[Cryptography] Secure software update protocol?

[Allen]

>
> How can software updates be secured so the distributor knows
> which customers have which updates, not permitting cloned
> devices, and not permitting the distributors to choose a
> particular customer to get a special "joejob" update?
>

Exploit: Distributor's updated messaging client loads and runs following
code: If user id == "joe", activate hook that stenographically embeds
encryption key in all outgoing encrypted messages.

Basically, every user would get the "joejob", but it would only be
activated for selected users.
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