> ## Documentation Index
> Fetch the complete documentation index at: https://docs.pfbridge.xyz/llms.txt
> Use this file to discover all available pages before exploring further.

# The BLS auth circuit

> Signature verification inside a proof — built, measured, and why the live flow verifies natively instead

Two ways exist to convince a contract that both traders signed:

1. **Verify the aggregated BLS signature directly on-chain** — Ethereum's
   precompiles and Soroban's host functions can run the pairing check
   natively.
2. **Verify it inside a zero-knowledge proof** — a circuit checks the
   signature and the chain only verifies the (fixed-size) proof.

v2 shipped **both**, and chose the first for the live flow.

## What was built

The auth circuit lives at
[`proof_circuits/auth`](https://github.com/Explore-Beyond-Innovations/ProofBridge/tree/main/proof_circuits/auth):
a Noir circuit that verifies an aggregated maker + bridger BLS12-381
signature over the order digest.

* Proves in **under 30 seconds** on the reference machine — the bar the
  deliverable set
* Four-case test pack: valid aggregate · single-signer aggregate ·
  invalid signature rejected · wrong public key rejected
* Verifier generation works for both chains

## Why the live flow verifies natively

Measured head-to-head, the native pairing check settles at a **fraction of
the gas** of verifying a proof of the same statement — and it runs on
Soroban in under two seconds inside a single transaction. Verifying
in-circuit would also add \~30 seconds of proving to every settlement for
no security gain: the statement being proven is exactly the check the
chain can already run.

So the `CounterpartyVerifier` contract on each chain performs the pairing
check natively at unlock, and the circuit stands as the **proven fallback
path** — if a future chain lacks BLS support, or a later design folds
signature checking into the deposit proof, the circuit is built, tested,
and benchmarked.

<Note>
  This is a deliberate engineering trade-off, not a de-scope: the circuit
  met its acceptance bar, and the cheaper path won on evidence.
</Note>