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Ensure cell indices are sorted in recover_cells_and_kzg_proofs #594

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Merged
asn-d6 merged 6 commits into from
Sep 3, 2025
+291 −14
Merged

Ensure cell indices are sorted in recover_cells_and_kzg_proofs #594

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7900b96
Fix recover_cells_and_kzg_proofs to support shuffled indices
jtraglia Aug 22, 2025
cc1064e
Check that cell indices are in ascending order
jtraglia Aug 26, 2025
d8c7e42
Remove now unnecessary duplicate check
jtraglia Aug 26, 2025
bfc068e
Update src/eip7594/eip7594.c
asn-d6 Aug 27, 2025
65ebfdd
recover_cells_and_kzg_proofs(): Don't introduce new index
asn-d6 Aug 29, 2025
022801d
fixup! recover_cells_and_kzg_proofs(): Don't introduce new index
asn-d6 Sep 2, 2025
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25 changes: 11 additions & 14 deletions src/eip7594/eip7594.c
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Original file line number Diff line number Diff line change
Expand Up @@ -187,12 +187,17 @@ C_KZG_RET recover_cells_and_kzg_proofs(
goto out;
}

/* Check that cell indices are valid */
for (size_t i = 0; i < num_cells; i++) {
/* Check that cell indices are valid */
if (cell_indices[i] >= CELLS_PER_EXT_BLOB) {
ret = C_KZG_BADARGS;
goto out;
}
/* Check that indices are in strictly ascending order */
if (i > 0 && cell_indices[i] <= cell_indices[i - 1]) {
ret = C_KZG_BADARGS;
goto out;
}
}

/* Do allocations */
Expand All @@ -210,19 +215,8 @@ C_KZG_RET recover_cells_and_kzg_proofs(
for (size_t i = 0; i < num_cells; i++) {
size_t index = cell_indices[i] * FIELD_ELEMENTS_PER_CELL;
for (size_t j = 0; j < FIELD_ELEMENTS_PER_CELL; j++) {
fr_t *ptr = &recovered_cells_fr[index + j];

/*
* Check if the field has already been set. If it has, there was a duplicate cell index
* and we can return an error. The compiler will optimize this and the overhead is
* practically zero.
*/
if (!fr_is_null(ptr)) {
ret = C_KZG_BADARGS;
goto out;
}

/* Convert the untrusted input bytes to a field element */
fr_t *ptr = &recovered_cells_fr[index + j];
size_t offset = j * BYTES_PER_FIELD_ELEMENT;
ret = bytes_to_bls_field(ptr, (const Bytes32 *)&cells[i].bytes[offset]);
if (ret != C_KZG_OK) goto out;
Expand All @@ -231,7 +225,10 @@ C_KZG_RET recover_cells_and_kzg_proofs(

if (num_cells == CELLS_PER_EXT_BLOB) {
/* Nothing to recover, copy the cells */
memcpy(recovered_cells, cells, CELLS_PER_EXT_BLOB * sizeof(Cell));
for (size_t i = 0; i < CELLS_PER_EXT_BLOB; i++) {
uint64_t index = cell_indices[i];
recovered_cells[index] = cells[i];
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@asn-d6 asn-d6 Aug 27, 2025 •
edited
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I see why this is a bit nicer than the blind memcpy, but IMO it's also a bit confusing.

It's confusing because at this point we are past all the initial checks and we also have num_cells == CELLS_PER_EXT_BLOB, and hence we MUST be certain that cell_indices = [0, 1, ..., CELLS_PER_EXT_BLOB-1].

The problem is that this new code introduces index and sorta implies it's a free ranging index, whereas in reality it should be very very constrainted.

For me, the following would be more descriptive:

Suggested change
for (size_t i = 0; i < CELLS_PER_EXT_BLOB; i++) {
uint64_t index = cell_indices[i];
recovered_cells[index] = cells[i];
for (size_t i = 0; i < CELLS_PER_EXT_BLOB; i++) {
/* At this point we know that all cells are in the right order */
assert(cell_indices[i] == i);
recovered_cells[i] = cells[i];

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@asn-d6 asn-d6 Aug 29, 2025

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Tried this after our discussion. It does not use an assert but it also does not introduce a new index: 65ebfdd

}
} else {
/* Perform cell recovery */
ret = recover_cells(recovered_cells_fr, cell_indices, num_cells, recovered_cells_fr, s);
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