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Wouldn't there be precedent in a Mafia case where their accountant used a code or something?
But they can get that information from a witness (i.e. the accountant if the accountant wasn't a defendant, or if the accountant got a plea bargain).
The key here is whether the defendant is being forced to give up self-incriminating information. It only protects the one person who's being asked to give it up. Any other person can provide the info and it doesn't fun afoul of the 5th.