> (= (mod-expt signature 3 modulus) message)
#t
How about to sign a simple one-digit message?
> (mod-expt 7 signing-exponent modulus)
This will result in a stack overflow.
Q: What kind of repetition does mod-expt use?
A: Linear recursion
Linear recursion must use more memory for each unfinished recursive call.
Q: Since linear iteration does not use extra memory, would it be a good approach for mod-expt?
A: Only if you are willing to wait 10180 years.