On Tue, Apr 5, 2016 at 9:56 AM, Nils Bruin <nbr...@sfu.ca> wrote: > Hi everyone, >> >> I was just doing a routine check for a Chaum-van Antwerpen Signature >> Scheme public parameter set(p=2q+1, p,q both odd primes) and in checking a >> small prime, it told me that it was composite. I'm not sure if it's just my >> build, but obviously I can't have this happening without understanding why. >> Any clues as to what could be wrong in my setup? >> >> thanks > > > No, this is confusing, but it behaves as documented. The problem is:
It is my fault I think, but it is terrible. A month ago (when there was that Diffie-Hellman prime-that-isnt-prime that Oracle planted), we had at least two people hitting this exact problem. And this is just what we explicitly heard about from people that mailed the list. I even hit this exact problem when preparing my lecture for a course yesterday. I think we should change is_prime for rational numbers, since people get confused by this so much. How? Pretty much any change at all would be better than the current situation. Options I can think of: - make is_prime([rational]) raise an error and tell the user what to do - look at what is_prime([rational]) does in Pari, Mathematica, etc., and do the same thing - make is_prime([rational]) return is_prime(ZZ([rational])) -- this would be a change that would potentially silently break some code, but is what many people expect. William > > sage: parent(q) > Rational Field > > so q is a rational number. In the rational numbers, non-zero elements are > units and hence not prime. > > Division of integers yields a rational number. If you want to get an integer > out, use // instead: > > sage: q=(p-1)//2 > sage: q.is_prime() > True > > -- > You received this message because you are subscribed to the Google Groups > "sage-support" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to sage-support+unsubscr...@googlegroups.com. > To post to this group, send email to sage-support@googlegroups.com. > Visit this group at https://groups.google.com/group/sage-support. > For more options, visit https://groups.google.com/d/optout. -- William (http://wstein.org) -- You received this message because you are subscribed to the Google Groups "sage-support" group. To unsubscribe from this group and stop receiving emails from it, send an email to sage-support+unsubscr...@googlegroups.com. To post to this group, send email to sage-support@googlegroups.com. Visit this group at https://groups.google.com/group/sage-support. For more options, visit https://groups.google.com/d/optout.
