Fix verifyWitness embedded extension-branch

src/index.ts

@@ -230,15 +230,21 @@ export function VerifyWitness(root: Buffer, key: Buffer, witness: Witness) {
         throw new VerificationError(`Key does not match the proof ${
             node.type}: expected ${node.key}, but got ${currentKey}`);
       }
-      cld = node.value;
+      cld = node.value as (Buffer | Buffer[][]);
       currentKey = currentKey.slice(node.key.length);
-      if (currentKey.length === 0) {
+      if (currentKey.length === 0 ||
+          (cld.length === 17 && currentKey.length === 1)) {
+        // The value is in an embedded branch. Extract it.
+        if (cld.length === 17) {
+          cld = (cld[currentKey[0]] as Buffer[])[1];
+          currentKey = currentKey.slice(1);
+        }
         if (idx !== witness.proof.length - 1) {
           throw new VerificationError(
               `Key length mismatch (${node.type}): expected ${
                   idx + 1} but got ${witness.proof.length}`);
         }
-        if (!cld.equals(witness.value!)) {
+        if (!(cld as Buffer).equals(witness.value!)) {
           throw new VerificationError(
               `Value mismatch: expected ${witness.value} but got ${cld}`);
         }

src/proof.spec.ts

@@ -78,4 +78,20 @@ describe(
         VerifyWitness(tree.root, Buffer.from('key2'), w2);
         VerifyWitness(tree.root, Buffer.from('key3'), w3);
       });
+
+
+      it('should create a merkle proof with an extension and embedded branch',
+         async () => {
+           await tree.put(Buffer.from('a'), Buffer.from('a'));
+           await tree.put(Buffer.from('b'), Buffer.from('b'));
+           await tree.put(Buffer.from('c'), Buffer.from('c'));
+
+           const w1 = await tree.get(Buffer.from('a'));
+           const w2 = await tree.get(Buffer.from('b'));
+           const w3 = await tree.get(Buffer.from('c'));
+
+           VerifyWitness(tree.root, Buffer.from('a'), w1);
+           VerifyWitness(tree.root, Buffer.from('b'), w2);
+           VerifyWitness(tree.root, Buffer.from('c'), w3);
+         });
     });