1 files changed, 114 insertions, 0 deletions
diff --git a/src/ark/board.js b/src/ark/board.js
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+++ b/src/ark/board.js
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+// Native-JS board: move onchain funds into the Ark.
+//
+// Mirrors bark's lib/src/board.rs BoardBuilder: the funding output pays a
+// taproot of musig(user, server) with a server-after-expiry sweep leaf (the
+// same shape as the checkpoint policy), the exit tx is cosigned via MuSig2
+// through RequestBoardCosign, and the resulting Vtxo<Full> has a single
+// Cosigned genesis transition anchored at the funding outpoint.
+
+import { hex, bech32m } from '@scure/base';
+import { schnorr } from '@noble/curves/secp256k1';
+import * as musig2 from '@scure/btc-signer/musig2';
+
+import { grpcCall, pbWriter, pbFields, concatBytes } from './proto.js';
+import { checkpointPolicyTaproot, pubkeyPolicyTaproot, taprootSighash, P2A_SCRIPT } from './send.js';
+
+const u16le = (n) => Uint8Array.of(n & 0xff, (n >> 8) & 0xff);
+const u32le = (n) => Uint8Array.of(n & 0xff, (n >> 8) & 0xff, (n >> 16) & 0xff, (n >>> 24) & 0xff);
+const u64le = (n) => {
+  const b = new Uint8Array(8);
+  new DataView(b.buffer).setBigUint64(0, BigInt(n), true);
+  return b;
+};
+
+export const boardFee = (amountSat, fees) =>
+  Math.max(fees.minFeeSat, fees.baseFeeSat + Math.floor((amountSat * fees.ppm) / 1_000_000));
+
+export const p2trAddress = (programXOnly, hrp = 'bcrt') =>
+  bech32m.encode(hrp, [1, ...bech32m.toWords(programXOnly)], 1023);
+
+// Build the board: funding output spec + exit tx + sighash.
+// `fundingOutpointRaw` (36B) is set after the funding tx is known.
+export function buildBoard({ userPub, serverPub, amountSat, feeSat, expiryHeight, exitDelta, fundingOutpointRaw }) {
+  const fundingTaproot = checkpointPolicyTaproot(userPub, serverPub, expiryHeight);
+  const exitTaproot = pubkeyPolicyTaproot(userPub, serverPub, exitDelta);
+
+  const exitTx = {
+    version: 3, locktime: 0,
+    inputs: [{ prevout: fundingOutpointRaw, sequence: 0 }],
+    outputs: [
+      { valueSat: amountSat - feeSat, scriptPubKey: exitTaproot.scriptPubKey },
+      { valueSat: feeSat, scriptPubKey: P2A_SCRIPT }, // board fee rides the anchor
+    ],
+  };
+  const fundingPrevout = { valueSat: amountSat, scriptPubKey: fundingTaproot.scriptPubKey };
+  const sighash = taprootSighash(exitTx, [fundingPrevout]);
+  return { fundingTaproot, exitTaproot, exitTx, sighash };
+}
+
+export async function requestBoardCosign(ark, { amountSat, fundingOutpointRaw, expiryHeight, userPub, pubNonce }) {
+  const w = pbWriter();
+  w.varintField(1, amountSat);
+  w.bytesField(2, fundingOutpointRaw);
+  w.varintField(3, expiryHeight);
+  w.bytesField(4, userPub);
+  w.bytesField(5, pubNonce);
+  const resp = await grpcCall(ark, 'bark_server.ArkService/RequestBoardCosign', w.finish());
+  const out = {};
+  for (const { field, value } of pbFields(resp)) {
+    if (field === 1) out.pubNonce = value;
+    if (field === 2) out.partialSig = value;
+  }
+  return out;
+}
+
+// MuSig2: combine our partial with the server's over the exit tx sighash.
+export function combineBoardSignature({ board, serverCosign, userNonces, vtxoKeys, serverPub }) {
+  const aggNonce = musig2.nonceAggregate([userNonces.public, serverCosign.pubNonce]);
+  const session = new musig2.Session(
+    aggNonce, board.fundingTaproot.sortedKeys, board.sighash, [board.fundingTaproot.tapTweak], [true],
+  );
+  const serverIdx = board.fundingTaproot.sortedKeys.findIndex(
+    (k) => hex.encode(k) === hex.encode(serverPub));
+  const noncesInKeyOrder = serverIdx === 0
+    ? [serverCosign.pubNonce, userNonces.public] : [userNonces.public, serverCosign.pubNonce];
+  if (!session.partialSigVerify(serverCosign.partialSig, noncesInKeyOrder, serverIdx)) {
+    throw new Error('server board partial signature is invalid');
+  }
+  const userPartial = session.sign(userNonces.secret, vtxoKeys.privkey);
+  const finalSig = session.partialSigAgg([userPartial, serverCosign.partialSig]);
+  if (!schnorr.verify(finalSig, board.sighash, board.fundingTaproot.outputXOnly)) {
+    throw new Error('combined board signature does not verify');
+  }
+  return finalSig;
+}
+
+// Vtxo<Full> bytes for the board: one Cosigned genesis transition.
+export function encodeBoardVtxo({ userPub, serverPub, amountSat, feeSat, expiryHeight, exitDelta,
+                                  fundingOutpointRaw, exitTxidInternal, finalSig }) {
+  const cosignedTransition = concatBytes(
+    Uint8Array.of(0x01),          // GenesisTransition::Cosigned
+    Uint8Array.of(0x02), userPub, serverPub, // LengthPrefixedVector [user, server]
+    finalSig,
+  );
+  return concatBytes(
+    u16le(2),                     // encoding version
+    u64le(amountSat - feeSat),
+    u32le(expiryHeight),
+    serverPub,
+    u16le(exitDelta),
+    fundingOutpointRaw,           // anchor point = funding utxo
+    Uint8Array.of(0x01),          // one genesis item
+    cosignedTransition,
+    Uint8Array.of(0x01, 0x00),    // nb_outputs=1, output_idx=0
+    u64le(feeSat),                // fee_amount
+    Uint8Array.of(0x00), userPub, // policy: Pubkey(user)
+    exitTxidInternal, u32le(0),   // point = exit_txid:0
+  );
+}
+
+export async function registerBoardVtxo(ark, vtxoBytes) {
+  const w = pbWriter();
+  w.bytesField(1, vtxoBytes);
+  await grpcCall(ark, 'bark_server.ArkService/RegisterBoardVtxo', w.finish());
+}