About two weeks ago, Apple made a change to their provisioning service to require a different authentication scheme for "free" Apple accounts (they return an error that mentions upgrading to "Xcode 7.3"); this broke Cydia Impactor for users without a paid Apple Developer account.
When Apple did this, I'd just arrived in Florida for a wedding (one of a dear friend and where I was a groomsman); even so, I stayed up ludicrously late the night before the ceremony rehearsal to understand the issue and try to triage how much it would hurt to fix Cydia Impactor.
(I then told myself I shouldn't be working on this at the wedding unless I had a fast fix (which I didn't), was being swamped with an audit deadline at work I needed to hit, frankly hate having to wade into the "eta wen" posts, and honestly wanted to see what other people found.)
Cydia Impactor (unlike, say, ReProvision) is intended to run on stock desktop platforms (macOS, Windows, and Linux); to support this new authentication scheme will require spending some time—I'd guess well over a week?—reverse engineering Apple's code to achieve interoperability.
(Alternatively, I could drop support for Linux—which honestly feels like "defeat" :(—to only support macOS and Windows, in which case I spend my time ripping apart iTunes to figure out where it is storing its keys on Windows; but this would be easy for Apple to repeatedly break.)
(On jailbroken iOS—or apparently on "a jailbroken Mac", where you disable SIP and patch amfid, similar to how we jailbreak iOS... I wonder how much longer it will be before we need exploits for that :/—one can directly use Apple's AuthKit to get the right authentication headers.)
(FWIW, I am actually impressed with the "creative workaround" (a plugin for Apple Mail) that @rileytestut came up with for @altstoreio's AltServer (which is largely based on the open source code for Cydia Impactor's core, ldid!) and am interested to see what he does for Windows.)
I actually am very interested in doing this work; but, as I have stated before, I also now have a day job where I'm in charge of technology for a company that absolutely must release its product within the next few weeks. I just can't take weeks off right now to work on Cydia :(.
If anyone else wants to do this work, I am *not* a gatekeeper: if there were a "walk-through" of GrandSlam, people like me can easily support it; the only existing references I've found are presentations by Vladimir Katalov (the CEO of @ElcomSoft...) and work by InflatableDonkey.
Regardless, I'm simply not in a position to dedicate time to this until mid-December; and I also do not have much good advice for users, other than to annoyingly note that if you buy an Apple developer account, Cydia Impactor works and you can install apps on hundreds of devices.
(Note: right now, Apple chose to only target people without paid Developer accounts; but they know as well as we that this is merely a speed bump; so, I wonder if they might intend to fully deprecate myacinfo due to security issues and are using free developers to derisk impact.)
(If myacinfo were to be deactivated entirely, that would also affect the deployment tools used by larger companies such as Facebook and Google, something Apple might enjoy rather than shy away from; if I were @FastlaneTools, I would be paying close attention to what is going on.)
