Receipt Validation and AirPlay 2

Chris Liscio:

I hear rumblings about some App Store receipt validation issues relating to MAC addresses on macOS Catalina. The commonly-used Apple sample code has busted logic that—in the presence of a strange condition on a (small?) number of systems—will fail to return a value.

[…]

I don’t know yet, but it manifests on my machine as a service match for both en0 as well as IOTimeSyncEthernetModernInterfaceAdapter (see attached screenshot.) The latter causes a second go-around in the while loop, causing the MAC address to get overwritten with NULL.

It makes no sense for Apple to provide buggy sample code for getting the MAC address when every app in the store needs to do this. With some other aspects of receipt validation, one could argue that you want each app to do it slightly differently to make them harder to crack. But the point of checking the MAC address is that you want to match it with the address provided by Apple when it generated the receipt. So you want to use the exact same logic they did.

Paul Haddad:

BTW I don’t think this is a Catalina issue, have seen reports of what sounds like this for several OS versions. Also seems to work if tested right after reboot, but breaks if tested later.

Chris Liscio:

The plot thickens!

After a reboot, my system no longer exhibits this “strange condition”.

Immediately after playing audio over AirPlay 2? BOOM—“strange condition”! 🤯

The “strange condition” appears to be the presence of IOTimeSyncEthernetModernInterfaceAdapter in ioreg.

Previously:

• Receipt Validation in Swift
• Apple Blacklists Its Own Ethernet Driver
• Modern Receipt Validation

Update (2019-10-22): Chris Liscio (tweet, James Thomson):

While playing audio using AirPlay 2, there are two devices that have a BSD Name equal to en0. Both the actual en0 interface, and a new device called IOTimeSyncEthernetModernInterfaceAdapter (or IOTimeSyncWiFiInterfaceAdapter, depending on your Mac) that is only present while AirPlay is active.

[…]

When copy_mac_address returns NULL, your receipt validation code fails, and probably results in your application calling exit(173) to ask the App Store for a new receipt. Unfortunately, a new receipt isn’t going to help, and you get stuck in a very strange loop.

For me, this resulted in the app repeatedly appearing & disappearing in the Dock for quite some time. Eventually, the system finally gave up and told me the app was “Damaged” and needed to be re-downloaded.

[…]

The keys to this alternate approach are as follows:

1. It specifies the kIOEthernetInterfaceClass, which only matches network interfaces. (Before you ask—yes, this includes Wi-Fi adapters.)
2. It also specifies that kIOPrimaryInterface is set to true, which matches the built-in, primary network interface.

AirPlay Mac Mac App Store macOS 10.15 Catalina Programming

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