1 /* Copyright 2014, Kenneth MacKay. Licensed under the BSD 2-clause license.
2
3 Modifications Copyright 2020, Espressif Systems (Shanghai) PTE LTD. Licensed under the BSD
4 2-clause license.
5 */
6
7 /* uECC_verify() calls a number of static functions form here and
8 uses other definitions, so we just build that whole source file here and then append
9 our modified version uECC_verify_antifault(). */
10 #include "micro-ecc/uECC.c"
11
12 /* Version of uECC_verify() which also copies message_hash into verified_hash,
13 but only if the signature is valid. Does this in an FI resistant way.
14 */
uECC_verify_antifault(const uint8_t * public_key,const uint8_t * message_hash,unsigned hash_size,const uint8_t * signature,uECC_Curve curve,uint8_t * verified_hash)15 int uECC_verify_antifault(const uint8_t *public_key,
16 const uint8_t *message_hash,
17 unsigned hash_size,
18 const uint8_t *signature,
19 uECC_Curve curve,
20 uint8_t *verified_hash) {
21 uECC_word_t u1[uECC_MAX_WORDS], u2[uECC_MAX_WORDS];
22 uECC_word_t z[uECC_MAX_WORDS];
23 uECC_word_t sum[uECC_MAX_WORDS * 2];
24 uECC_word_t rx[uECC_MAX_WORDS];
25 uECC_word_t ry[uECC_MAX_WORDS];
26 uECC_word_t tx[uECC_MAX_WORDS];
27 uECC_word_t ty[uECC_MAX_WORDS];
28 uECC_word_t tz[uECC_MAX_WORDS];
29 const uECC_word_t *points[4];
30 const uECC_word_t *point;
31 bitcount_t num_bits;
32 bitcount_t i;
33 #if uECC_VLI_NATIVE_LITTLE_ENDIAN
34 uECC_word_t *_public = (uECC_word_t *)public_key;
35 #else
36 uECC_word_t _public[uECC_MAX_WORDS * 2];
37 #endif
38 uECC_word_t r[uECC_MAX_WORDS], s[uECC_MAX_WORDS];
39 wordcount_t num_words = curve->num_words;
40 wordcount_t num_n_words = BITS_TO_WORDS(curve->num_n_bits);
41
42 rx[num_n_words - 1] = 0;
43 r[num_n_words - 1] = 0;
44 s[num_n_words - 1] = 0;
45
46 #if uECC_VLI_NATIVE_LITTLE_ENDIAN
47 bcopy((uint8_t *) r, signature, curve->num_bytes);
48 bcopy((uint8_t *) s, signature + curve->num_bytes, curve->num_bytes);
49 #else
50 uECC_vli_bytesToNative(_public, public_key, curve->num_bytes);
51 uECC_vli_bytesToNative(
52 _public + num_words, public_key + curve->num_bytes, curve->num_bytes);
53 uECC_vli_bytesToNative(r, signature, curve->num_bytes);
54 uECC_vli_bytesToNative(s, signature + curve->num_bytes, curve->num_bytes);
55 #endif
56
57 /* r, s must not be 0. */
58 if (uECC_vli_isZero(r, num_words) || uECC_vli_isZero(s, num_words)) {
59 return 0;
60 }
61
62 /* r, s must be < n. */
63 if (uECC_vli_cmp(curve->n, r, num_n_words) != 1 ||
64 uECC_vli_cmp(curve->n, s, num_n_words) != 1) {
65 return 0;
66 }
67
68 /* Calculate u1 and u2. */
69 uECC_vli_modInv(z, s, curve->n, num_n_words); /* z = 1/s */
70 u1[num_n_words - 1] = 0;
71 bits2int(u1, message_hash, hash_size, curve);
72 uECC_vli_modMult(u1, u1, z, curve->n, num_n_words); /* u1 = e/s */
73 uECC_vli_modMult(u2, r, z, curve->n, num_n_words); /* u2 = r/s */
74
75 /* Calculate sum = G + Q. */
76 uECC_vli_set(sum, _public, num_words);
77 uECC_vli_set(sum + num_words, _public + num_words, num_words);
78 uECC_vli_set(tx, curve->G, num_words);
79 uECC_vli_set(ty, curve->G + num_words, num_words);
80 uECC_vli_modSub(z, sum, tx, curve->p, num_words); /* z = x2 - x1 */
81 XYcZ_add(tx, ty, sum, sum + num_words, curve);
82 uECC_vli_modInv(z, z, curve->p, num_words); /* z = 1/z */
83 apply_z(sum, sum + num_words, z, curve);
84
85 /* Use Shamir's trick to calculate u1*G + u2*Q */
86 points[0] = 0;
87 points[1] = curve->G;
88 points[2] = _public;
89 points[3] = sum;
90 num_bits = smax(uECC_vli_numBits(u1, num_n_words),
91 uECC_vli_numBits(u2, num_n_words));
92
93 point = points[(!!uECC_vli_testBit(u1, num_bits - 1)) |
94 ((!!uECC_vli_testBit(u2, num_bits - 1)) << 1)];
95 uECC_vli_set(rx, point, num_words);
96 uECC_vli_set(ry, point + num_words, num_words);
97 uECC_vli_clear(z, num_words);
98 z[0] = 1;
99
100 for (i = num_bits - 2; i >= 0; --i) {
101 uECC_word_t index;
102 curve->double_jacobian(rx, ry, z, curve);
103
104 index = (!!uECC_vli_testBit(u1, i)) | ((!!uECC_vli_testBit(u2, i)) << 1);
105 point = points[index];
106 if (point) {
107 uECC_vli_set(tx, point, num_words);
108 uECC_vli_set(ty, point + num_words, num_words);
109 apply_z(tx, ty, z, curve);
110 uECC_vli_modSub(tz, rx, tx, curve->p, num_words); /* Z = x2 - x1 */
111 XYcZ_add(tx, ty, rx, ry, curve);
112 uECC_vli_modMult_fast(z, z, tz, curve);
113 }
114 }
115
116 uECC_vli_modInv(z, z, curve->p, num_words); /* Z = 1/Z */
117 apply_z(rx, ry, z, curve);
118
119 /* v = x1 (mod n) */
120 if (uECC_vli_cmp(curve->n, rx, num_n_words) != 1) {
121 uECC_vli_sub(rx, rx, curve->n, num_n_words);
122 }
123
124 /* Anti-FI addition. Copy message_hash into verified_hash, but do it in a
125 way that it will only happen if v == r (ie, rx == r)
126 */
127 const uECC_word_t *mhash_words = (const uECC_word_t *)message_hash;
128 uECC_word_t *vhash_words = (uECC_word_t *)verified_hash;
129 unsigned hash_words = hash_size / sizeof(uECC_word_t);
130 for (unsigned int w = 0; w < hash_words; w++) {
131 /* note: using curve->num_words here to encourage compiler to re-read this variable */
132 vhash_words[w] = mhash_words[w] ^ rx[w % curve->num_words] ^ r[w % curve->num_words];
133 }
134 /* Curve may be longer than hash, in which case keep reading the rest of the bytes */
135 for (int w = hash_words; w < curve->num_words; w++) {
136 vhash_words[w % hash_words] |= rx[w] ^ r[w];
137 }
138
139 /* Accept only if v == r. */
140 return (int)(uECC_vli_equal(rx, r, num_words));
141 }
142