otpcli/base32.c

/*
 * otpcli - command-line interface for HOTP and TOTP
 * Written in 2021 by Lucas
 *
 * To the extent possible under law, the author(s) have dedicated all
 * copyright and related and neighboring rights to this software to the
 * public domain worldwide. This software is distributed without any
 * warranty.
 *
 * You should have received a copy of the CC0 Public Domain Dedication
 * along with this software. If not, see
 * <http://creativecommons.org/publicdomain/zero/1.0/>.
 */

#include <limits.h>
#include <stdint.h>
#include <string.h>

#include "base32.h"

#define B32_VALID(a) (('A' <= (a) && (a) <= 'Z') || ('2' <= (a) && (a) <= '7'))
#define B32_DECODE_CHAR(a) ((a) <= '7' ? (a) - '2' + 26 : (a) - 'A')

static int b32_last_chunk_length[] = { 0, -1, 1, -1, 2, 3, -1, 4 };

static void
b32_decode_chunk(unsigned char out[5], const char in[8])
{
	uint64_t x = 0;

	x |= (uint64_t)B32_DECODE_CHAR(in[0]) << 35;
	x |= (uint64_t)B32_DECODE_CHAR(in[1]) << 30;
	x |= (uint64_t)B32_DECODE_CHAR(in[2]) << 25;
	x |= (uint64_t)B32_DECODE_CHAR(in[3]) << 20;
	x |= (uint64_t)B32_DECODE_CHAR(in[4]) << 15;
	x |= (uint64_t)B32_DECODE_CHAR(in[5]) << 10;
	x |= (uint64_t)B32_DECODE_CHAR(in[6]) << 5;
	x |= (uint64_t)B32_DECODE_CHAR(in[7]);

	out[0] = (x >> 32) & 0xff;
	out[1] = (x >> 24) & 0xff;
	out[2] = (x >> 16) & 0xff;
	out[3] = (x >> 8) & 0xff;
	out[4] = x & 0xff;
}

int
b32_decoded_len(const char *s, size_t n)
{
	const char *it, *t;
	size_t r, eqc;

	for (it = s; *it != '\0' && B32_VALID(*it); it++)
		continue;
	if (*it != '\0' && *it != '=')
		return -1;

	for (t = it; *t == '=' && t - it < 8; t++)
		continue;
	eqc = t - it;
	if (*t != '\0' || t - it == 8 || b32_last_chunk_length[eqc] == -1)
		return -1;

	r = n / 8 * 5 + b32_last_chunk_length[eqc == 0 ? n % 8 : eqc];
	return r > INT_MAX ? -1 : r;
}

int
b32_decode(unsigned char *out, size_t outlen, const char *in, size_t inlen)
{
	unsigned char tout[5];
	char tin[8];
	size_t i, t, eqc, last_chunk;

	t = inlen;
	while (in[inlen - 1] == '=')
		inlen--;
	eqc = t - inlen;
	last_chunk = b32_last_chunk_length[eqc == 0 ? inlen % 8 : eqc];

	while (inlen >= 8 && outlen >= 5) {
		b32_decode_chunk(out, in);
		in += 8;
		inlen -= 8;
		out += 5;
		outlen -= 5;
	}

	if (inlen > 0) {
		if (outlen == 0)
			return 0;

		memcpy(tin, in, inlen);
		memset(tin + inlen, 'A', sizeof(tin) - inlen);
		b32_decode_chunk(tout, tin);
		memcpy(out, tout, outlen);
	}

	return outlen >= last_chunk;
}