Last time I checked there is only one even prime number... If a number is even and greater than 2 it cannot be prime. Though Petrich is right. Proving Sum -> Even is easy. Even -> Sum is not.
Look, RSA is built on the fact in the private key (d,n) and the public key (e,n) d and e are exponential inverses of each other mod n. If x^d or x^e are congruent to 1 then d = phi(n) or e = phi(n) respectively... Take a look at Euler's Theorem which is a generalize form of Fermat's Little...