Theory Of Numbers
In Number theory Primes play a very important role.They are the keys of Code and Cryptography.
Fist time we used Primes was in the Problem of factorisation.
They can only be divided by 1 or Themselves ;
That's why we call them Primes
Primes as Locks :
As you can see in above explanation Primes are the builing blocks of any natural number.
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Source : KhanAcademy.com