Diffie Hellman c# implementation

What you implemented is not Diffie-Hellman, and has no strength at all. You are being confused with the use of the '^' character.

In C-like programming languages, '^' is the operator for a bitwise exclusive-or (a "XOR").

When writing mathematics in ASCII, it is customary to denotes exponentiation with the '^' character -- and it is not a XOR at all ! This notation comes from LaTeX, a typesetting system which is the de facto standard among mathematicians. In this message, I can use HTML tags and write 'g^a' to say "g to the power a", but if I were to write in plain ASCII (e.g. on Usenet), I would have to write: 'g^a'.

Moreover, Diffie-Hellman uses modular exponentation on big integers -- typical size being 1024 bits or more. 32-bit or 64-bit integers will not be enough to achieve any kind of security, and, in any case, modular exponentiation is not what pow() implements (in algebraic terms, you want to work over a finite field, not plain integers or real numbers). In C# (.NET 4.0), you would want to use the System.Numerics.BigInteger class, and in particular its ModPow() method. But first, if you ever want to do that, you first have to understand the underlying mathematics. You can begin by reading the first three chapters of the Handbook of Applied Cryptography (no relation whatsoever with Schneier's "Applied Cryptography", and, in my view, the "Handbook" is a far more useful book). It may seem a bit harsh, but you cannot hope to implement Diffie-Hellman properly unless you master the mathematics described in those first three chapters.

(And, of course, implementing cryptographic algorithms has other pitfalls, related to side-channel leaks, so even if you do understand what happens mathematically, making your own implementation is not necessarily a good idea.)

Diffie-Hellman is based on modular exponentiation, so by using a different function in this code you haven't implemented Diffie-Hellman at all but something else.

Also the 63/64-bit numbers you're using are too small in any case.

You should read a basic text on cryptography, e.g. Schneier's Applied Cryptography.