There is a little known type of number that everyone has heard of, but few people understand what it really is. It is called a prime number. This is a number which can only be divided by 1 and itself without producing a decimal.

For example, 11 is a prime number because no other number can divide it evenly except itself and 1. The interesting thing is that there is no sure way to determine if a number is prime unless we divide it by all of the numbers smaller than it. Actually we only have to go up to its square root, but that can be a lot of numbers to crunch if you are talking about numbers with many digits.

Mathematicians have struggled for centuries to understand primes, and they have developed ways to test numbers based on probabilistic models to determine if they are likely prime. So we can repeat these tests over and over to be nearly certain that a number is prime. Yet a problem still arises. Carmichael numbers are rare numbers that are not actually prime, but appear to be when we test them. So still, the only way to absolutely sure is to divide it by all of its possible factors.

Since this is the only way to be completely sure a number is prime, if we pick a super big number with perhaps thousands of digits, even a computer takes a long time to figure out if a number is prime. Cryptologists take advantage of this by taking two huge prime numbers and multiplying them together. The two primes constitute a code that conceals information such as credit card numbers or secret military plans.

If the number is big enough, even computers will never be able to figure out its factors, but if the receiver knows just one factor, he can easily find the other. Since he will have both at this point, he can decipher the message. The funny thing is, that we can advertise the the product of the two primes to the world because they have almost no chance of figuring out its factors. This is how most of your private information is coded today.

So what's a prime number good for? It's good for you!