Summary
Bn254G2Point in barretenberg-rs is missing serde_bytes annotations on its [Vec<u8>; 2] fields, causing msgpack serialization to produce arrays of integers instead of bin32 blobs. The C++ bbapi expects bin32 (via uint256_t::msgpack_unpack), so bn254_g2_mul calls fail with a deserialization abort.
Reported by Porco from Obsidion.
Details
Bn254G1Point works correctly because its Vec<u8> fields use #[serde(with = "serde_bytes")]:
pub struct Bn254G1Point {
#[serde(with = "serde_bytes")]
pub x: Vec<u8>,
#[serde(with = "serde_bytes")]
pub y: Vec<u8>,
}Bn254G2Point is missing the equivalent annotation:
pub struct Bn254G2Point {
pub x: [Vec<u8>; 2], // no serde annotation
pub y: [Vec<u8>; 2], // no serde annotation
}Without the annotation, rmp-serde serializes each Vec<u8> element as a msgpack array of integers rather than a bin32 blob.
Root cause
The Rust code generator (rust_codegen.ts) handles serde_bytes for bytes primitives and serde_array4_bytes for [Vec<u8>; 4] arrays, but has no handling for the field2 primitive type which maps to [Vec<u8>; 2].
Fix
- Add a
serde_array2_bytes module (same pattern as existing serde_array4_bytes)
- Add
needsSerdeArray2Bytes() detection for field2 primitive types in rust_codegen.ts
- Annotate
Bn254G2Point fields with #[serde(with = "serde_array2_bytes")]
Affected versions
Every published version from 4.1.0-rc.2 through 5.0.0-private.20260319. There are no tests for bn254_g2_mul which is why this was never caught.
Summary
Bn254G2Pointinbarretenberg-rsis missingserde_bytesannotations on its[Vec<u8>; 2]fields, causing msgpack serialization to produce arrays of integers instead ofbin32blobs. The C++bbapiexpectsbin32(viauint256_t::msgpack_unpack), sobn254_g2_mulcalls fail with a deserialization abort.Reported by Porco from Obsidion.
Details
Bn254G1Pointworks correctly because itsVec<u8>fields use#[serde(with = "serde_bytes")]:Bn254G2Pointis missing the equivalent annotation:Without the annotation,
rmp-serdeserializes eachVec<u8>element as a msgpack array of integers rather than abin32blob.Root cause
The Rust code generator (
rust_codegen.ts) handlesserde_bytesforbytesprimitives andserde_array4_bytesfor[Vec<u8>; 4]arrays, but has no handling for thefield2primitive type which maps to[Vec<u8>; 2].Fix
serde_array2_bytesmodule (same pattern as existingserde_array4_bytes)needsSerdeArray2Bytes()detection forfield2primitive types inrust_codegen.tsBn254G2Pointfields with#[serde(with = "serde_array2_bytes")]Affected versions
Every published version from 4.1.0-rc.2 through 5.0.0-private.20260319. There are no tests for
bn254_g2_mulwhich is why this was never caught.