The IRS is very open about this. A fellow HN commenter pointed me to this document [1], on which you should search for "Bribes" and "Stolen Property". Even if you don't want to trust to the IRS's confidentiality rules (for example, it won't share your tax return with law enforcement without a court order), it's still common practice to come up with another description for your income in order to pay tax on it.
[1] https://www.irs.gov/publications/p17/ch12.html#en_US_2016_pu...
Once upon a time it was the best excuse for those who had undisclosed income sources (other than prostitution). Recently tax offices started demanding proofs, like names of the clients.
Huh? That makes no sense. Why would naming a client proove anything? And why would sex workers know names of their clients?
It's not because the receipt proves anything. It's because if there was ever any question, it gives a validator something to actually validate, to call up the putative place of business and confirm the transaction and the size.
It's one of those deals where the mere threat of being able to validate tends to keep the system humming along fairly well, even to the point that many of the participants have forgotten the justification for the system. Presumably there's someone out there routinely forging receipts for fun and profit but it must not be that big a fraud considered across the entire system.
Same thing here.