Function to encrypt a string using pointers

I need to write a function that gets a string of chars and a number N, the function will return in the same pointer the encrypted string. The function will encrypt a string by the following rules: 1. Reverse the string, f.e: if the string is Amnon the result would be nonmA. 2. After the reverse each letter needs to be replaced by the value of N, f.e: if N=3 then instead of nonmA the result would be qrqpD.

I had no problem doing the reverse part but Im struggling with switching each letter. here is the code I wrote so far:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void StRreverse(char* mystr, int N);

void StRreverse(char* mystr, int N)
{
    int c, length, n;
    char *begin, *end, temp;
    length = strlen(mystr);
    begin = mystr;
    end = mystr;
    for (c = 0; c < length - 1; c++)
    {
        end++;
    }
    for (c = 0; c < length / 2; c++)
    {
        temp = *end;
        *end = *begin;
        *begin = temp;
        begin++;
        end--;
    }
}
void main()
{
    char string[100];

    printf("Enter some text\n");
    gets(string);

    StRreverse(string);
    printf("Reverse of entered string is \"%s\".\n", string);
    system("pause");
}