Large integers in C++

I am writing a program that deals with large integers: more than 8 digits. 
Eventually, I hope to use integers with about 100 digits. So far, I have 
tried using long and unsigned long as the variable type. However, the 
results of calculations become inaccurate when integers with 9 or more 
digits are used.

The calculations themselves are fairly simple: squaring, addition and 
subtraction in modular arithmetic.

Any help much appreciated.