There's precedent regarding the 5th Amendment being invoked in taxation cases. The amount of your income isn't privileged although the source can be.
A careful reading of Sullivan and Garner, therefore, is that the self-incrimination privilege can be employed to protect the taxpayer from revealing the information as to an illegal source of income, but does not protect him from disclosing the amount of his income.
If the IRS made the argument that they needed the keys in order to establish the
amount of your income, you probably couldn't use the 5th Amendment to get around having to comply with that demand even though you could successfully use it to avoid disclosing the nature of any illegal activity by which you acquired that income (I learned today that you can likely enter "5th Amendment" on your tax return when disclosing income from illegal activities).