Have you been getting the right answers with your code? It looks wrong to me.
1 2 3 4 5 6 7 8 9
longlong cryption ( constlonglong msg, constlonglong n, constint DorE ) // Encryption with two keys.
longlong Ptte = msg; // Ptte = P to the e
for ( int i = 0; i < DorE - 1; ++i )
Ptte *= msg; // ( a = a * b ) == ( a *= b )
return Ptte % n;
Inlining a function doesn't really do much. If you have a good compiler it will be able to figure that out itself.
Making the variables you pass constant could help your compiler speed things up, possibly.
You don't need to calculate and store Ptte % n before returning it, you can just return it.
I don't know why you need long long's, but if you don't need them, smaller variables will be faster (on 32bit computers).