### Re: [Cfrg] Prime 630*(427!+1)+1 for classic DH?

Greg Rose <ggr@seer-grog.net> Wed, 05 April 2017 23:20 UTC

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From: Greg Rose <ggr@seer-grog.net>

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Date: Wed, 05 Apr 2017 16:20:26 -0700

Cc: Dan Brown <danibrown@blackberry.com>, "cfrg@irtf.org" <cfrg@irtf.org>

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To: Greg Rose <ggr@seer-grog.net>

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Subject: Re: [Cfrg] Prime 630*(427!+1)+1 for classic DH?

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On Apr 5, 2017, at 13:32 , Greg Rose <ggr@seer-grog.net> wrote: > > >> On Apr 5, 2017, at 12:39 , Dan Brown <danibrown@blackberry.com> wrote: >> [snip]... for the choice 427!+1 of large prime factor of p-1. [snip] >> > > Since a surprisingly large number of people think that N!+1 must be prime (referring back to the proof that there are an infinite number of primes) I checked. It is. Not that I ever doubted you, Dan... A number of people responded off-list asking why I said this, and/or how I checked primality. Quoting: https://primes.utm.edu/notes/proofs/infinite/euclids.html > Theorem. > There are infinitely many primes. > Proof. > Suppose that p1=2 < p2 = 3 < ... < pr are all of the primes. Let P = p1p2...pr+1 and let p be a prime dividing P; then p can not be any of p1, p2, ..., pr, otherwise p would divide the difference P-p1p2...pr=1, which is impossible. So this prime p is still another prime, and p1, p2, ..., pr would not be all of the primes. > It is a common mistake to think that this proof says the product p1p2...pr+1 is prime. The proof actually only uses the fact that there is a prime dividing this product. I calculated 427!+1 using bc, edited it to remove continuation lines, and pasted the result into https://www.alpertron.com.ar/ECM.HTM. If it had been composite, it might have taken a long time when asked to factor it, but it takes very little time to come back and say that it's prime (that is, it is its only non-trivial factor). However, I then proceeded with the obvious experiment to check the original number: (quoting Dan's original mail) > Is the prime p=630*(427!+1)+1 vulnerable to the SNFS, or some variant of SNFS? I think not, but I could easily be very wrong. Somewhat to my dismay, it did NOT immediately pronounce the number to be prime. 20 minutes later it's still trying to factor it. I don't have anything to hand that will just check primality... perhaps someone else should check it. (Or I will try harder when I have something better than my laptop, which will be a while). It's also possible I made some sort of transcription error, and if so I apologize. Someone who has Mathematica or Maple lying around? Greg.

- [Cfrg] Prime 630*(427!+1)+1 for classic DH? Dan Brown
- Re: [Cfrg] Prime 630*(427!+1)+1 for classic DH? Greg Rose
- Re: [Cfrg] Prime 630*(427!+1)+1 for classic DH? Greg Rose
- Re: [Cfrg] Prime 630*(427!+1)+1 for classic DH? Greg Rose
- Re: [Cfrg] Prime 630*(427!+1)+1 for classic DH? Travis Finkenauer
- Re: [Cfrg] Prime 630*(427!+1)+1 for classic DH? Travis Finkenauer
- Re: [Cfrg] Prime 630*(427!+1)+1 for classic DH? Anna (Amy) Johnston
- Re: [Cfrg] Prime 630*(427!+1)+1 for classic DH? Dan Brown
- Re: [Cfrg] Prime 630*(427!+1)+1 for classic DH? Anna (Amy) Johnston
- Re: [Cfrg] Prime 630*(427!+1)+1 for classic DH? Watson Ladd