Thread: Put this algorithm into code

  1. #1
    Registered User
    Join Date
    Jun 2020

    Put this algorithm into code

    I am trying to put this algorithm into code for my autokey cipher program.

    If (keyIsNotExhaust) Encryption = currChar + key; else Encryption = currChar + prevChar;

    If (keyIsNotExhaust) decryption = currChar - key; else decryption = currChar - decryption;

    This is the code I have so far:

    #include <stdio.h>
    char key[100];
    unsigned int easyendecrypt(char c);
    int main(void) {
        int c;
        while ((c = getc(stdin)) !=EOF) {
            putc(easyendecrypt(c), stdout);
        return 0;
    unsigned int easyendecrypt(char c) {
        char k;
    #ifdef DECRYPT
        fprintf(stderr, "decrypting with %s\n",key);
        k = 'A' + (c - key);
        key[c] = k;
        return k;
        fprintf(stderr,"encrypting with %s\n",key);
        k = key + (c + key - 2 * 'A');
        return k;

  2. #2
    Lurking whiteflags's Avatar
    Join Date
    Apr 2006
    United States
    So the idea of this is fine, I guess.

    > char key[100];
    Here you declare key as an array and it's empty.

    > k = 'A' + (c - key);
    Then you start using the array name of the key to decrypt

    > key[c] = k;
    And also store the plaintext somewhere in the key?

    And you do similar things when you encrypt:

    > k = key + (c + key - 2 * 'A');

    In pretty much all of these cases, you probably only intended to use part of the key in a loop, which right now, it doesn't look like you're attempting to do at all.

    Also you really need to separate plaintext, key, and ciphertext. Each of these things need their own array, perhaps even of the same size (to start with). You cannot store plaintext in the key array without overwriting part of the key.

  3. #3
    Registered User
    Join Date
    Jun 2020
    Is this how an autokey cipher would work or is there an easier way.

  4. #4
    and the hat of int overfl Salem's Avatar
    Join Date
    Aug 2001
    The edge of the known universe
    Well somewhere, you need to initialise your key string.
    char key[] = "hello";

    Then you need an index and a count.
    int key_index = 0;
    int key_count = 5;

    > If (keyIsNotExhaust) Encryption = currChar + key; else Encryption = currChar + prevChar;
    The first part of this being something like
    if ( key_index < key_count ) Encryption = currChar + key[key_index]; key_index += 1;
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

Popular pages Recent additions subscribe to a feed

Similar Threads

  1. Sudoku how to include algorithm into this code?
    By maestorm in forum C Programming
    Replies: 8
    Last Post: 05-22-2014, 04:05 AM
  2. algorithm to code.
    By mast3124mind in forum C Programming
    Replies: 6
    Last Post: 03-27-2013, 07:51 PM
  3. Algorithm code problem
    By Djur0 in forum C Programming
    Replies: 11
    Last Post: 01-22-2012, 02:11 PM
  4. Understanding Genetic Algorithm Code!!
    By L3munoz in forum C Programming
    Replies: 2
    Last Post: 10-25-2010, 06:53 AM
  5. need source code for Cohen sutherland algorithm
    By specks5317 in forum C++ Programming
    Replies: 1
    Last Post: 10-17-2002, 08:24 PM

Tags for this Thread