Merge pull request #8746 from BitGo/WCI-385/fix-eddsa-mpcv2-get-reduced-key-share

fix(sdk-lib-mpc): add keyShare to EdDSA MPCv2 getReducedKeyShare

Author: Marzooqa

modules/sdk-lib-mpc/src/tss/eddsa-mps/dkg.ts

@@ -234,14 +234,18 @@ export class DKG {
   }
 
   /**
-   * Returns a CBOR-encoded reduced representation containing the public key.
+   * Returns a CBOR-encoded ReducedKeyShare buffer containing the party's opaque
+   * signing key share in the `keyShare` field. This buffer is private key material.
+   * The caller encrypts it and stores it as `reducedEncryptedPrv` on the key card QR code.
    */
   getReducedKeyShare(): Buffer {
-    if (!this.sharePk) {
+    if (!this.keyShare || !this.sharePk || !this.shareChaincode) {
       throw Error('DKG session not initialized');
     }
     const reducedKeyShare: EddsaReducedKeyShare = {
+      keyShare: Array.from(this.keyShare),
       pub: Array.from(this.sharePk),
+      rootChainCode: Array.from(this.shareChaincode),
     };
     return Buffer.from(encode(reducedKeyShare));
   }

modules/sdk-lib-mpc/src/tss/eddsa-mps/types.ts

@@ -3,7 +3,9 @@ import { isLeft } from 'fp-ts/Either';
 import * as t from 'io-ts';
 
 export const ReducedKeyShareType = t.type({
+  keyShare: t.array(t.number),
   pub: t.array(t.number),
+  rootChainCode: t.array(t.number),
 });
 
 export type EddsaReducedKeyShare = t.TypeOf<typeof ReducedKeyShareType>;

modules/sdk-lib-mpc/test/unit/tss/eddsa/dkg.ts

@@ -205,12 +205,38 @@ describe('EdDSA MPS DKG', function () {
       );
       assert(Buffer.isBuffer(bitgoReduced) && bitgoReduced.length > 0, 'BitGo reduced key share should be non-empty');
 
-      const userPub = Buffer.from(MPSTypes.getDecodedReducedKeyShare(userReduced).pub).toString('hex');
-      const backupPub = Buffer.from(MPSTypes.getDecodedReducedKeyShare(backupReduced).pub).toString('hex');
-      const bitgoPub = Buffer.from(MPSTypes.getDecodedReducedKeyShare(bitgoReduced).pub).toString('hex');
+      const userDecoded = MPSTypes.getDecodedReducedKeyShare(userReduced);
+      const backupDecoded = MPSTypes.getDecodedReducedKeyShare(backupReduced);
+      const bitgoDecoded = MPSTypes.getDecodedReducedKeyShare(bitgoReduced);
+
+      const userPub = Buffer.from(userDecoded.pub).toString('hex');
+      const backupPub = Buffer.from(backupDecoded.pub).toString('hex');
+      const bitgoPub = Buffer.from(bitgoDecoded.pub).toString('hex');
 
       assert.strictEqual(userPub, backupPub, 'User and backup should have same public key in reduced share');
       assert.strictEqual(backupPub, bitgoPub, 'Backup and BitGo should have same public key in reduced share');
+
+      // keyShare must be present and non-empty (opaque WASM bincode needed for DSG)
+      assert(userDecoded.keyShare.length > 0, 'User reduced share must include keyShare');
+      assert(backupDecoded.keyShare.length > 0, 'Backup reduced share must include keyShare');
+      assert(bitgoDecoded.keyShare.length > 0, 'BitGo reduced share must include keyShare');
+
+      // rootChainCode must be 32 bytes
+      assert.strictEqual(userDecoded.rootChainCode.length, 32, 'User rootChainCode must be 32 bytes');
+      assert.strictEqual(backupDecoded.rootChainCode.length, 32, 'Backup rootChainCode must be 32 bytes');
+      assert.strictEqual(bitgoDecoded.rootChainCode.length, 32, 'BitGo rootChainCode must be 32 bytes');
+
+      // All parties derive the same chaincode
+      assert.strictEqual(
+        Buffer.from(userDecoded.rootChainCode).toString('hex'),
+        Buffer.from(backupDecoded.rootChainCode).toString('hex'),
+        'User and backup should have same rootChainCode'
+      );
+      assert.strictEqual(
+        Buffer.from(backupDecoded.rootChainCode).toString('hex'),
+        Buffer.from(bitgoDecoded.rootChainCode).toString('hex'),
+        'Backup and BitGo should have same rootChainCode'
+      );
     });
   });