feat: add ECDSA signing to enclave for x402/EVM support

[tiero]

Problem

The x402 protocol requires EIP-3009 transferWithAuthorization — an off-chain ECDSA (secp256k1) signature on EIP-712 typed data. This lets a facilitator execute the USDC transfer on-chain on behalf of the client.

The clw.cash enclave currently only supports Schnorr signing for Bitcoin/Ark. The signMessage method in RemoteSignerIdentity explicitly throws on ECDSA:

// remote-signer-identity/src/remoteSignerIdentity.ts:107
if (signatureType === "ecdsa") {
  throw new Error("ECDSA signing is not supported by RemoteSignerIdentity");
}

What's needed

1. Enclave: Add ECDSA signing endpoint (e.g. /sign-ecdsa) alongside the existing Schnorr endpoint. Same secp256k1 key, different signature scheme.
2. API: Expose ECDSA signing via a new route or extend the existing /sign endpoint with a signatureType parameter.
3. RemoteSignerIdentity: Remove the ECDSA throw, route ECDSA requests to the new endpoint.
4. Derive Ethereum address: The existing compressed secp256k1 pubkey can derive an Ethereum address (keccak256(uncompressedPubKey[1:])[-20:]). Expose this so the agent knows its EVM address.

x402 flow once ECDSA is available

Agent requests URL → gets 402 Payment Required
  → parse PAYMENT-REQUIRED header (amount, token, chain, recipient)
  → swap BTC → USDC to agent's own EVM address (via existing LendaSwap infra)
  → construct EIP-712 typed data for EIP-3009 TransferWithAuthorization
  → hash client-side, send digest to enclave for ECDSA signing
  → retry request with PAYMENT-SIGNATURE header
  → facilitator settles the USDC transfer on-chain
  → agent gets the resource

New CLI command: cash pay <url>

Once ECDSA signing is available, implement:

• cash pay <url> — fetch URL, handle 402 automatically, return response body
• Uses @x402/fetch wrapper or custom implementation
• Integrates with existing cash send swap infrastructure for the BTC→USDC funding step

Files affected

• enclave/src/index.ts — new ECDSA signing route
• api/src/index.ts — expose ECDSA signing
• remote-signer-identity/src/remoteSignerIdentity.ts — support ECDSA
• cli/src/commands/pay.ts — new command (after ECDSA is available)

Reference

• x402 spec: https://github.com/coinbase/x402
• EIP-3009: https://eips.ethereum.org/EIPS/eip-3009
• EIP-712: https://eips.ethereum.org/EIPS/eip-712
• x402 EVM mechanism uses @x402/evm package with ExactEvmScheme

[viftode4]

This can give the planned cash pay <url> path a live Base exact fixture once ECDSA signing lands, while exercising a swap-specific buyer workflow rather than a synthetic 402 endpoint.

The paid resource is a read-only Base swap preflight: 0.01 USDC per successful call. It returns execute / decision, reasons and risk flags, Base block freshness, gas-cap comparison, and token/router bytecode checks. It does not execute a swap; the caller should still enforce a fresh quote and minOut.

Exact URL:

https://5-9-107-124.nip.io/swap-preflight?token_in=0x4200000000000000000000000000000000000006&token_out=0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913&router=0x2626664c2603336E57B271c5C0b26F421741e481&amount_in=1000000000000000&max_gas_gwei=1&source=claw_cash_5

Copy-paste x402 invocation (uses only your own funded Base wallet key):

test -n "${X402_PAY_PRIVATE_KEY:-}" && uv run --isolated --no-project \
  --with 'x402[evm]==2.17.0' --with 'requests>=2.32' --with 'eth-account>=0.13' \
  python -c 'import os,requests; from eth_account import Account; from x402 import x402ClientSync; from x402.http.clients.requests import wrapRequestsWithPayment; from x402.mechanisms.evm.exact.client import ExactEvmScheme; u="https://5-9-107-124.nip.io/swap-preflight?token_in=0x4200000000000000000000000000000000000006&token_out=0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913&router=0x2626664c2603336E57B271c5C0b26F421741e481&amount_in=1000000000000000&max_gas_gwei=1&source=claw_cash_5"; c=x402ClientSync(); c.register("eip155:8453",ExactEvmScheme(Account.from_key(os.environ["X402_PAY_PRIVATE_KEY"]))); r=wrapRequestsWithPayment(requests.Session(),c).get(u,timeout=45); print(r.status_code,r.text); r.raise_for_status()'

If useful, one concrete acceptance test would be: fund/sign through the new enclave path, call this URL, then assert HTTP 200 plus the returned preflight JSON. A pasted receipt/JSON would confirm the real buyer path; no integration is expected otherwise.

[tiero]

PRs are welcome! @viftode4