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Old 2019-10-09, 19:46   #1
baih's Avatar
Jun 2019

2·17 Posts
Default Condition on composite numbers easily factored (Challenger)

q prime numbre

p prime numbre
and q>p
lets c = qp

e= 2^p mod q

if we know e we can factore (c)

ok letz try with c and e

Last fiddled with by baih on 2019-10-09 at 19:51
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Old 2019-10-10, 11:14   #2
R. Gerbicz
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"Robert Gerbicz"
Oct 2005

58116 Posts

With "a mod n" we denote the smallest non-negative residue modulo n.

We're given e=2^p mod q and calculate (in polynomial time) r=2^n mod n

What is r mod q ?

Easy: r mod q=(2^n mod n) mod q=2^n mod q=(2^q)^p mod q=2^p mod q=e (used Fermat's little theorem).

So we know that r mod q=e, hence r-e is divisible by q, so q | gcd(r-e,n), and if this is not n, then
we could factorize: because q=gcd(r-e,n) and with a division p=n/q.

The (only) hole in this proof is that (in rare cases) it is possible that n=gcd(r-e,n), for example this is the case for n=341.

To see that it is really working:


(one possible output)
? [623981947, 805922797]
? %42 = 805922797
ps. we have not used that q>p.

Last fiddled with by R. Gerbicz on 2019-10-10 at 11:16 Reason: typo
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