An illegal prime is a prime number which represents information forbidden by law to possess or distribute. It is a matter of debate whether the number itself is therefore illegal, and under what circumstances.

The first illegal prime to be announced, when interpreted a particular way, describes a computer program which bypasses copyright protection schemes on some DVDs. Because that program has been found illegal by courts in the United States of America, this has produced debate about whether the number itself could be considered illegal.

This question has never been tested in court, and it is possible that the number itself and its possession would be found to be legal, but not a particular interpretation of it."
[wikipedia]

Die Neuaxiomatisierung der natürlichen Zahlen in einer Weise, die alle illegalen Zahlen ausläßt, könnte wohl zur größten Herausforderung werden, vor welche die Mathematik im 21. Jahrhundert gestellt wird...