What is the illegal prime number?

Excuse me, could you clarify what you mean by the term "illegal prime number"? Prime numbers, as we know, are numbers greater than 1 that have no divisors other than 1 and themselves. They are fundamental building blocks of number theory and have been studied for centuries. However, the notion of an "illegal" prime number isn't a standard concept in mathematics or cryptography. Is there a specific context or application where this term is used that I'm not aware of? Could you elaborate on your question to help me better understand what you're asking?