policy: make unstructured Taproot annexes standard #36466

 283 |  
 284 | -        // witness stuffing detected
 285 | +        // P2A inputs cannot carry an annex and are exempt from annex opt-in.
 286 |          if (prevScript.IsPayToAnchor()) {
 287 | -            return false;
 288 | +            if (!tx.vin[i].scriptWitness.IsNull()) return false;

petertodd commented at 11:00 AM on October 8, 2026:

You should re-add the "witness stuffing detected" comment here to explain what this line is doing.

 284 | -        // witness stuffing detected
 285 | +        // P2A inputs cannot carry an annex and are exempt from annex opt-in.
 286 |          if (prevScript.IsPayToAnchor()) {
 287 | -            return false;
 288 | +            if (!tx.vin[i].scriptWitness.IsNull()) return false;
 289 | +            ++anchor_inputs;

petertodd commented at 11:00 AM on October 8, 2026:

...and the comment about P2A inputs and the annex probably makes more sense here.

 291 |          }
 292 |  
 293 | +        // We don't care if witness for this input is empty, since it must not be bloated.
 294 | +        // If the script is invalid without witness, it would be caught sooner or later during validation.
 295 | +        if (tx.vin[i].scriptWitness.IsNull())
 296 | +            continue;

petertodd commented at 11:03 AM on October 8, 2026:

I'm confused by this fragment. It doesn't look like it's related to the annex stuff at all. Rather it's just skipping further checks if scriptWitness is empty.

 323 | @@ -320,13 +324,17 @@ bool IsWitnessStandard(const CTransaction& tx, const CCoinsViewCache& mapInputs)
 324 |  
 325 |          // Check policy limits for Taproot spends:
 326 |          // - MAX_STANDARD_TAPSCRIPT_STACK_ITEM_SIZE limit for stack item size
 327 | -        // - No annexes
 328 | +        // - Annexes must be marker-only or start with the unstructured data prefix

petertodd commented at 11:04 AM on October 8, 2026:

It'd be more clear to say that "Annexes must be zero-length, or start with the unstructured data prefix, 0x00"

 335 | +                const auto& annex = SpanPopBack(stack);
 336 | +                // A marker-only annex opts in without carrying data. Otherwise, 0x00
 337 | +                // identifies unstructured data; other prefixes remain reserved.
 338 | +                // The existing MAX_STANDARD_TX_WEIGHT limit includes annex weight.
 339 | +                if (annex.size() > 1 && annex[1] != 0x00) return false;
 340 | +                ++annex_inputs;

petertodd commented at 11:07 AM on October 8, 2026:

We should explain why the unstructured marker is being used here: 0x00 is reserved, so that future consensus-related usage of the annex can use a different prefix (quite possibly an entire encoding scheme).

 355 | @@ -348,7 +356,10 @@ bool IsWitnessStandard(const CTransaction& tx, const CCoinsViewCache& mapInputs)
 356 |              }
 357 |          }
 358 |      }
 359 | -    return true;
 360 | +    // Signatures commit only to the annex of their own input. Requiring all non-P2A inputs
 361 | +    // to opt in prevents a participant from introducing an annex into a transaction
 362 | +    // whose other participants did not opt in. Non-P2A inputs without witness also count.
 363 | +    return annex_inputs == 0 || annex_inputs == tx.vin.size() - anchor_inputs;

petertodd commented at 11:09 AM on October 8, 2026:

You should use the term "transaction pinning attack" here, so people have something to google.