Something I created a long time ago while familiarizing myself with bitwise operators.

ezcrpyt.c

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


void encrypt (char * msg, int msg_len, char * key, int key_len) {
    int i, p;
	char c;
	for (i = 0; i < msg_len; i++) {
		c = msg[i];
		p = key [i % key_len] % (msg_len - i) + i;
		msg[i] = msg[p];
		msg[p] = c;
		msg[i] ^= key[p % key_len];
	}
}

void decrypt (char * msg, int msg_len, char * key, int key_len) {
	int i, p;
	char c;
	for (i = msg_len - 1; i > -1; --i) {
		p = key [i % key_len] % (msg_len - i) + i;
		msg[i] ^= key[p % key_len];		
		c = msg[i];
		msg[i] = msg[p];
		msg[p] = c;
	}
}

void print (char * message, int len) {
	int i;
	for (i = 0; i < len; i++) {
		printf("%c", message[i]);
	}
	printf("\n");
}

void encrypt_file (char * fname, char * key) {
	FILE *f = fopen(fname, "rb");
	fseek(f, 0, SEEK_END);
	long pos = ftell(f);
	fseek(f, 0, SEEK_SET);

	char *bytes = (char *)malloc(pos);
	fread(bytes, pos, 1, f);
	fclose(f);

	encrypt(bytes, pos, key, strlen(key));

	FILE *f2 = fopen(fname, "wb");
	fwrite(bytes, pos, 1, f2);
	fclose(f2);

	free(bytes);
}

void decrypt_file (char * fname, char * key) {
	FILE *f = fopen(fname, "rb");
	fseek(f, 0, SEEK_END);
	long pos = ftell(f);
	fseek(f, 0, SEEK_SET);

	char *bytes = (char *)malloc(pos);
	fread(bytes, pos, 1, f);
	fclose(f);

	decrypt(bytes, pos, key, strlen(key));

	FILE *f2 = fopen(fname, "wb");
	fwrite(bytes, pos, 1, f2);
	fclose(f2);

	free(bytes);
}

int main (int args, char ** argv) {

	int i, type = -1, state = 0, pass = -1, file = -1;
	for (i = 1; i < args; i++) {
		if (!strcmp(argv[i], "-f")) {
			state = 0;
		} else if (!strcmp(argv[i], "-p") || !strcmp(argv[i], "-k")) {
			state = 1;
		} else if (!strcmp(argv[i], "-e")) {
			type = 0;
		} else if (!strcmp(argv[i], "-d")) {
			type = 1;
		} else if (state == 0) {
			file = i;
		} else if (state == 1) {
			pass = i;
		}
	}

	if (file == -1) {
		printf ("no file '-f'\n");
		return 1;
	}

	if (pass == -1) {
		printf ("need encrypt '-p' or '-k'");
		return 1;
	}

	if (type == -1) {
		printf("did not define type encrypt '-e' or decrypt '-d'");
	}

	printf("file: %s\nkey: %s\n", argv[file], argv[pass]);

	if (type == 0) {
		encrypt_file(argv[file], argv[pass]);
	} else if (type == 1) {
		decrypt_file(argv[file], argv[pass]);
	}


	return 0;

	
	decrypt_file("test.txt", "test");
	return 0;

	char pwd[] = "password";
	char message[] = "this is a wonderful test that I have created";
	int len = strlen(message);
	encrypt(message, len, pwd, strlen(pwd));
	decrypt(message, len, pwd, strlen(pwd));
	print(message, len);
}