How can my server securely authenticate iPhone in-app purchase? - iphone

Question

Look at Apple's diagram for the server purchase model.
In step #9, how can the server know that it is really talking with an iPhone that is entitled to the purchase, and that Eve is not performing a replay with a dishonestly obtained receipt?
The receipt may be valid, but that doesn't prove that the sender is the entitled party.
Is there any notion of a device certificate on the iPhone that can be used to sign the receipt?
Is there any way to bind the receipt to the device, or bind the receipt to both the iTunes account and to the device, so the server can validate?

Answer 1

Apple-Provided Vulnerable Approach
The server can authenticate a purchase by doing the following:
The iPhone application receives a transactionReceipt after the purchase. Have the iPhone base64 encode it (You can use this open-source addition to NSData) and send it to your server. (You could even send it as-is and have the server base64 encode it before validation.)
Have your server send a JSON request with the single key receipt-data with the base64 encoded transactionReceipt to https://buy.itunes.apple.com/verifyReceipt using an HTTP POST. (For directions on how to do this in various server-side languages see this site)
The server will respond with a JSON object with two keys: status which is an integer and receipt which is the receipt repeated.
If status is zero, the receipt is valid should be accepted, a non-zero value means the receipt isn't valid.
Secure Additions to Apple's Approach
However, there are a few security implications. A user could use another user's receipt since devices aren't tied to receipts, or a user could use another product's receipt since the server doesn't verify the product id of the receipt. To ensure this doesn't happen you should also do the following:
When you first get the receipt in the application, immediately send it to your server along with the device's UUID over a secure channel such as HTTPS or an SSL socket. Do not store it anywhere, leave it in memory.
On your server, store the UUID and receipt pair in a database.
When a device sends a UUID and receipt pair, verify with your database that the receipt has not already been used, and make sure the receipt is actually for your product by checking the receipt's product id. The receipt is just a JSON object, so your server can read the contents by decoding the receipt from base64.
Return a response to the device over the secure channel telling it whether the purchase is:
Authenticated as new (wasn't in DB and was valid)
Authenticated in the past (Same UUID and receipt pair was already in DB)
Denied due to wrong product id
Denied due to having already used the receipt with another UUID.
Since the receipt is only ever in memory on the device, and your application uses the device's UUID (can be spoofed by jailbroken devices, see comments), and all purchases of your product are logged with the device's UUID on your server in a secure manner; a user could not use another user's receipt to verify the purchase, nor could they use a receipt from another product, since you check for that.
You can also validate other fields from the receipt if you want to verify other details of the transaction. For example, if your product is a subscription, you'll like want to look at the transaction date as well.
Also, users cannot pretend to be your server by having the device on a private network with a host of the same name as yours, since they won't have your SSL certificate.
Failure Considerations
Since failure might occur between when the user's device gets the receipt and verifying it with your server (for example if the user looses connectivity, or your server is down for maintenance), you should also let the user "re-authorize". Re-authorizing should get the receipt from the store (using a Restored Transaction) and re-send it to the server just as though this was a new purchase. This should rarely need to be used, but should be available to save the user having to re-buy the product in the case of network failure.
Multiple Devices Consideration
This means that if a user wants to use an application on more than one device, they will have to purchase the product multiple times. This might be the desired effect, but you should likely inform your users before they purchase since they might expect to be able to use the content across devices they have associated with their account.
If the receipt also contains the iTunes account information, authentication could use that to allow users to share content between all their devices (but not their friends').

Answer 2

I do not think that can bind the receipt to the device.
My understanding is that you are allowed to install an application on multiple devices without extra cost. Binding it to the device would mean that if you for example upgrade/change your phone you would need to purchase all the apps again.

Answer 3

I believe if you can't read the user's apple ID, your only protection against piracy would be keeping track(server-side of course) of the number of download requests per transaction_id and limit those if they go over a certain value.
So if you limit it to say 50, that gives a reasonable margin for the user to deploy the app and it's contents on multiple devices and restore several times, but makes it hard for whoever wants to distribute a pirated version with a valid receipt for unlimited restores. Of course they can just distribute a version with all your content, but that's nothing you can do about that and at least they're not taxing your servers.

Answer 4

UDID Does Not Work Anymore
Beniot answer is great, however, these days, as mentioned by Joe D'Andrea, UDID is deprecated and the last time I tried, an App that used the call to get the UDID failed to pass validation during upload to iTunes.
Time-limited Receipts as Alternative to Receipt Counters
To add on to hloupyhonza's answer, besides having a "download request" counter for a particular receipt, you can just limit the receipt validity by time. I found anything between 12 to 24 hours reasonable.
This method also allows the purchaser to use the purchase on any other device he owns as long as he logs into the App Store with the same Apple ID. Note: Each time Restore Purchases is done, Apple returns a completely new receipt (with details of the original receipt contained) - this allows purchases to be restored past the time limit we set for a particular receipt.
Preventing "Off-The-Shelf" Hacks
To prevent typical "Googled" hacking solutions (my data shows this constitutes almost all of IAP hacking attempts), I use a checksum (pick your favorite algorithm, doesn't matter unless you want to make it watertight) of the following concatenation:
receipt-data json string
A shared secret key
Validation success status code.
The App will verify the checksum returned by our validation server. This is not watertight though, as the hacker may retrieve the shared key from your App's binary. But it has prevented all "off-the-shelf" hacks thus far and that's good enough for my use.