Scytale encryption (ancient greek technique) in C - https://en.wikipedia.org/wiki/Scytale

scytale.c

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

void scytale(char* plain, int key, char* encrypted) {
	int length = strlen(plain);
	for(int i = 0; i < key; i++) {
		for(int ii = i; ii < length; ii += key) {
			encrypted[strlen(encrypted)] = plain[ii];
			printf("col:%d row:%d current:'%c'\n",
				i, ii, plain[ii]);
		}
	}
}

void main() {
    char plain[] = "This is some text to encrypt";
    char* encrypted = calloc(1, strlen(plain)+1);
    char* decrypted = calloc(1, strlen(plain)+1);
    
    int key = 4;
    
    printf("Plain text: %s\n", plain);
    
    scytale(plain, key, encrypted);
    
    printf("Encrypted text: %s\n", encrypted);
    
    if(!strlen(plain) % n) {
        scytale(encrypted, strlen(plain) / key, decrypted);
        
        printf("Decrypted: %s\n", decrypted);
        
        if(!strcmp(plain, decrypted)) {
            puts("Encryption/decryption successful!");
        } else {
            puts("Encryption/decryption failed!");
        }
    }
    
    free(encrypted);
    free(decrypted);
}