[caf]

You don't need to deliver bytecode, just a new seed for the algorithm. Even 64 bits is more than sufficient to ensure that they can't enumerate all possible seeds.

[zzzcpan]

Seeds don't impose human costs of reverse engineering though. Which could be important in some cases, since we are up against state actors.

But yeah, having seeds sharded per id/phone_number same way I proposed above could make it pretty much unblockable.