---
paths:
  - "licensing-service-startos/licensing-service/src/crypto/**"
  - "licensing-service-startos/licensing-service/tests/crosscheck/**"
  - "tests/crosscheck/**"
  - "licensing-client-rust/**"
  - "licensing-client-ts/**"
  - "licensing-client-python/**"
  - "licensing-client-go/**"
---

# Crypto & the LIC1 wire format

Ed25519 signing. The **LIC1 key byte layout** (defined in `src/crypto/mod.rs`) is
THE contract that all four SDKs (Rust, TS, Python, Go) implement. Two payload
layouts exist: legacy **v1** and current **v2** — both must keep validating.

## Change with extreme care

Any change to the byte layout, field order, or signing input ripples to four
SDKs and every license key already issued. Before changing it:

1. Update `src/crypto/` and every SDK in lockstep.
2. Update the cross-language fixtures.

## Cross-language verification

- **Top-level `tests/crosscheck/`** — an independent Python reference signer
  (`reference_signer.py`, using `cryptography` for Ed25519) plus a TS runner
  (`run_ts.mjs`) assert byte-for-byte agreement across Rust + TS + Python on both
  v1 and v2 layouts. Agreement here is strong evidence the format is correct, not
  just internally self-consistent.
- **`licensing-service/tests/crosscheck/`** — wire-format fixtures the four SDKs
  cross-verify, run via the Rust `crosscheck` suite (`cargo test --test crosscheck`).