In mathematics, a semiprime (also called biprime or 2-almost prime, or pq number) is a natural number that is the product of two (not necessarily distinct) prime numbers. The first few semiprimes are 4, 6, 9, 10, 14, 15, 21, 22, 25, 26, ... .
Semiprimes are highly useful in the area of cryptography and number theory, the most notable examples of which being public key cryptography (ie. RSA) and pseudo-random number generators (ie. BBS/Blum-Blum-Shub). The main rationale for the use of such numbers is the inherent difficulty (at least presently) of integer factorization, especially as the size of the numbers increase. Put simply, it's rather simple to see that 35 can be factored into the product of 5 and 7, but if you can factor this large semiprime, you win $200,000.
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