C implementation of the railfence cipher

railfence.c

#include <stdlib.h>
#include <stdio.h>

void railfence_encipher(int key, const char *plaintext, char *ciphertext);
void railfence_decipher(int key, const char *ciphertext, char *plaintext);

int main(int argc, char *argv[]){
    /* the string to encipher / decipher */
    char *plaintext = "defendtheeastwallofthecastle";
    /* allocate some space for results of enciphering/deciphering */
    char *ciphertext = malloc(strlen(plaintext)+1);
    char *result = malloc(strlen(plaintext)+1);
    
    /* the following lines do all the enciphering and deciphering */
    railfence_encipher(3, plaintext,ciphertext);
    railfence_decipher(3, ciphertext,result);
    /* print our results */
    printf("-->original:   %s\n-->ciphertext: %s\n-->plaintext:  %s\n",
            plaintext,ciphertext,result);
    
    free(ciphertext);
    free(result);
    
    system("PAUSE");
    return 0;
}

/*******************************************************************
void railfence_encipher(int key, const char *plaintext, char *ciphertext)
- Uses railfence transposition cipher to encipher some text.
- takes a key, string of plaintext, result returned in ciphertext.
- ciphertext should be an array the same size as plaintext.
- The key is the number of rails to use
*******************************************************************/
void railfence_encipher(int key, const char *plaintext, char *ciphertext){
    int line, i, skip, length = strlen(plaintext), j=0,k=0;    
    for(line = 0; line < key-1; line++){
        skip = 2*(key - line - 1); 
        k=0;
        for(i = line; i < length;){
            ciphertext[j] = plaintext[i];
            if((line==0) || (k%2 == 0)) i+=skip;
            else i+=2*(key-1) - skip;  
            j++;   k++;
        }
    }
    for(i=line; i<length; i+=2*(key-1)) ciphertext[j++] = plaintext[i];
    ciphertext[j] = '\0'; /* Null terminate */  
}

/*******************************************************************
void railfence_decipher(int key, const char *ciphertext, char *plaintext)
- Uses railfence transposition cipher to decipher some text.
- takes a key, string of ciphertext, result returned in plaintext.
- plaintext should be an array the same size as plaintext.
- The key is the number of rails to use
*******************************************************************/
void railfence_decipher(int key, const char *ciphertext, char *plaintext){
    int i, length = strlen(ciphertext), skip, line, j, k=0;
    for(line=0; line<key-1; line++){
        skip=2*(key-line-1);	  
        j=0;
        for(i=line; i<length;){
            plaintext[i] = ciphertext[k++];
            if((line==0) || (j%2 == 0)) i+=skip;
            else i+=2*(key-1) - skip;  
            j++;        
        }
    }
    for(i=line; i<length; i+=2*(key-1)) plaintext[i] = ciphertext[k++];
    plaintext[length] = '\0'; /* Null terminate */  
}