Major interop fixes discovered by IETF test vectors:
1. VarInt migration: All MLS TLS struct serialization now uses
QUIC-style VarInt encoding for opaque<V> and vector<V> fields,
matching OpenMLS and mls-rs wire format. Added readVarInt(),
readOpaqueVarInt(), readVectorVarInt() to TlsReader and
putVectorVarInt() to TlsWriter.
2. SecretTree left/right derivation: Fixed tree secret splitting
to use "left"/"right" as context strings per RFC 9420 Section 9,
instead of byte(0)/byte(1).
3. LeafNode parent_hash: Added parent_hash<V> field for COMMIT
source per RFC 9420 Section 7.2. The COMMIT case is NOT empty -
it includes a parent_hash opaque field.
4. MLS-Exporter: Added ByteArray overload for raw byte labels
(test vectors use non-UTF-8 label bytes).
Test results: 32/41 passing (78%), up from 25/41 (61%).
Newly passing: SecretTree (2), TreeValidation deserialization (1),
TreeValidation resolution (1), TreeKem deserialization (1),
Commit deserialization (1), RatchetTree deserialization (1).
https://claude.ai/code/session_01NocQDWj2Y92FugjfgazzL3
Claude
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