feat: add needsXpubsForColdSigning capability method

modules/abstract-utxo/src/abstractUtxoCoin.ts

@@ -7,6 +7,7 @@ import {
   AddressCoinSpecific,
   BaseCoin,
   BitGoBase,
+  ColdTransactionPrebuild,
   CreateAddressFormat,
   ExtraPrebuildParamsOptions,
   HalfSignedUtxoTransaction,
@@ -17,6 +18,7 @@ import {
   IRequestTracer,
   isTriple,
   IWallet,
+  Keychain,
   KeychainsTriplet,
   KeyIndices,
   MismatchedRecipient,
@@ -1068,6 +1070,17 @@ export abstract class AbstractUtxoCoin extends BaseCoin implements Musig2Partici
     return false;
   }
 
+  prepareColdTransaction<T extends BaseTransactionPrebuild>(
+    txPrebuild: T,
+    keychains: Triple<Keychain>
+  ): T & ColdTransactionPrebuild {
+    // PSBTs already carry BIP-32 derivation info inline, so offline signers do not need the extra information
+    if (typeof txPrebuild.txHex === 'string' && hasPsbtMagic(stringToBufferTryFormats(txPrebuild.txHex, ['hex']))) {
+      return txPrebuild as T & ColdTransactionPrebuild;
+    }
+    return super.prepareColdTransaction(txPrebuild, keychains);
+  }
+
   getRecoveryProvider(apiToken?: string): RecoveryProvider {
     return forCoin(this.getChain(), apiToken);
   }

modules/sdk-core/src/bitgo/baseCoin/baseCoin.ts

@@ -13,7 +13,7 @@ import { signMessage } from '../bip32util';
 import { NotImplementedError } from '../../account-lib';
 import { BitGoBase } from '../bitgoBase';
 import { Enterprises } from '../enterprise';
-import { Keychains, KeyIndices } from '../keychain';
+import { ApiKeyShare, Keychain, Keychains, KeyIndices } from '../keychain';
 import { Markets } from '../market';
 import { PendingApprovals } from '../pendingApproval';
 import { IWallet, Wallet, Wallets } from '../wallet';
@@ -22,6 +22,7 @@ import {
   BaseBroadcastTransactionOptions,
   BaseBroadcastTransactionResult,
   BuildNftTransferDataOptions,
+  ColdTransactionPrebuild,
   DeriveKeyWithSeedOptions,
   ExtraPrebuildParamsOptions,
   FeeEstimateOptions,
@@ -57,6 +58,7 @@ import {
   PopulatedIntent,
   PrebuildTransactionWithIntentOptions,
   TokenTransferRecipientParams,
+  Triple,
 } from '../utils';
 
 export abstract class BaseCoin implements IBaseCoin {
@@ -244,6 +246,44 @@ export abstract class BaseCoin implements IBaseCoin {
     return false;
   }
 
+  /**
+   * Prepare a built transaction for cold (offline) signing by attaching the
+   * xpubs / xpub derivation paths the offline signer (OVC) needs in order to
+   * sign it. Coins whose tx format already carries this information inline
+   * (e.g. PSBT) can override this to skip attaching it.
+   * @param txPrebuild The built transaction JSON returned by the platform
+   * @param keychains The user/backup/bitgo keychains for this wallet, in that order
+   * @returns The txPrebuild JSON, augmented with `pubs` and `xpubsWithDerivationPath`
+   */
+  prepareColdTransaction<T extends TransactionPrebuild>(
+    txPrebuild: T,
+    keychains: Triple<Keychain>
+  ): T & ColdTransactionPrebuild {
+    const [user, backup, bitgo] = keychains;
+    const sources: Array<[ApiKeyShare['from'], Keychain]> = [
+      ['user', user],
+      ['backup', backup],
+      ['bitgo', bitgo],
+    ];
+    const pubs: string[] = [];
+    const xpubsWithDerivationPath: NonNullable<ColdTransactionPrebuild['xpubsWithDerivationPath']> = {};
+    for (const [source, keychain] of sources) {
+      // MPC keychains only have a commonKeychain, not a pub - skip those.
+      if (keychain.pub) {
+        pubs.push(keychain.pub);
+        xpubsWithDerivationPath[source] = {
+          xpub: keychain.pub,
+          derivedFromParentWithSeed: keychain.derivedFromParentWithSeed,
+        };
+      }
+    }
+    return {
+      ...txPrebuild,
+      pubs,
+      xpubsWithDerivationPath,
+    };
+  }
+
   /**
    * Returns the factor between the base unit and its smallest subdivison
    * @return {number}

modules/sdk-core/src/bitgo/baseCoin/iBaseCoin.ts

@@ -2,7 +2,7 @@ import { BaseTokenConfig, BaseCoin as StaticsBaseCoin } from '@bitgo/statics';
 import BigNumber from 'bignumber.js';
 import { IRequestTracer } from '../../api';
 import { IEnterprises } from '../enterprise';
-import { IKeychains, Keychain } from '../keychain';
+import { ApiKeyShare, IKeychains, Keychain } from '../keychain';
 import { IMarkets } from '../market';
 import { IPendingApprovals } from '../pendingApproval';
 import { InitiateRecoveryOptions } from '../recovery';
@@ -14,7 +14,7 @@ import { TokenEnablement } from '@bitgo/public-types';
 import { Hash } from 'crypto';
 import { TransactionType } from '../../account-lib';
 import { IInscriptionBuilder } from '../inscriptionBuilder';
-import { MessageStandardType, MPCTx, PopulatedIntent, TokenTransferRecipientParams, TokenType } from '../utils';
+import { MessageStandardType, MPCTx, PopulatedIntent, TokenTransferRecipientParams, TokenType, Triple } from '../utils';
 import type { SignableTransaction } from '../utils/tss/baseTypes';
 import { IWebhooks } from '../webhook/iWebhooks';
 
@@ -442,6 +442,16 @@ export interface TransactionPrebuild extends BaseSignable {
   txInfo?: unknown;
 }
 
+export interface XpubWithDerivationPath {
+  xpub: string;
+  derivedFromParentWithSeed?: string;
+}
+
+export interface ColdTransactionPrebuild {
+  pubs?: string[];
+  xpubsWithDerivationPath?: Partial<Record<ApiKeyShare['from'], XpubWithDerivationPath>>;
+}
+
 export interface Message extends BaseSignable {
   messageRaw: string;
   messageEncoded?: string;
@@ -672,6 +682,10 @@ export interface IBaseCoin {
   supportsDeriveKeyWithSeed(): boolean;
   isEVM(): boolean;
   supportsBlsDkg(): boolean;
+  prepareColdTransaction<T extends TransactionPrebuild>(
+    txPrebuild: T,
+    keychains: Triple<Keychain>
+  ): T & ColdTransactionPrebuild;
   getBaseFactor(): number | string;
   baseUnitsToBigUnits(baseUnits: string | number): string;
   bigUnitsToBaseUnits(bigUnits: string | number): string;