# why RSA uses Semiprime numbers? [duplicate]

Why does RSA use semiprime numbers? Why not just use any big number ??
What is the advantage of the two original numbers being prime?

Because factoring any big number will be difficult

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Factoring a big number which has less than two big factors is much easier. –  CodesInChaos Dec 7 '13 at 10:28
@user16...: Example: Try and factor $2^{1337}$. That's a very big number, but its definitely not hard to factor! –  figlesquidge Dec 7 '13 at 10:31
Note: RSA has been extended to moduli with more than 2 factors (eg. 3), although that's not as common. –  AnotherTest Dec 7 '13 at 13:17