Apple offers two documents about receipt validation, with apparently contradictory statements.
In "Verifying Store Receipts":
Note: On iOS, the contents and format of the store receipt is private and subject to change. Your application should not attempt to parse the receipt data directly.
Yet, in "In-App Purchase Receipt Validation on iOS" sample code is provided in which the store receipt is parsed and verified, as part of a "mitigation strategy" for a security vulnerability:
// Check the validity of the receipt. If it checks out then also ensure the transaction is something
// we haven't seen before and then decode and save the purchaseInfo from the receipt for later receipt validation.
- (BOOL)isTransactionAndItsReceiptValid:(SKPaymentTransaction *)transaction
{
if (!(transaction && transaction.transactionReceipt && [transaction.transactionReceipt length] > 0))
{
// Transaction is not valid.
return NO;
}
// Pull the purchase-info out of the transaction receipt, decode it, and save it for later so
// it can be cross checked with the verifyReceipt.
NSDictionary *receiptDict = [self dictionaryFromPlistData:transaction.transactionReceipt];
NSString *transactionPurchaseInfo = [receiptDict objectForKey:@"purchase-info"];
NSString *decodedPurchaseInfo = [self decodeBase64:transactionPurchaseInfo length:nil];
NSDictionary *purchaseInfoDict = [self dictionaryFromPlistData:[decodedPurchaseInfo dataUsingEncoding:NSUTF8StringEncoding]];
NSString *transactionId = [purchaseInfoDict objectForKey:@"transaction-id"];
NSString *purchaseDateString = [purchaseInfoDict objectForKey:@"purchase-date"];
NSString *signature = [receiptDict objectForKey:@"signature"];
// Convert the string into a date
NSDateFormatter *dateFormat = [[NSDateFormatter alloc] init];
[dateFormat setDateFormat:@"yyyy-MM-dd HH:mm:ss z"];
NSDate *purchaseDate = [dateFormat dateFromString:[purchaseDateString stringByReplacingOccurrencesOfString:@"Etc/" withString:@""]];
if (![self isTransactionIdUnique:transactionId])
{
// We've seen this transaction before.
// Had [transactionsReceiptStorageDictionary objectForKey:transactionId]
// Got purchaseInfoDict
return NO;
}
// Check the authenticity of the receipt response/signature etc.
BOOL result = checkReceiptSecurity(transactionPurchaseInfo, signature,
(__bridge CFDateRef)(purchaseDate));
if (!result)
{
return NO;
}
// Ensure the transaction itself is legit
if (![self doTransactionDetailsMatchPurchaseInfo:transaction withPurchaseInfo:purchaseInfoDict])
{
return NO;
}
// Make a note of the fact that we've seen the transaction id already
[self saveTransactionId:transactionId];
// Save the transaction receipt's purchaseInfo in the transactionsReceiptStorageDictionary.
[transactionsReceiptStorageDictionary setObject:purchaseInfoDict forKey:transactionId];
return YES;
}
If I understand correctly, if I verify the receipt my app could stop working when Apple decides to change the format of the receipt.
And if I don't verify the receipt, I'm not following Apple's "mitigation strategy" and my app is vulnerable to attacks.
Damned if I do, damned if I don't. Is there something I'm missing?