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# Factoring vs Prime identification

Hey folks,

I generated two random primes, using random_prime(2^256)

Which gave me:

26743933906960470604491354271488742656120020729367854162490438790852133849203


and

58989902932261902911492570960628926646065206682060380716310751283003413744077


Multiplying them to get a semiprime I get:

1577622065198425994274982187337138762115683359284991921617675178559462773099557341124448864625540436189444788629927033127980383957266963291159527952420631


So maybe this is my miscomprehension, but when I try to run:

factor(on the semiprime) -- it takes a long time, never completing.

When I run

isPrime(on the semiprime) -- it instantly returns false.

If indeed sage generates proper primes (non pseudoprimes) how can isPrime be so efficient. Does verifying that a number ISNT prime, not require identifying a factor? If so, why does factorisation take so long?

Thanks

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## 1 Answer

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Yes, checking that a number is not prime can be very easy, without factoring it: you just check that it does not satisfy some condition that all primes satisfy.

For example, if $n$ is a prime, then for any integer $a$, we have $a^n$ is congruent to $a$ modulo $n$.

Another way to say that is that if $n$ is an integer, and if there is any integer $a$ such that $a^n$ is not congruent to $a$ modulo $n$, then $n$ is not prime. Finding such an $a$ can be very cheap but it would not give a way to factor $n$.

See the notion of probable prime and pseudo-prime.

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## Comments

thank you sir!

( 2019-10-26 11:20:32 +0200 )edit

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Asked: 2019-10-06 19:16:44 +0200

Seen: 356 times

Last updated: Oct 07 '19