/*
  Author:  Pate Williams (c) 1997

  Exercise 1.10
  "Decrypt the following ciphertext, obtained from
  the Autokey Cipher, by using exhaustive key
  search:
  MALVVMAFBHBUQPTSOXALTGVWWRG" -Douglas R. Stinson-
  See "Cryptography: Theory and Practice" by
  Douglas R. Stinson page 43.
*/

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

int main(void)
{
  char answer[256];
  char ciphertext[] = "MALVVMAFBHBUQPTSOXALTGVWWRG";
  int found = 0;
  long count = strlen(ciphertext), i, j, x, y;

  for (i = 0; i < count; i++)
    ciphertext[i] -= (char) 'A';
  for (i = 0; !found && i < 26; i++) {
    x = (ciphertext[0] - i) % 26;
    if (x < 0) x += 26;
    printf("%c", x + 'A');
    for (j = 1; j < count; j++) {
      y = (ciphertext[j] - x) % 26;
      if (y < 0) y += 26;
      printf("%c", y + 'A');
      x = y;
    }
    printf("\n");
    printf("another key = %c (n or y)? ", i + 'A');
    scanf("%s", answer);
    found = tolower(answer[0]) == 'n';
  }
  return 0;
}