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All prime numbers are odd numbers. If one were even, it would be divisible by two and would, therefore, not be prime.
Prime numbers are helpful in cryptography because it is MUCH easier to calculate the product (multiplication) of two prime numbers than to do the reverse process (find the prime factors of a big number). The bigger the prime numbers are, the higher the difference in time between calculating the product, or factoryzing this product back into the two prime numbers. When person A wants to tell B a secret, they could agree on two great prime numbers (in a secret way) and later use the product to communicate. A and B could easely calculate the other's factor because they know their own factor. Anyone else would have to try to factorize the huge prime number without any knowledge which would take, ideally, longer than 4.6 billion years (the age of the Earth). This is a VERY simplified answer and more can be found by googling around.
No, there are no prime numbers that are also square numbers. Prime numbers are only divisible by 1 and themselves, while square numbers have integer square roots. Since the square root of a prime number is not an integer, a prime number cannot be a square number.
2 would be a counterexample to the conjecture that prime numbers are odd. 2 is a prime number but it is the only even prime number.
Prime numbers