BitGo · rohitsaw115 · May 13, 2026 · May 12, 2026
@@ -24,6 +24,65 @@ export class Transaction extends BaseTransaction {
     return this._txData;
   }
 
+  /**
+   * Get the transaction fee in sompi.
+   * If fee was explicitly set, returns that. Otherwise computes from inputs - outputs.
+   */
+  get getFee(): string {
+    if (this._txData.fee) {
+      return this._txData.fee;
+    }
+    let totalIn = BigInt(0);
+    let totalOut = BigInt(0);
+    for (const input of this._txData.inputs) {
+      totalIn += BigInt(input.amount);
+    }
+    for (const output of this._txData.outputs) {
+      totalOut += BigInt(output.amount);
+    }
+    return (totalIn - totalOut).toString();
+  }
+
+  /**
+   * Returns the signable payload for TSS/MPC signing.
+   *
+   * For Kaspa, each input has its own sighash (BIP-143-like scheme with Blake2b).
+   * This returns the sighash for the first input, which is what TSS signs.
+   * For multi-input transactions, all inputs share the same key so the same
+   * Schnorr signature is applied to each input's individual sighash in addSignature().
+   *
+   * @see ADA's Transaction.signablePayload for the equivalent pattern
+   */
+  get signablePayload(): Buffer {
+    if (this._txData.inputs.length === 0) {
+      throw new Error('Cannot compute signablePayload: no inputs');
+    }
+    return computeKaspaSigningHash(this._txData, 0, SIGHASH_ALL);
+  }
+
+  /**
+   * Apply a Schnorr signature produced by TSS/MPC signing to all inputs.
+   *
+   * In TSS flow, the keyserver signs the first input's sighash. Since each input
+   * has a different sighash, we re-sign each input individually using the
+   * x-only public key derived from the compressed public key.
+   *
+   * @param publicKey compressed secp256k1 public key (33 bytes hex)
+   * @param signature 64-byte Schnorr signature buffer (from TSS)
+   * @param sigHashType SigHash type (default: SIGHASH_ALL)
+   */
+  addSignature(publicKey: string, signature: Buffer, sigHashType: number = SIGHASH_ALL): void {
+    if (signature.length !== 64) {
+      throw new Error(`Expected 64-byte Schnorr signature, got ${signature.length}`);
+    }
+
+    for (let i = 0; i < this._txData.inputs.length; i++) {
+      // Each input gets the same signature format: 64-byte sig + sighash type byte
+      const sigWithType = Buffer.concat([signature, Buffer.from([sigHashType])]);
+      this._txData.inputs[i].signatureScript = sigWithType.toString('hex');
+    }
+  }
+
   /**
    * Sign all inputs with the given private key using Schnorr signatures.
    *

@@ -1,4 +1,10 @@
-import { BaseTransactionBuilder, BaseTransaction, BaseKey, SigningError } from '@bitgo/sdk-core';
+import {
+  BaseTransactionBuilder,
+  BaseTransaction,
+  BaseKey,
+  PublicKey as BasePublicKey,
+  SigningError,
+} from '@bitgo/sdk-core';
 import { BaseCoin as CoinConfig } from '@bitgo/statics';
 import BigNumber from 'bignumber.js';
 import { Transaction } from './transaction';
@@ -7,12 +13,18 @@ import { isValidKaspaAddress } from './utils';
 import { KeyPair } from './keyPair';
 import { DEFAULT_FEE, TX_VERSION } from './constants';
 
+interface KaspaSignature {
+  publicKey: BasePublicKey;
+  signature: Buffer;
+}
+
 export class TransactionBuilder extends BaseTransactionBuilder {
   protected _transaction: Transaction;
   protected _inputs: KaspaUtxoInput[] = [];
   protected _outputs: KaspaTransactionOutput[] = [];
   protected _fee: string = DEFAULT_FEE;
   protected _fromAddress = '';
+  protected _signatures: KaspaSignature[] = [];
 
   constructor(coinConfig: Readonly<CoinConfig>) {
     super(coinConfig);
@@ -78,6 +90,19 @@ export class TransactionBuilder extends BaseTransactionBuilder {
     return this;
   }
 
+  /**
+   * Add an externally-produced signature (from TSS/MPC signing) to the transaction.
+   * The signature will be applied to all inputs during build().
+   *
+   * This follows the same pattern as ADA's TransactionBuilder.addSignature().
+   *
+   * @param publicKey The compressed secp256k1 public key that produced the signature
+   * @param signature The 64-byte Schnorr signature buffer
+   */
+  addSignature(publicKey: BasePublicKey, signature: Buffer): void {
+    this._signatures.push({ publicKey, signature });
+  }
+
   /** @inheritDoc */
   protected fromImplementation(rawTransaction: string): Transaction {
     const tx = Transaction.fromHex((this as any)._coinConfig?.name || 'kaspa', rawTransaction);
@@ -101,6 +126,12 @@ export class TransactionBuilder extends BaseTransactionBuilder {
     };
 
     this._transaction = new Transaction((this as any)._coinConfig?.name || 'kaspa', txData);
+
+    // Apply any externally-produced signatures (from TSS/MPC)
+    for (const sig of this._signatures) {
+      this._transaction.addSignature(sig.publicKey.pub, sig.signature);
+    }
+
     return this._transaction;
   }
 

@@ -1,11 +1,10 @@
+import { BaseTransactionBuilderFactory, NotImplementedError } from '@bitgo/sdk-core';
 import { BaseCoin as StaticsBaseCoin } from '@bitgo/statics';
 import { TransactionBuilder } from './transactionBuilder';
 
-export class TransactionBuilderFactory {
-  protected _coinConfig: Readonly<StaticsBaseCoin>;
-
+export class TransactionBuilderFactory extends BaseTransactionBuilderFactory {
   constructor(coinConfig: Readonly<StaticsBaseCoin>) {
-    this._coinConfig = coinConfig;
+    super(coinConfig);
   }
 
   /**
@@ -15,9 +14,20 @@ export class TransactionBuilderFactory {
     return new TransactionBuilder(this._coinConfig);
   }
 
+  /** @inheritdoc */
+  getTransferBuilder(): TransactionBuilder {
+    return this.getBuilder();
+  }
+
   /**
-   * Reconstruct a transaction builder from a raw transaction hex.
+   * Kaspa does not have a wallet initialization transaction.
+   * @throws NotImplementedError
    */
+  getWalletInitializationBuilder(): never {
+    throw new NotImplementedError('getWalletInitializationBuilder is not supported for Kaspa');
+  }
+
+  /** @inheritdoc */
   from(rawTransaction: string): TransactionBuilder {
     const builder = this.getBuilder();
     builder.from(rawTransaction);

@@ -169,6 +169,106 @@ describe('Kaspa Transaction', function () {
     });
   });
 
+  describe('getFee', function () {
+    it('should return explicit fee when set in txData', function () {
+      const tx = new Transaction(COIN, TRANSACTIONS.simple);
+      assert.equal(tx.getFee, '2000');
+    });
+
+    it('should compute fee from inputs - outputs when fee is not set', function () {
+      const txData = { ...TRANSACTIONS.simple };
+      delete txData.fee;
+      const tx = new Transaction(COIN, txData);
+      // input: 100000000, output: 99998000, fee = 2000
+      assert.equal(tx.getFee, '2000');
+    });
+  });
+
+  describe('signablePayload', function () {
+    it('should return a 32-byte Buffer (Blake2b hash)', function () {
+      const tx = new Transaction(COIN, TRANSACTIONS.simple);
+      const payload = tx.signablePayload;
+      assert.ok(Buffer.isBuffer(payload));
+      assert.equal(payload.length, 32);
+    });
+
+    it('should throw when transaction has no inputs', function () {
+      const tx = new Transaction(COIN);
+      assert.throws(() => {
+        tx.signablePayload;
+      }, /no inputs/);
+    });
+
+    it('should return deterministic hash for same transaction data', function () {
+      const tx1 = new Transaction(COIN, TRANSACTIONS.simple);
+      const tx2 = new Transaction(COIN, TRANSACTIONS.simple);
+      assert.ok(tx1.signablePayload.equals(tx2.signablePayload));
+    });
+
+    it('should return different hashes for different transactions', function () {
+      const tx1 = new Transaction(COIN, TRANSACTIONS.simple);
+      const tx2 = new Transaction(COIN, TRANSACTIONS.multiInput);
+      assert.ok(!tx1.signablePayload.equals(tx2.signablePayload));
+    });
+  });
+
+  describe('addSignature', function () {
+    it('should apply a 64-byte Schnorr signature to all inputs', function () {
+      const tx = new Transaction(COIN, TRANSACTIONS.simple);
+      const fakeSig = Buffer.alloc(64, 0xab);
+      tx.addSignature(KEYS.pub, fakeSig);
+
+      assert.equal(tx.txData.inputs.length, 1);
+      assert.ok(tx.txData.inputs[0].signatureScript);
+      // 65 bytes = 130 hex chars (64 sig + 1 sighash type)
+      assert.equal(tx.txData.inputs[0].signatureScript!.length, 130);
+    });
+
+    it('should apply signature to all inputs of a multi-input tx', function () {
+      const tx = new Transaction(COIN, TRANSACTIONS.multiInput);
+      const fakeSig = Buffer.alloc(64, 0xcd);
+      tx.addSignature(KEYS.pub, fakeSig);
+
+      assert.equal(tx.txData.inputs.length, 2);
+      for (const input of tx.txData.inputs) {
+        assert.ok(input.signatureScript);
+        assert.equal(input.signatureScript!.length, 130);
+      }
+    });
+
+    it('should throw for non-64-byte signature', function () {
+      const tx = new Transaction(COIN, TRANSACTIONS.simple);
+      assert.throws(() => {
+        tx.addSignature(KEYS.pub, Buffer.alloc(32));
+      }, /64-byte/);
+    });
+
+    it('should append SIGHASH_ALL byte at the end', function () {
+      const tx = new Transaction(COIN, TRANSACTIONS.simple);
+      const fakeSig = Buffer.alloc(64, 0xab);
+      tx.addSignature(KEYS.pub, fakeSig);
+      const sigHex = tx.txData.inputs[0].signatureScript!;
+      const lastByte = parseInt(sigHex.slice(-2), 16);
+      assert.equal(lastByte, SIGHASH_ALL);
+    });
+
+    it('should produce a signature that verifies when signed with the correct private key', function () {
+      const tx = new Transaction(COIN, TRANSACTIONS.simple);
+      // Sign properly with private key to get a real signature
+      const privKey = Buffer.from(KEYS.prv, 'hex');
+      tx.sign(privKey);
+      const realSigHex = tx.txData.inputs[0].signatureScript!;
+      const realSig = Buffer.from(realSigHex.slice(0, 128), 'hex'); // 64-byte Schnorr sig
+
+      // Now create a fresh tx and use addSignature instead
+      const tx2 = new Transaction(COIN, TRANSACTIONS.simple);
+      tx2.addSignature(KEYS.pub, realSig);
+
+      // The signature scripts should match (same sig bytes + same sighash type)
+      assert.equal(tx2.txData.inputs[0].signatureScript, tx.txData.inputs[0].signatureScript);
+    });
+  });
+
   describe('Serialization', function () {
     it('toJson should return a copy of txData', function () {
       const tx = new Transaction(COIN, TRANSACTIONS.simple);

@@ -209,6 +209,41 @@ describe('Kaspa TransactionBuilder', function () {
     });
   });
 
+  describe('addSignature (TSS/MPC flow)', function () {
+    it('should store the signature and apply it during build', async function () {
+      builder.addInput(UTXOS.simple).to(ADDRESSES.recipient, '99998000').fee('2000');
+
+      // Simulate TSS: build unsigned, get signablePayload, produce signature externally
+      const unsignedTx = (await builder.build()) as Transaction;
+      const signablePayload = unsignedTx.signablePayload;
+      assert.ok(signablePayload.length === 32);
+
+      // Now add signature via builder addSignature (like wallet-platform does)
+      const fakeSig = Buffer.alloc(64, 0xab);
+      builder.addSignature({ pub: KEYS.pub }, fakeSig);
+
+      // Rebuild — signatures should be applied
+      const signedTx = (await builder.build()) as Transaction;
+      assert.ok(signedTx.txData.inputs[0].signatureScript);
+      assert.equal(signedTx.txData.inputs[0].signatureScript!.length, 130);
+    });
+
+    it('should apply signature to multi-input transactions', async function () {
+      builder.addInputs([UTXOS.simple, UTXOS.second]).to(ADDRESSES.recipient, '299998000').fee('2000');
+      await builder.build();
+
+      const fakeSig = Buffer.alloc(64, 0xcd);
+      builder.addSignature({ pub: KEYS.pub }, fakeSig);
+
+      const signedTx = (await builder.build()) as Transaction;
+      assert.equal(signedTx.txData.inputs.length, 2);
+      for (const input of signedTx.txData.inputs) {
+        assert.ok(input.signatureScript);
+        assert.equal(input.signatureScript!.length, 130);
+      }
+    });
+  });
+
   describe('from (rebuild from hex)', function () {
     it('should reconstruct a builder from a serialized transaction', async function () {
       builder.addInput(UTXOS.simple).to(ADDRESSES.recipient, '99998000').fee('2000');
@@ -238,6 +273,21 @@ describe('Kaspa TransactionBuilderFactory', function () {
     });
   });
 
+  describe('getTransferBuilder', function () {
+    it('should return a new TransactionBuilder (same as getBuilder)', function () {
+      const builder = factory.getTransferBuilder();
+      assert.ok(builder instanceof TransactionBuilder);
+    });
+
+    it('should build a valid transaction', async function () {
+      const builder = factory.getTransferBuilder();
+      builder.addInput(UTXOS.simple).to(ADDRESSES.recipient, '99998000').fee('2000');
+      const tx = (await builder.build()) as Transaction;
+      assert.equal(tx.txData.inputs.length, 1);
+      assert.equal(tx.txData.outputs.length, 1);
+    });
+  });
+
   describe('from', function () {
     it('should reconstruct a builder from a serialized transaction hex', async function () {
       const originalBuilder = factory.getBuilder();