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RSA-210 has been factored

N = 245246644900278211976517663573088018467026787678332759743414451715061600830038587216952208399332071549103626827191679864079776723243005600592035631246561218465817904100131859299619933817012149335034875870551067

in two 105-digit prime numbers.


How did we know RSA-210 was a semiprime?

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In RSA $n = p \times q$, where $p$ and $q$ are picked using two large primes by algorithm, RSA Algotithm. This is known as the Integer Factorization Problem. – Amzoti Oct 7 '13 at 12:57

No one "knew" that RSA-210 was semiprime. Not even those running the contest know what the factors of the number are. It's just that they assume their algorithms for generating the contest numbers are correct (to enough probability) and so the numbers they post are almost certainly semiprime.

It's only after factoring that everyone else knows a number is semiprime, unless some incredible result comes up in number theory algorithms which isn't likely.

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Do you mean by "almost certainly" that their algorithm for generating the contest numbers was probabilistic or that they cannot be sure (as one never can) that their implementation/algorithm did not contain a bug? – JiK Oct 7 '13 at 14:30
Probabilistic algorithm for generating 2 primes. – DanielV Oct 7 '13 at 14:31

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