I am just a beginner of computer science. I learned something about running time but I can’t be sure what I understood is right. So please help me.
So integer factorization is currently not a polynomial time problem but primality test is. Assume the number to be checked is n. If we run a program just to decide whether every number from 1 to sqrt(n) can divide n, and if the answer is yes, then store the number. I think this program is polynomial time, isn’t it?
One possible way that I am wrong would be a factorization program should find all primes, instead of the first prime discovered. So maybe this is the reason why.
However, in public key cryptography, finding a prime factor of a large number is essential to attack the cryptography. Since usually a large number (public key) is only the product of two primes, finding one prime means finding the other. This should be polynomial time. So why is it difficult or impossible to attack?
Casual descriptions of complexity like “polynomial factoring algorithm” generally refer to the complexity with respect to the size of the input, not the interpretation of the input. So when people say “no known polynomial factoring algorithm”, they mean there is no known algorithm for factoring N-bit natural numbers that runs in time polynomial with respect to N. Not polynomial with respect to the number itself, which can be up to 2^N.