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c-kzg-4844
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Fix compute_kzg_proof_impl() when `z` is inside the domain (#111) 

* Fix compute_kzg_proof_impl() when `z` is inside the domain

* Satisfy linter D:
This commit is contained in:
George Kadianakis 2023-02-01 21:32:39 +02:00 committed by GitHub
parent e5fa8c7ee1
commit e36c11dfe2
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2 changed files with 56 additions and 4 deletions
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9
src/c_kzg_4844.c
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@ -1103,7 +1103,9 @@ C_KZG_RET compute_kzg_proof_impl(KZGProof *out, const Polynomial *polynomial, co
for (i = 0; i < FIELD_ELEMENTS_PER_BLOB; i++) {
if (fr_equal(z, &roots_of_unity[i])) {
/* We are asked to compute a KZG proof inside the domain */
m = i + 1;
inverses_in[i] = FR_ONE;
continue;
}
// (p_i - y) / (ω_i - z)
@ -1122,15 +1124,20 @@ C_KZG_RET compute_kzg_proof_impl(KZGProof *out, const Polynomial *polynomial, co
q.evals[--m] = FR_ZERO;
for (i = 0; i < FIELD_ELEMENTS_PER_BLOB; i++) {
if (i == m) continue;
// (p_i - y) * ω_i / (z * (z - ω_i))
/* Build denominator: z * (z - ω_i) */
blst_fr_sub(&tmp, z, &roots_of_unity[i]);
blst_fr_mul(&inverses_in[i], &tmp, z);
}
ret = fr_batch_inv(inverses, inverses_in, FIELD_ELEMENTS_PER_BLOB);
if (ret != C_KZG_OK) goto out;
for (i = 0; i < FIELD_ELEMENTS_PER_BLOB; i++) {
if (i == m) continue;
/* Build numerator: ω_i * (p_i - y) */
blst_fr_sub(&tmp, &polynomial->evals[i], &y);
blst_fr_mul(&tmp, &tmp, &roots_of_unity[i]);
/* Do the division: (p_i - y) * ω_i / (z * (z - ω_i)) */
blst_fr_mul(&tmp, &tmp, &inverses[i]);
blst_fr_add(&q.evals[m], &q.evals[m], &tmp);
}

51
src/test_c_kzg_4844.c
View File

@ -421,7 +421,8 @@ static void test_log_2_byte__expected_values(void) {
while (true) {
/*
* Corresponds to the index of the highest bit set in the byte.
* Adapted from https://graphics.stanford.edu/~seander/bithacks.html#IntegerLog.
* Adapted from
* https://graphics.stanford.edu/~seander/bithacks.html#IntegerLog.
*/
byte b = i;
int r, shift;
@ -442,7 +443,7 @@ static void test_log_2_byte__expected_values(void) {
// Tests for compute_kzg_proof
///////////////////////////////////////////////////////////////////////////////
static void test_compute_and_verify_kzg_proof(void) {
static void test_compute_and_verify_kzg_proof__succeeds_round_trip(void) {
C_KZG_RET ret;
Bytes48 proof;
Bytes32 z, y;
@ -487,6 +488,49 @@ static void test_compute_and_verify_kzg_proof(void) {
ASSERT_EQUALS(ok, 1);
}
static void test_compute_and_verify_kzg_proof__succeeds_within_domain(void) {
const int SAMPLES = 25;
for (int i = 0; i < SAMPLES; i++) {
C_KZG_RET ret;
Blob blob;
KZGCommitment c;
Polynomial poly;
Bytes48 proof;
Bytes32 z, y;
fr_t y_fr, z_fr;
bool ok;
get_rand_blob(&blob);
ret = blob_to_kzg_commitment(&c, &blob, &s);
ASSERT_EQUALS(ret, C_KZG_OK);
ret = blob_to_polynomial(&poly, &blob);
ASSERT_EQUALS(ret, C_KZG_OK);
z_fr = s.fs->roots_of_unity[i];
bytes_from_bls_field(&z, &z_fr);
/* Compute the proof */
ret = compute_kzg_proof(&proof, &blob, &z, &s);
ASSERT_EQUALS(ret, C_KZG_OK);
/* Now evaluate the poly at `z` to learn `y` */
ret = evaluate_polynomial_in_evaluation_form(&y_fr, &poly, &z_fr, &s);
ASSERT_EQUALS(ret, C_KZG_OK);
/* Now also get `y` in bytes */
bytes_from_bls_field(&y, &y_fr);
/* Finally verify the proof */
ret = verify_kzg_proof(&ok, &c, &z, &y, &proof, &s);
ASSERT_EQUALS(ret, C_KZG_OK);
/* The proof should verify! */
ASSERT_EQUALS(ok, 1);
}
}
///////////////////////////////////////////////////////////////////////////////
// Main logic
///////////////////////////////////////////////////////////////////////////////
@ -532,7 +576,8 @@ int main(void) {
RUN(test_reverse_bits__some_bits_are_one);
RUN(test_reverse_bits__all_bits_are_one);
RUN(test_log_2_byte__expected_values);
RUN(test_compute_and_verify_kzg_proof);
RUN(test_compute_and_verify_kzg_proof__succeeds_round_trip);
RUN(test_compute_and_verify_kzg_proof__succeeds_within_domain);
teardown();
return TEST_REPORT();