Add base32 decoder

Makefile

@@ -17,7 +17,7 @@
 P =	otpcli
 V =	0.0
 
-HDR =	err.h strtonum.h otp.h
+HDR =	base32.h err.h strtonum.h otp.h
 OBJ =	cli.o ${HDR:.h=.o}
 SRC =	${OBJ:.o=.c}
 

base32.c

@@ -0,0 +1,101 @@
+/*
+ * otpcli - CLI utility for generating OTPs
+ *
+ * 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;
+}

base32.h

@@ -0,0 +1,2 @@
+int b32_decoded_len(const char *, size_t);
+int b32_decode(unsigned char *, size_t, const char *, size_t);