From 9f443bc5aad2146dc7785b00be21a3781d376927 Mon Sep 17 00:00:00 2001
From: George Kadianakis <desnacked@riseup.net>
Date: Tue, 31 Jan 2023 09:33:02 +0100
Subject: [PATCH] test_compute_kzg_proof() now also verifies the KZG proof
 (#101)

Co-authored-by: Justin Traglia <95511699+jtraglia@users.noreply.github.com>
---
 src/c_kzg_4844.c      | 12 +++---------
 src/c_kzg_4844.h      |  5 +++++
 src/test_c_kzg_4844.c | 34 ++++++++++++++++++++++++++++++----
 3 files changed, 38 insertions(+), 13 deletions(-)

diff --git a/src/c_kzg_4844.c b/src/c_kzg_4844.c
index 765ac88..f1050f4 100644
--- a/src/c_kzg_4844.c
+++ b/src/c_kzg_4844.c
@@ -34,12 +34,6 @@
 #define STATIC static
 #endif /* defined(UNIT_TESTS) */
 
-///////////////////////////////////////////////////////////////////////////////
-// Types
-///////////////////////////////////////////////////////////////////////////////
-
-typedef struct { fr_t evals[FIELD_ELEMENTS_PER_BLOB]; } Polynomial;
-
 ///////////////////////////////////////////////////////////////////////////////
 // Constants
 ///////////////////////////////////////////////////////////////////////////////
@@ -623,7 +617,7 @@ STATIC void hash_to_bls_field(fr_t *out, const Bytes32 *b) {
  * @retval C_KZG_OK      Deserialization successful
  * @retval C_KZG_BADARGS Input was not a valid scalar field element
  */
-static C_KZG_RET bytes_to_bls_field(fr_t *out, const Bytes32 *b) {
+STATIC C_KZG_RET bytes_to_bls_field(fr_t *out, const Bytes32 *b) {
     blst_scalar tmp;
     blst_scalar_from_lendian(&tmp, b->bytes);
     if (!blst_scalar_fr_check(&tmp)) return C_KZG_BADARGS;
@@ -697,7 +691,7 @@ static C_KZG_RET bytes_to_kzg_proof(g1_t *out, const Bytes48 *b) {
  * @retval C_KZG_OK      Deserialization successful
  * @retval C_KZG_BADARGS Invalid input bytes
  */
-static C_KZG_RET blob_to_polynomial(Polynomial *p, const Blob *blob) {
+STATIC C_KZG_RET blob_to_polynomial(Polynomial *p, const Blob *blob) {
     C_KZG_RET ret;
     for (size_t i = 0; i < FIELD_ELEMENTS_PER_BLOB; i++) {
         ret = bytes_to_bls_field(&p->evals[i], (Bytes32 *)&blob->bytes[i * BYTES_PER_FIELD_ELEMENT]);
@@ -908,7 +902,7 @@ static void compute_powers(fr_t *out, fr_t *x, uint64_t n) {
  * @retval C_KZG_OK Evaluation successful
  * @retval C_KZG_MALLOC Memory allocation failed
  */
-static C_KZG_RET evaluate_polynomial_in_evaluation_form(fr_t *out, const Polynomial *p, const fr_t *x, const KZGSettings *s) {
+STATIC C_KZG_RET evaluate_polynomial_in_evaluation_form(fr_t *out, const Polynomial *p, const fr_t *x, const KZGSettings *s) {
     C_KZG_RET ret;
     fr_t tmp;
     fr_t *inverses_in = NULL;
diff --git a/src/c_kzg_4844.h b/src/c_kzg_4844.h
index 5370877..3baf530 100644
--- a/src/c_kzg_4844.h
+++ b/src/c_kzg_4844.h
@@ -124,10 +124,15 @@ C_KZG_RET compute_kzg_proof(KZGProof *out,
                             const Bytes32 *z_bytes,
                             const KZGSettings *s);
 
+typedef struct { fr_t evals[FIELD_ELEMENTS_PER_BLOB]; } Polynomial;
+
 #ifdef UNIT_TESTS
 
 void hash_to_bls_field(fr_t *out, const Bytes32 *b);
 void bytes_from_bls_field(Bytes32 *out, const fr_t *in);
+C_KZG_RET evaluate_polynomial_in_evaluation_form(fr_t *out, const Polynomial *p, const fr_t *x, const KZGSettings *s);
+C_KZG_RET blob_to_polynomial(Polynomial *p, const Blob *blob);
+C_KZG_RET bytes_to_bls_field(fr_t *out, const Bytes32 *b);
 
 #endif
 
diff --git a/src/test_c_kzg_4844.c b/src/test_c_kzg_4844.c
index 49b61da..a88e7a0 100644
--- a/src/test_c_kzg_4844.c
+++ b/src/test_c_kzg_4844.c
@@ -187,23 +187,49 @@ static void test_blob_to_kzg_commitment__succeeds_consistent_commitment(void) {
 // Tests for compute_kzg_proof
 ///////////////////////////////////////////////////////////////////////////////
 
-static void test_compute_kzg_proof(void) {
+static void test_compute_and_verify_kzg_proof(void) {
     C_KZG_RET ret;
     Bytes48 proof;
-    Bytes32 z;
+    Bytes32 z, y;
     KZGCommitment c;
     Blob blob;
+    Polynomial poly;
+    fr_t y_fr, z_fr;
+    bool ok;
 
+    /* Some preparation */
     get_rand_field_element(&z);
     get_rand_blob(&blob);
 
     ret = blob_to_kzg_commitment(&c, &blob, &s);
     ASSERT_EQUALS(ret, C_KZG_OK);
 
+    /* Compute the proof */
     ret = compute_kzg_proof(&proof, &blob, &z, &s);
     ASSERT_EQUALS(ret, C_KZG_OK);
 
-    // XXX now verify it!
+    /* Now let's attempt to verify the proof */
+    /* First convert the blob to field elements */
+    ret = blob_to_polynomial(&poly, &blob);
+    ASSERT_EQUALS(ret, C_KZG_OK);
+
+    /* Also convert z to a field element */
+    ret = bytes_to_bls_field(&z_fr, &z);
+    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);
 }
 
 ///////////////////////////////////////////////////////////////////////////////
@@ -234,7 +260,7 @@ int main(void) {
     RUN(test_blob_to_kzg_commitment__fails_x_greater_than_modulus);
     RUN(test_blob_to_kzg_commitment__succeeds_point_at_infinity);
     RUN(test_blob_to_kzg_commitment__succeeds_consistent_commitment);
-    RUN(test_compute_kzg_proof);
+    RUN(test_compute_and_verify_kzg_proof);
     teardown();
 
     return TEST_REPORT();