Simplify missing cell handling in recovery

src/common/fr.c

@@ -50,18 +50,6 @@ bool fr_is_one(const fr_t *p) {
     return a[0] == 1 && a[1] == 0 && a[2] == 0 && a[3] == 0;
 }
 
-/**
- * Test whether the operand is null (all 0xff's).
- *
- * @param[in]   p   The field element to be checked
- *
- * @retval  true    The element is null
- * @retval  false   The element is not null
- */
-bool fr_is_null(const fr_t *p) {
-    return fr_equal(p, &FR_NULL);
-}
-
 /**
  * Divide a field element by another.
  *

src/common/fr.h

@@ -48,11 +48,6 @@ static const fr_t FR_ONE = {
     {0x00000001fffffffeL, 0x5884b7fa00034802L, 0x998c4fefecbc4ff5L, 0x1824b159acc5056fL}
 };
 
-/** This used to represent a missing element. It's an invalid value. */
-static const fr_t FR_NULL = {
-    {0xffffffffffffffffL, 0xffffffffffffffffL, 0xffffffffffffffffL, 0xffffffffffffffffL}
-};
-
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 // Public Functions
 ////////////////////////////////////////////////////////////////////////////////////////////////////
@@ -63,7 +58,6 @@ extern "C" {
 
 bool fr_equal(const fr_t *a, const fr_t *b);
 bool fr_is_one(const fr_t *p);
-bool fr_is_null(const fr_t *p);
 void fr_div(fr_t *out, const fr_t *a, const fr_t *b);
 void fr_pow(fr_t *out, const fr_t *a, uint64_t n);
 void fr_from_uint64(fr_t *out, uint64_t n);

src/eip7594/eip7594.c

@@ -206,9 +206,9 @@ C_KZG_RET recover_cells_and_kzg_proofs(
     ret = new_g1_array(&recovered_proofs_g1, CELLS_PER_EXT_BLOB);
     if (ret != C_KZG_OK) goto out;
 
-    /* Initialize all cells as missing */
+    /* Initialize all cells to zero */
     for (size_t i = 0; i < FIELD_ELEMENTS_PER_EXT_BLOB; i++) {
-        recovered_cells_fr[i] = FR_NULL;
+        recovered_cells_fr[i] = FR_ZERO;
     }
 
     /* Populate recovered_cells_fr with available cells at the right places */

src/eip7594/recovery.c

@@ -196,7 +196,6 @@ static bool is_in_array(const uint64_t *arr, size_t arr_size, uint64_t value) {
  *
  * @remark `reconstructed_data_out` and `cells` can point to the same memory.
  * @remark The array `cells` must be in the correct order (according to cell_indices).
- * @remark Missing cells in `cells` should be equal to FR_NULL.
  */
 C_KZG_RET recover_cells(
     fr_t *reconstructed_data_out,
@@ -280,17 +279,7 @@ C_KZG_RET recover_cells(
      * P(x) is the polynomial we want to reconstruct (degree FIELD_ELEMENTS_PER_BLOB - 1).
      */
     for (size_t i = 0; i < FIELD_ELEMENTS_PER_EXT_BLOB; i++) {
-        if (fr_is_null(&cells_brp[i])) {
-            /*
-             * We handle this situation differently because FR_NULL is an invalid value. The right
-             * hand side, vanishing_poly_eval[i], will always be zero when cells_brp[i] is null, so
-             * the multiplication would still be result in zero, but we shouldn't depend on blst
-             * handling invalid values like this.
-             */
-            extended_evaluation_times_zero[i] = FR_ZERO;
-        } else {
-            blst_fr_mul(&extended_evaluation_times_zero[i], &cells_brp[i], &vanishing_poly_eval[i]);
-        }
+        blst_fr_mul(&extended_evaluation_times_zero[i], &cells_brp[i], &vanishing_poly_eval[i]);
     }
 
     /*