gistfile1.c

/* Polyalphabetic encryption - Enigma-like. */

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

char *alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";

unsigned long rand = 42;

#define RNG_M 4294967296
#define RNG_C 12345
#define RNG_A 1103515245

unsigned long rng() {
	rand = (rand * RNG_A + RNG_C) % RNG_M;
	return rand;
}

void gen_table(char *dst, char *src) {
	int iteration = 0;
	unsigned char g, uniq[36];
	memset(&uniq[0], 0, 36);
	while (iteration < 36) {
		g = rng() % 36;
		if (uniq[g] == 0) {
			if (src != 0)
				src[iteration] = g;
			if (dst != 0)
				dst[g] = iteration;
			uniq[g] = 1;
			iteration++;
		}
	}
}

void gen_encypher_table(char *t) {
	gen_table(&t[0], 0L);
}

void gen_decypher_table(char *t) {
	gen_table(0L, &t[0]);
}

void cypher(char *p, int r, void (*table_fn)(char *)) {
	char buffer[36];
	int val;
	rand = r;
	while (*p) {
		table_fn(&buffer[0]);
		if (*p >= 'A' && *p <= 'Z') {
			val = *p - 'A';
		} else if (*p >= '0' && *p <= '9') {
			val = 26 + (*p - '0');
		}
		*p = alphabet[buffer[val]];
		++p;
	}
}

void test_gen_table() {
	int i, j;
	char buffer[36];
	for (i = 0; i < 30; i++) {
		gen_table(&buffer[0], 0L);
		for (j = 0; j < 36; j++) {
			putchar(alphabet[buffer[j]]);
		}
		putchar('\n');
	}
}

int main(int argc, char **argv) {
	int r, s;
	char *plaintext;
	char mode;
	void (*table)(char *);

	if (argc < 3) {
		printf("usage: polyalphabetic [mode - e/d] [plaintext] [starting rotor position]");
		return 0;
	}

	mode = *argv[1];
	plaintext = argv[2];
	sscanf(argv[3], "%d", &r); /* rotor starting position */

	if (mode == 'e')
		table = gen_encypher_table;
	else
		table = gen_decypher_table;

	cypher(plaintext, r, table);

	printf(plaintext);
}