/*
  Author:  Pate Williams (c) 1997

  Exercise III.1.3
  "In the 27-letter alphabet (with blank = 26), use
  the affine enciphering transformation with key
  a = 13, b = 9 to encipher the message "HELP ME".
  -Neal Koblitz-
  See "A Course in Number Theory and Cryptography"
  by Neal Koblitz second edition page 62.
*/

#include <stdio.h>
#include <string.h>

int main(void)
{
  char plaintext[16] = "HELP ME";
  long a = 13, b = 9, c, i, p = 27;
  printf("%s\n", plaintext);
  for (i = 0; i < strlen(plaintext); i++) {
    c = plaintext[i];
    if (c == ' ') c = 26; else c -= 'A';
    c = (a * c + b) % p;
    printf("%c", c + 'A');
  }
  printf("\n");
  return 0;
}