Self modifying hash encryption algorithm

Question

I am looking for an algorithm to change it's keys every period and can still be deciphered.
Basically I am looking for a way to maintain a secured link between mobile clients and a stationary server in such a way that even if you intercepted the hash or even the authentication credentials themselves they would change on both sides every period.
Does it ring a bell to anyone?
Is there a better way to ensure that even if you will intercept the authentication credentials somehow it will only be valid to a certain request from a certain user?

Answer 1

I suggest just using SSL. It is designed to be reasonably resistant to man-in-the-middle attacks, which is what I assume is your concern.

Or maybe Kerberos.

Do not code encryption algorithms yourself.