1// SPDX-License-Identifier: GPL-2.0 OR MIT
2/*
3 * Copyright (C) 2015-2020 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
4 *
5 * This is a specialized constant-time base64/hex implementation that resists side-channel attacks.
6 */
7
8#include <string.h>
9#include "encoding.h"
10
11static inline void encode_base64(char dest[static 4], const uint8_t src[static 3])
12{
13	const uint8_t input[] = { (src[0] >> 2) & 63, ((src[0] << 4) | (src[1] >> 4)) & 63, ((src[1] << 2) | (src[2] >> 6)) & 63, src[2] & 63 };
14
15	for (unsigned int i = 0; i < 4; ++i)
16		dest[i] = input[i] + 'A'
17			  + (((25 - input[i]) >> 8) & 6)
18			  - (((51 - input[i]) >> 8) & 75)
19			  - (((61 - input[i]) >> 8) & 15)
20			  + (((62 - input[i]) >> 8) & 3);
21
22}
23
24void key_to_base64(char base64[static WG_KEY_LEN_BASE64], const uint8_t key[static WG_KEY_LEN])
25{
26	unsigned int i;
27
28	for (i = 0; i < WG_KEY_LEN / 3; ++i)
29		encode_base64(&base64[i * 4], &key[i * 3]);
30	encode_base64(&base64[i * 4], (const uint8_t[]){ key[i * 3 + 0], key[i * 3 + 1], 0 });
31	base64[WG_KEY_LEN_BASE64 - 2] = '=';
32	base64[WG_KEY_LEN_BASE64 - 1] = '\0';
33}
34
35static inline int decode_base64(const char src[static 4])
36{
37	int val = 0;
38
39	for (unsigned int i = 0; i < 4; ++i)
40		val |= (-1
41			    + ((((('A' - 1) - src[i]) & (src[i] - ('Z' + 1))) >> 8) & (src[i] - 64))
42			    + ((((('a' - 1) - src[i]) & (src[i] - ('z' + 1))) >> 8) & (src[i] - 70))
43			    + ((((('0' - 1) - src[i]) & (src[i] - ('9' + 1))) >> 8) & (src[i] + 5))
44			    + ((((('+' - 1) - src[i]) & (src[i] - ('+' + 1))) >> 8) & 63)
45			    + ((((('/' - 1) - src[i]) & (src[i] - ('/' + 1))) >> 8) & 64)
46			) << (18 - 6 * i);
47	return val;
48}
49
50bool key_from_base64(uint8_t key[static WG_KEY_LEN], const char *base64)
51{
52	unsigned int i;
53	volatile uint8_t ret = 0;
54	int val;
55
56	if (strlen(base64) != WG_KEY_LEN_BASE64 - 1 || base64[WG_KEY_LEN_BASE64 - 2] != '=')
57		return false;
58
59	for (i = 0; i < WG_KEY_LEN / 3; ++i) {
60		val = decode_base64(&base64[i * 4]);
61		ret |= (uint32_t)val >> 31;
62		key[i * 3 + 0] = (val >> 16) & 0xff;
63		key[i * 3 + 1] = (val >> 8) & 0xff;
64		key[i * 3 + 2] = val & 0xff;
65	}
66	val = decode_base64((const char[]){ base64[i * 4 + 0], base64[i * 4 + 1], base64[i * 4 + 2], 'A' });
67	ret |= ((uint32_t)val >> 31) | (val & 0xff);
68	key[i * 3 + 0] = (val >> 16) & 0xff;
69	key[i * 3 + 1] = (val >> 8) & 0xff;
70
71	return 1 & ((ret - 1) >> 8);
72}
73
74void key_to_hex(char hex[static WG_KEY_LEN_HEX], const uint8_t key[static WG_KEY_LEN])
75{
76	unsigned int i;
77
78	for (i = 0; i < WG_KEY_LEN; ++i) {
79		hex[i * 2] = 87U + (key[i] >> 4) + ((((key[i] >> 4) - 10U) >> 8) & ~38U);
80		hex[i * 2 + 1] = 87U + (key[i] & 0xf) + ((((key[i] & 0xf) - 10U) >> 8) & ~38U);
81	}
82	hex[i * 2] = '\0';
83}
84
85bool key_from_hex(uint8_t key[static WG_KEY_LEN], const char *hex)
86{
87	uint8_t c, c_acc, c_alpha0, c_alpha, c_num0, c_num, c_val;
88	volatile uint8_t ret = 0;
89
90	if (strlen(hex) != WG_KEY_LEN_HEX - 1)
91		return false;
92
93	for (unsigned int i = 0; i < WG_KEY_LEN_HEX - 1; i += 2) {
94		c = (uint8_t)hex[i];
95		c_num = c ^ 48U;
96		c_num0 = (c_num - 10U) >> 8;
97		c_alpha = (c & ~32U) - 55U;
98		c_alpha0 = ((c_alpha - 10U) ^ (c_alpha - 16U)) >> 8;
99		ret |= ((c_num0 | c_alpha0) - 1) >> 8;
100		c_val = (c_num0 & c_num) | (c_alpha0 & c_alpha);
101		c_acc = c_val * 16U;
102
103		c = (uint8_t)hex[i + 1];
104		c_num = c ^ 48U;
105		c_num0 = (c_num - 10U) >> 8;
106		c_alpha = (c & ~32U) - 55U;
107		c_alpha0 = ((c_alpha - 10U) ^ (c_alpha - 16U)) >> 8;
108		ret |= ((c_num0 | c_alpha0) - 1) >> 8;
109		c_val = (c_num0 & c_num) | (c_alpha0 & c_alpha);
110		key[i / 2] = c_acc | c_val;
111	}
112
113	return 1 & ((ret - 1) >> 8);
114}
115
116bool key_is_zero(const uint8_t key[static WG_KEY_LEN])
117{
118	volatile uint8_t acc = 0;
119
120	for (unsigned int i = 0; i < WG_KEY_LEN; ++i) {
121		acc |= key[i];
122		asm volatile("" : "=r"(acc) : "0"(acc));
123	}
124	return 1 & ((acc - 1) >> 8);
125}
126