Caesar Cipher

caesar_cipher.c

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

char cap(char c) {
	if(c >= 'a' && c <= 'z') c -= 32;
	return c;
}

void addToSet(char* set, int* len, char c) {
	int i = 0;
	while(i < *len && set[i] != '\0') {
		if(set[i] == c) break; // Found
		++i;
	}
	// Not found
	if((i == 36 || set[i] == '\0') && set[i] != c) set[(*len)++] = c;
}

char* caesarEncrypt(char* msg, int msg_len, char* abc, char* cae_abc) {
	char* enc_msg = (char*) malloc(msg_len + 1);
	enc_msg[msg_len] = '\0';
	int i;
	for(i = 0; i < msg_len; ++i) {
		int c_idx = 0;
		while(c_idx < 36 && abc[c_idx] != cap(msg[i])) ++c_idx;

		enc_msg[i] = cae_abc[c_idx];
	}
	return enc_msg;
}

// argv[1] Key, argv[2] Message
int main(int argc, char *argv[]){
	char def_abc[36]= "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
	char enc_abc[36]; // new alphabet
	char new_key[36]; // new key
	int i; for(i = 0; i < 36; ++i) enc_abc[i] = new_key[i] = '\0'; // Clear the strings

	// Count message length
		int m_len = 0; while(argv[2][m_len] != '\0') ++m_len;

	// Count key length and capitalize it
		int key_len = 0; i = 0;
		while(argv[1][i] != '\0') addToSet(new_key, &key_len, cap(argv[1][i++]));

	// Create the encrypted alphabet
		int enc_abc_len = key_len;
		for(i = 0; i < key_len; ++i) enc_abc[i] = new_key[i]; // Copy the key to start
		for(i = 0; i < 36 && enc_abc_len < 36; ++i) addToSet(enc_abc, &enc_abc_len, def_abc[i]); // Copy the default alphabet
		printf("Encrypted Alphabet:\t%s\n\n", enc_abc);
	
	// Encrypt and print
		char* enc_msg = caesarEncrypt(argv[2], m_len, def_abc, enc_abc);
		printf("Encrypted Message:\t%s\n", enc_msg);
		printf("De-encrypted Message:\t%s\n", caesarEncrypt(enc_msg, m_len, enc_abc, def_abc));
	return 0;
}