Are you sure they are riveted? Because I just took apart the front chainwheel assembly on a peugeot mixte, and it was bolted together. Not with normal chainring bolts, but with weird short solid bolts, with a 10mm head and with the head on the inside of the chainring.

One warning: If this applies to you, those bolts are SOFT, so make sure they are soaked in penetrine or WD40 before you try to take them out, because they will round off in a second, as I found out