feat(abstract-lightning): add utility to compute BIP32 derivation index

Add a utility function that computes a BIP32 derivation index from a seed
string. The function uses SHA256 to generate a deterministic index value
within the valid range for BIP32 derivation paths.

Issue: BTC-2202

Co-authored-by: llm-git <[email protected]>

Author: davidkaplanbitgo

modules/abstract-lightning/src/lightning/lightningUtils.ts

@@ -226,3 +226,12 @@ export function deriveTatSharedSecret(coinName: 'lnbtc' | 'tlnbtc', userXprv: st
   const userAuthHdNode = utxolib.bip32.fromBase58(userXprv);
   return sdkcore.getSharedSecret(userAuthHdNode, publicKey);
 }
+
+/**
+ * Given a seed, compute a BIP32 derivation index.
+ * 0 <= index < 4294967295 (largest 4 byte number)
+ * @param seed
+ */
+export function computeBip32DerivationIndexFromSeed(seed: string): number {
+  return Buffer.from(utxolib.crypto.sha256(Buffer.from(seed, 'utf8'))).readUint32BE(0);
+}

modules/abstract-lightning/test/unit/lightning/lightningUtils.ts

@@ -15,6 +15,7 @@ import {
   deriveLightningServiceSharedSecret,
   deriveMiddlewareSharedSecret,
   deriveTatSharedSecret,
+  computeBip32DerivationIndexFromSeed,
 } from '../../../src/lightning';
 
 import * as sdkcore from '@bitgo/sdk-core';
@@ -125,4 +126,10 @@ describe('lightning utils', function () {
     const secret = deriveTatSharedSecret('tlnbtc', userXprv);
     assert.deepStrictEqual(secret, expectedSecret);
   });
+
+  it(`computeBip32DerivationIndexFromSeed`, function () {
+    const seed1 = 'The Times 03/Jan/2009 Chancellor on brink of second bailout for banks.';
+    const seed2 = 'The Times 04/Jan/2009 Chancellor on brink of second bailout for banks.';
+    assert.notDeepStrictEqual(computeBip32DerivationIndexFromSeed(seed1), computeBip32DerivationIndexFromSeed(seed2));
+  });
 });