cipher.c

#include <stdio.h>

int find_index(char s[], char element)
{
  int i;

  for (i = 0; s[i] != '\0'; i++)
  {
    if (s[i] == element)
    {
      return i;
    }
  }

  return -1;
}

int main()
{
  char c;

  char plain[]  = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ ";
  char cipher[] = "fAOjrhNKknMpIivUGmPxlQDeCTFJyt RLSWsdEXcYoawqZzbBugVH";

  while ((c = getchar()) != EOF)
  {
    int i = find_index(cipher, c);
    if (i >= 0)
    {
      putchar(plain[i]);
    }
    else
    {
      putchar(c);
    }
  }

  return 0;
}