Possible to recreate GPG using pen and paper?

[Harry Rickards]

Would it be possible to do the same job that GPG does (using all the  
same algorithms) simply using a pen and paper? You can do simple  
public key crypto with RSA, by choosing two primes and doing a  
multitude of stuff with them. I understand that it will take a while  
to actually encrypt/decrypt something, and you'll need a calculator as  
well, but it would be fun to try all the same.

Thanks
Harry Rickards

P.S Not signed as I'm currently using an iPhone.