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crypto/shamir: Fix edge case when shamir_interpolate() is called with duplicate share indices which are equal to the result index.

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This commit is contained in:
Andrew Kozlik 2019-05-02 18:44:56 +02:00
parent 0a99ad3f13
commit 223868f552
2 changed files with 22 additions and 11 deletions
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21
crypto/shamir.c
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@ -69,7 +69,7 @@ static void unbitslice(uint8_t *r, const uint32_t x[8], size_t len) {
static void bitslice_setall(uint32_t r[8], const uint8_t x) {
size_t idx;
for (idx = 0; idx < 8; idx++) {
r[idx] = ((int32_t)((x & (1 << idx)) << (31 - idx))) >> 31;
r[idx] = -((x >> idx) & 1);
}
}
@ -271,14 +271,6 @@ bool shamir_interpolate(uint8_t *result, uint8_t result_index,
if (len > SHAMIR_MAX_LEN) return false;
/* The code below assumes that none of the share_indices are equal to
* result_index. We need to treat that as a special case. */
for (i = 0; i < share_count; i++)
if (share_indices[i] == result_index) {
memcpy(result, share_values[i], len);
return true;
}
/* Collect the x and y values */
for (i = 0; i < share_count; i++) {
bitslice_setall(xs[i], share_indices[i]);
@ -294,8 +286,15 @@ bool shamir_interpolate(uint8_t *result, uint8_t result_index,
/* Use Lagrange basis polynomials to calculate the secret coefficient */
for (i = 0; i < share_count; i++) {
memcpy(denom, x, sizeof(denom));
gf256_add(denom, xs[i]);
/* The code below assumes that none of the share_indices are equal to
* result_index. We need to treat that as a special case. */
if (share_indices[i] != result_index) {
memcpy(denom, x, sizeof(denom));
gf256_add(denom, xs[i]);
} else {
bitslice_setall(denom, 1);
gf256_add(secret, ys[i]);
}
for (j = 0; j < share_count; j++) {
if (i == j) continue;
memcpy(tmp, xs[i], sizeof(uint32_t[8]));

12
crypto/tests/test_check.c
View File

@ -5120,6 +5120,18 @@ START_TEST(test_shamir) {
7,
true},
{{0},
1,
{5, 1, 1},
{
{129, 18, 104, 86, 236, 73, 176},
{91, 188, 226, 91, 254, 197, 225},
{69, 53, 151, 204, 224, 37, 19},
},
3,
7,
false},
{{0},
255,
{3, 12, 3},