[plt-scheme] RSA encryption algorithm

Hi.

I need some help here. I'm new to Scheme and now I'm supposed to build a RSA
encryption algorithm.

I have some guidelines to build it and I'll post them now.

1. Write a procedure "prime?" that given a natural (positive) number returns
true if that number is a prime number or false otherwise.

2. Write a procedure "calc-e" that receives an argument n and returns a
natural number, between 1 and n, that is a coprime of n.

3. Write a procedure "calc-d" that given 2 arguments, e and n, returns a
natural number d, such that there is a number k to which the following
expression is true:

d . e = 1 + k . n

4. Write a procedure "RSA" that receives 2 natural (positive) numbers, k and
l, and generates a public key and it's respective private key that uses de
k-th and the l-th prime numbers as the initial prime numbers. The keys
should be displayed in the screen in an adequate form

5. Write a procedure "encoder" that receives 2 natural numbers that
correspond to the public and private key and returns a procedure that the
argument is de encoder or decoder that uses the received key.

I'm not able to use every primitive procedure in Scheme. I'm only able to
use these procedures:

Numerical operations;
Predicates;
Logical operations;
abs, cond, else, if, lambda, let, let*;
begin, display, newline;

I guess that's all of them...

So far I have this:

;Begin prime?

(define (prime? k)
  (if (<= k 1)
      "Error: Insert number over 1."
   (= k (smallest-divisor k))))

(define (smallest-divisor k)
  (divisor k 2))

(define (divisor k test-divisor)
  (cond ((< k (sqrt test-divisor)) k)
        (( divisor? test-divisor k) test-divisor)
        (else (divisor k (+ test-divisor 1)))))

(define (divisor? a b)
  (= (remainder b a) 0))

;End prime?

;Begin calc-e

(define (a n)
  (gcd (- n 1) n))

(define (calc-e n)
  (if (= (a n) 1)
      (- n 1)
      (a ( - n 1))))

;End calc-e

If there's anyone that could help me out here I would appreciate it.

Regards.
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