I am developing a project for sending an encrypted data over a LAN using RSA. i should have two large prime numbers say P and Q and then say that n = P*Q. then say that a value phi =(P-1)*(Q-1)
and find a value e such that 1<e<phi and the gcd(e,phi)=1.
the problem is i should have a value d such that
e*d mod phi = 1 . i always get an error saying that arthimitic operation flows.
 i have done this

Dim p,q As Long
Dim e1,d As Long
Dim n,phi As Long
Dim x, y As Long
        Dim i As Long


        Randomize()

        x = 10000 + Rnd() * 10000

        While (Not isprime(x))

            x = 10000 + Rnd() * 10000

        End While
        p = x


        Randomize()

        y = 10000 + Rnd() * 10000
        While (Not isprime(y))

            y = 10000 + Rnd() * 10000
        End While
        q = y


        n = p * q

        phi = (p - 1) * (q - 1)


        Dim e11 As Long
        Randomize()

        e11 = 1 + Rnd() * phi


        While (gcd(e11, phi) <> 1)

            e11 = e11 + 1
        End While
        e1 = e11

        Dim d1 As Long
        Randomize()

        d1 = 1 + Rnd() * phi

        While (e1 * d1 Mod phi <> 1)'here the problem occurs
            d1 = d1 + 1
        End While
        d = d1

For a start off, I wouldn't use Longs because they only hold integer numbers. You should be using doubles, or decimals, for floating point operations like this. Try changing all your variables to doubles and I don't think you'll get Arithmetic overflows any more.