, I recommend browsing the complete code for more details.
