Hello everyone!

I am trying to write a program in C for the RSA encryption. I have come this far and can't seem to go further. I don't understand why it is not giving me the desired results. My program style may be a bit naive i am just starting off with C. but please i do need some help.
Any nip and tucking or helpful changes?
I have tried to translate the comments into english.

// FinalProject.


// RSA ENCRYTION REQUIREMENTS
// 'p' and 'q' Primenumbers. n= p*q
// 'e' encryption key 'd' decryption key. [n, e] Public key pair, known to all. [n, d] Private key
//pair known to user alice bob or whoever.
// Encryption function C= M^e mod n. Decryption function M= C^d mod n  ; M = message in ASCII code. C= Coded message
// e is greater than  1 und has no prime factors with z in common.
//(e*d)mod z=1





//#include <conio.h>
#include <stdio.h>
#include "stdafx.h"
 

#define scanf fflush(NULL); scanf
const int maxnum=100;

long z, n, e, d, C, M;

/*Defining functions*/
/*** PRIME GEN ***/
/*is supposed to generate all prime numbers from one to 10000*/
void primegen()
{
int zahlen[10000]; //Array der zu überprüfenden Zahlen
int zaehler = 2; //Zählvariable
int izaehler; //Funktionsinterner Zähler
int x;

	zahlen[0] = 0; //Exception
    zahlen[1] = 0; //Exception
	for (x=2;x<(sizeof(zahlen)/sizeof(int));x++)
	{
		zahlen[x] = 4;
	}
    for(zaehler;zaehler <= (sizeof(zahlen)/sizeof(int));zaehler++)
		/*All elements of the array are examined*/
    {
        if(zahlen[zaehler]==4) //Wenn des jeweilige Element der Zählvariable 0 ist, dann ist das jeweilige Element eine Primzahl
        {
            printf("%i\n",zaehler); //Ausgabe
            for(izaehler = 1;(izaehler*zaehler) < (sizeof(zahlen)/sizeof(int));++izaehler) //Setze alle Elemente, die das ganzzahlige Vielfache der Primzahl sind auf 1 -&gt; Dh. dass diese keine Primzahlen sind
            {
                zahlen[(izaehler*zaehler)] = 1;
            }
        }
    }
    //getchar();
    
}

/***ENCRYPTION FUNCTION***/
void ziffern()
{
	int i; //for schleife.
	C=1; 
		for (i=0; i<e; i++)
		{
			C=C*M%n;
			C=C%n;
			// text wird auf dem Bildschirm gezeigt. Text displayed on screen
		}
		printf("\n\t Chiffrierter Text: %ld\n", C);
//[(a mod b)*(b mod n)]mod n = (a*b)mod n. 
//1st step.C* M mod n, 2nd Step C*C *M mod n,... eth Step C^e M mod n. 

}

/***DECRYPTION FUNCTION***/
void entziffern()
{
	int i;
	M=1;
	for (i=0; i<d; i++)
		M=M*C%n;
		M=M%n;
		printf("\n\t Dechiffrierter Text: %ld\n", M);

}

/***PRIME NUMBER CHECK***/
/*the user chooses prime numbers from the list of numbers generated by PRIMEGEN and in case he makes an error, the NEXTPRIME finds the nearest prime number*/
long nextPrime(long prePrime) 
{
	/*prePrime is the supposed prime number given in by the user.*/
    bool isPrime = true;      
    long start = prePrime; /*this variable is just meant to hold prePrime for a while*/

    while (isPrime)  
    {
     if (prePrime == 0)
       break;
     else if (prePrime == 1)
       break;

  	   for (long i = 2; i <= prePrime; i++)
    	 {
  		 if (start == i)
      		 break;
   
  		 else if ((prePrime % i) == 0) /*Factor different from 1 found i.e. not prime!*/
     		  {
  		   prePrime++; /*next number is examined*/
    		   i = 2; /*loop is restarted at the initial point.*/
      		 }
     }

 printf( "Number entered is invalid! The nearest prime ist %ld: prePrime ") ;

 isPrime = false; //break loop!
    }
    /*nextPrimeNum = prePrime;*/
	return (prePrime);
}
/***HIGHEST COMMON FACTOR***/
/*using this to make sure ‘e’ has no factors in common with ‘z’*/
long ggT (long zahl1, long zahl2)
{
	long x, y, hilf;
	long rest=x%y; 

	if(zahl2>zahl1)
	{
	hilf=zahl1;
	zahl1=zahl2;
	zahl2=hilf;
	}

	if (zahl2>0)
	{
	x=zahl1;
	y=zahl2;
		while(rest != 0)
		{
		rest = x%y;
		x=y;
		y=rest;
		printf("Steps: ggT(%ld, ld%) = %ld\n", x, y, rest);

		printf ("ggT (%ld, %ld) = %ld \n", zahl1, zahl2, x);
/*HCF of #1 and #2 equals x*/
		}

	}
	else
	{
	printf ("Ungültige Eingabe! \n");
	}
return (x);

}



// User prompt
/***MAIN PROGRAM***/

void main()
{

	long p, q, i;
	char wunsch;
	
	printf("primegen: ");
	primegen();


printf("Choose the first random prime numbers from the list of numbers: \t"); 
	
	scanf("%ld ",&p);
	p=nextPrime(p);

printf("Choose the second random prime numbers from the list of numbers \t"); 

	scanf("%ld ",&q);
	q=nextPrime(q);

	

	
		z = (p-1)*(q-1);
		n = p * q;
		printf("\n\t Z = %ld", z);

printf("Please give in a value for e\t: " );
scanf("%ld ",&e);
/*if e value is invalid,the next valid e is choosen*/
	do{
for(int i=2; i<=e; i++)
	{ 
	e++;
	ggT(e, z);
	i=2;
	}
}while (ggT(e,z)!=1);
printf ("Eingabe ungültig. Den Wert %ld wurde für e ausgesucht!", e); 

/*Because e*d mod z =1, we set i to 1, and calculate the value of d.*/

	d=1; /*starting point, d=0 makes no sense here*/
	do
	{
		i=(d*e)%z;
		d++;

	}while(i!=1);
	d=d-1;

	//User prompt entcrypt or decrypt
	
	
	printf ("\n\n encrypt or decrypt?(Z\\E):");
	wunsch = getchar();
	switch (getchar()) {
	case 'z': case 'Z':
		printf ("\n\n Enter plain text\t:");
		scanf("%ld", &M);
		ziffern();
	break;
	case 'e': case 'E':
		printf("\n\n Enter encrypted text\t: ");
		scanf("%ld", &C);
		entziffern();
	break;
  default : printf("\n Enter z oder e \n");
//getch();  // member of conio.h read xter directly from the console without buffer and without echo.
	}
	//return 0;
}
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