Another C program

shednik

Hello again,

So next programming assignment was for the autokey cipher, I have it working sort of...If I test my own strings it works and well as the example in my book. However when I try to complete an exhaustive key search on a piece of ciphertext from the review problems this string specifically doesn't produce the proper output. Below is my code, I'm just trying to see if I'm missing something or if I'm just losing it.

/* Autokey Cipher
   Joe Hlasnik
*/

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#define  MAXCHARS 1024
#define  MAXS    4

void encrypt();
void decrypt();
char *letters[26] = {"A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T","U","V","W","X","Y","Z"};

int main()
{
    char response[MAXS],plaintext[MAXCHARS];
    
    printf("Autokey Cipher Program\n\nWould you like to (E)ncrypt or (D)ecrypt a message or (Q)uit.  ");
    scanf("%s",response);
    
    while (response[0] != 'q' && response[0] != 'Q') {
          switch (response[0]) {
                 case 'e':
                 case 'E':
                      encrypt();
                      break;
                 case 'd':
                 case 'D':
                      decrypt();
                      break;
                 default:
                         printf("error: invalid command\n");
                         break;
          }//end of switch
    printf("Would you like to (E)ncrypt or (D)ecrypt a message or (Q)uit.  ");
    scanf("%s",response);
    }//end of while
    return 0;
}//end of main


void encrypt()
{
     int i=0,k,z,tempk;
     char msg[MAXCHARS];
     char letter[MAXS];
     
     printf("Please enter the plain text to encrypt in all CAPS and press enter\n");
     scanf ("%s",msg);
  
     printf("Please enter the alpha key(k) in CAPS you would like to use  ");
     scanf ("%s",letter);
  
     for (k=0;k<26;k++)
         if (strcmp(letters[k],letter)==1) //compare to convert k
            break;
     k--;//step back one
    
     for (z=0;z<strlen(msg);z++)
         msg[z] = msg[z] - 'A';
           
     while (msg[i]!= '\0') {
           tempk = msg[i];
           if ((msg[i] + k)>26)
              msg[i] = (msg[i] + k) - 26;
           else
               msg[i] = msg[i] + k;
           k = tempk;
           i++;
           }
           
     for (k=0;k<strlen(msg);k++)
         msg[k] = msg[k] + 'A';
     printf("Encrypted Text: %s\n",msg);
}
void decrypt()
{
     int i=0,k,z;
     char msg[MAXCHARS];
     char letter[MAXS];
     printf("Please enter the ciper text to decrypt in all CAPS and press enter\n");
     scanf ("%s",msg);
     printf("Please enter the alpha key(k) in CAPS you would like to use  ");
     scanf ("%s",letter);
     for (k=0;k<26;k++)
         if (strcmp(letters[k],letter)==1) //compare to convert k
            break;
     k--;//step back one
    
     for (z=0;z<strlen(msg);z++)
         msg[z] = msg[z] - 'A';
     //printf("%s\n",msg);
     while (msg[i]!= '\0') {
           if ((msg[i] - k)<0)
              msg[i] = (msg[i] - k) + 26;
           else
               msg[i] = msg[i] - k;
           k = msg[i];
           i++;
           }
    
     for (k=0;k<strlen(msg);k++)
         msg[k] = msg[k] + 'A';
     printf("Decrypted Text: %s\n",msg);
    //MALVVMAFBHBUQPTSOXALTGVWWRG
}                     
                     

Ok so the working one I tried from the book is 'RENDEZVOUS' with k = I = ciphertext: ZVRQHDUJIM

The book ciphertext: MALVVMAFBHBUQPTSOXALTGVWWRG
Output when I try k = A ; M, k = B ; L so it's only producing the first char for output.

Output below

Autokey Cipher Program

Would you like to (E)ncrypt or (D)ecrypt a message or (Q)uit.  E
Please enter the plain text to encrypt in all CAPS and press enter
RENDEZVOUS
Please enter the alpha key(k) in CAPS you would like to use  I
Encrypted Text: ZVRQHDUJIM
Would you like to (E)ncrypt or (D)ecrypt a message or (Q)uit.  D
Please enter the ciper text to decrypt in all CAPS and press enter
ZVRQHDUJIM
Please enter the alpha key(k) in CAPS you would like to use  I
Decrypted Text: RENDEZVOUS
Would you like to (E)ncrypt or (D)ecrypt a message or (Q)uit.  D
Please enter the ciper text to decrypt in all CAPS and press enter
MALVVMAFBHBUQPTSOXALTGVWWRG
Please enter the alpha key(k) in CAPS you would like to use  C
Decrypted Text: K
Would you like to (E)ncrypt or (D)ecrypt a message or (Q)uit.  D
Please enter the ciper text to decrypt in all CAPS and press enter
MALVVMAFBHBUQPTSOXALTGVWWRG
Please enter the alpha key(k) in CAPS you would like to use  B
Decrypted Text: L
Would you like to (E)ncrypt or (D)ecrypt a message or (Q)uit.  E
Please enter the plain text to encrypt in all CAPS and press enter
JOEJOE
Please enter the alpha key(k) in CAPS you would like to use  X
Encrypted Text: GXSNXS
Would you like to (E)ncrypt or (D)ecrypt a message or (Q)uit.  D
Please enter the ciper text to decrypt in all CAPS and press enter
GXSNXS
Please enter the alpha key(k) in CAPS you would like to use  X
Decrypted Text: JOEJOE
Would you like to (E)ncrypt or (D)ecrypt a message or (Q)uit.

Let me know if anyone has any idea? Slowhand here's some good practice for you

tiersten

It is because there is an A as the second character of MALVVMAFBHBUQPTSOXALTGVWWRG.

Your code is subtracting 'A' from each character of the message. 'A' - 'A' = 0 which therefore null terminates the string and stops further processing later on. Rewrite your code to avoid doing that.