Help Me with an autocipher key in C!!!

Hello,
Can somebody please help me with this code. I've gotten somewhat an idea of what it can be but it is still giving me errors. I also don't understand the algorithm of an autokey cipher so could someone please explain in the program.THANKS!!!

Code:

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


char key[];

unsigned int easyendecrypt(char c);

int main(void) {
    int c;
    while ((c = getc(stdin)) !=EOF) {
        putc(easyendecrypt(c), stdout);
    }
    printf("\n");
    return 0;
}

unsigned int easyendecrypt(char c) {
    char k;

#ifdef DECRYPT
    fprintf(stderr, "decrypting with %s\n",key);
    k = 'A' + (c - key);
    key = k;
    return k;
#else
    fprintf(stderr,"encrypting with %s\n",key);
    k = key + (c + key - 2 * 'A');
    key=c;
    return k;
#endif
}

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Well first, decide whether your global variable 'key' is a single char, or an array of chars.

It is an array so I’m trying to input that all though out my code

Right, so you need several things.

1. Your key needs a size.
char key[100];

2. Whenever you write a single char to the array, you need a subscript.
key[some_integer_counter] = k;

3. If you want to print the key using %s, you need to make sure there is a '\0' character in the right place.
key[some_integer_counter] = '\0';

ok, what about my logic of how autokeycipher works. Is this how it would happen to encrypt and decrypt?

> Is this how it would happen to encrypt and decrypt?
Have you tested it?

At some point, you'll have been given some example inputs and expected outputs.

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

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

Return encryption in the encryption part or decryption in decryption part

store the encryption/decryption results in an array

This is the logic I'm trying to go by but I'm having trouble putting that into my code above.

Quote:

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algorithm of an autokey cipher

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Autokey cipher - Wikipedia

Code:

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

#define maxLength 256

#define key "KEYCODE"
#define message "autokey encryption"

#define encrypt(a,b) (((((a-64)+(b-64))%26)+65))

int main(int args, char *argv[]) {
   
    int count, start = 0;
    char toEncode[maxLength], noSpaces[maxLength];

    count = 0;
    for (int i = 0; i < (strlen(message)); i++ ) {
        if (message[i] != ' ') {
            noSpaces[count++] = toupper(message[i]);
        }
    } noSpaces[count] = '\0';

    int textLength = count, keyLength = strlen(key);

    count = 0;
    for (int i = 0; i < (textLength); i++ ) {
        if (i < keyLength) {
            toEncode[count++] = toupper(key[i]);
        } else {
            if (start == 0) start = i;
            toEncode[count++] = noSpaces[i-start];
        }
    } toEncode[count] = '\0';

    for (int i=0; i<(textLength); i++ ) {
        printf("%c", encrypt( toEncode[i], noSpaces[i]) );
    }

    return 0;
}

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Code 4 Life: Autokey Cipher
Practical Cryptography

The guys have given you good advice, but we already have this software, so I post it directly. This is called encryption, please do NOT change the terms! It becomes very hard to figure out what someone is talking about. Especially Americans have this tendency. For example what is [MF, its main function, but written like this, who can guess....We are considering releasing a GUI version of this software with which you can encrypt everything, even .exe files. Test it for yourself to be sure. I don't know if advertisement is allowed on this site, so we can advertise our online shop.

Code:

---

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
                                                    /* Function declaration. */
void Encrypt(char * FILENAME, char * NEW_FILENAME);

int main ()
{
    char f1[100];  // array big enough to hold expected input
    char f2[100];  // array big enough to hold expected input

    printf ("Please enter file for processing:\n");
    scanf  ("%99s", f1);    // set max characters to read to prevent buffer overflow
    printf ("Please enter the name of the file after processing:\n");
    scanf  ("%99s", f2);

    Encrypt(f1, f2);
    return 0;
}

void Encrypt(char * FILENAME, char * NEW_FILENAME)
{
        printf("Processing started.\n");
        FILE *inFile;
        FILE *outFile;
        char key[50];        // array big enough to hold expected input
        char process[50];    // array big enough to hold expected input
        int keylen, keyidx;

        int Byte;      // fgetc returns int, so use an int
        int newByte;

        printf ("Please enter 'encryption or decryption'\n");
        scanf  ("%49s", process);
        printf ("Please enter the key\n");
        scanf  ("%49s", key);
        keylen = strlen(key);
        keyidx = 0;    // starting index into key
        printf("Opening files\n");

        inFile = fopen(FILENAME,"rb");
        outFile = fopen(NEW_FILENAME, "wb");

        if(inFile == NULL)
        {
            printf("Error: Can't Open inFile\n");
        }
        else if(outFile == NULL)
        {
            printf("Error: Can't open outFile\n");
        }
        else
        {
                printf("File Opened, Encrypting\n");
                while((Byte = fgetc(inFile)) != EOF)    // read a byte, check if EOF
                {
                        if (!strcmp(process,"encryption"))
                        {
                                newByte = Byte + key[keyidx];    // use key index
                                if (newByte > 255) newByte -= 256;  // check for overflow
                        }
                        else if (!strcmp(process,"decryption"))
                        {
                                newByte = Byte - key[keyidx];
                                if (newByte < 0) newByte += 256;
                        }
                        else
                        {
                            newByte = Byte;
                        }
                        fputc(newByte, outFile);
                        keyidx++;
                        // loop to the start of the key if needed
                        if (keyidx >= keylen) keyidx = 0;
                }
        }
        if (inFile != NULL) fclose(inFile);    // close your files when you're done
        if (outFile != NULL) fclose(outFile);
}

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Quote:

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Originally Posted by Carry for cents

we already have this software, so I post it directly. This is called encryption, please do NOT change the terms! It becomes very hard to figure out what someone is talking about.

---

The term "autokey cipher" refers to a particular kind of encryption algorithm.

Quote:

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Originally Posted by Carry for cents

We are considering releasing a GUI version of this software with which you can encrypt everything, even .exe files. Test it for yourself to be sure.

---

A quick glance shows that you have implemented a kind of substitution cipher of the "XOR cipher" variety, although the mechanism used is addition rather than XOR. This category of ciphers is equivalent to a one-time pad -- hence unbreakable -- when you have at least as much key material as plaintext, but with insufficient key material the encryption would be rather weak. Since having as much key material as plaintext is usually not practical, in practice what you have is usually just suitable for a student exercise rather than commercial software.

Since your example is not an autokey cipher, it is irrelevant to this discussion.

Quote:

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Originally Posted by Carry for cents

I don't know if advertisement is allowed on this site, so we can advertise our online shop.

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You're free to add an advertisement to your forum signature or to post in the Projects and Job Recruitment forum, but I think you will need to do much more research about cryptography before you go into the business of writing and selling software to securely encrypt stuff.

Thank you very much for this positive critic. I have not given the full version of the software, only an example for students. In fact I don't intend to ever post the full software, only part of it. Also yes, I know the key is important, I was considering adding a key within the software itself, but I decided its better to leave it to the user, and add a warning that the quality depends on the key length.

As for the rest, I was left with the impression that this is what the thread starter wants. It looks the same as here. Maybe I am wrong. Sorry if its so.

Quote:

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Originally Posted by Carry for cents

Also yes, I know the key is important, I was considering adding a key within the software itself, but I decided its better to leave it to the user, and add a warning that the quality depends on the key length.

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I'm not sure what to make of this: either you know your cryptography and are just trolling (with a second account), or you don't know your cryptography and so have no clue as to why "considering adding a key within the software itself" is hilariously painful to those of us who do have some background in cryptography.

Quote:

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Originally Posted by Carry for cents

As for the rest, I was left with the impression that this is what the thread starter wants. It looks the same as here. Maybe I am wrong. Sorry if its so.

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That article you linked to does indeed demonstrate an autokey cipher. Your sample program doesn't. This would normally be a Good Thing since showing students a sample that isn't exactly what they are trying to do forces them to learn rather than just copy, but that wasn't how you presented your program in post #9.