Need a datatype for large integers or RSA code

Does anyone know where I can get an implementation of a datatype for large integers (1000s of digits)?
I need to write a couple small programs that will use RSA or some other form of public key encryption (thus my need for large integers). I planned to write the RSA code myself but if anyone knows of a free package that implements RSA in VB (or an ocx or something else I can use in VB) please let me know.
Please email me if you have any ideas : [email protected]
Thanks,
Lisa