[MattPalmer1086]

Sure, you can't use built in multiplication, but it isn't a very big hurdle. Just use repeated squares, it's fairly trivial to implement. I've worked on software that did this on very low power mobile payment devices.

[SkiFire13]

Repeated squares is a way to implement exponentiation, not multiplication.

[MattPalmer1086]

Oops, yes, I meant exponentiation. Which you need (mod n) in RSA.