Problem using ChargeAssetTxPayment SignedExtension

[jsidorenko]

I was trying to check the ChargeAssetTxPayment SignedExtension and noticed an issue with the data encoding/decoding, which prevents us from using such an extension.

In the beginning, I've noticed the tx goes through and works well when I use the KeyPair and submit it in this way:

const keyring = new Keyring({ type: 'sr25519' });
const alice = keyring.addFromUri('//Alice');
const charlie = keyring.addFromUri('//Charlie');
await api.tx.balances.transfer(charlie.address, native(100)).signAsync(alice, { assetId: 1 });

Next, I tried to use a similar approach and inject a signer (could be from web extension or simulated like in the code below):

const signer = new SingleAccountSigner(registry, alice);
await api.tx.balances.transfer(charlie.address, native(100)).signAsync(alice.address, { signer, assetId: 1 });

And that tx fails!
By logging the received data in the node, I discovered that instead of assetId=1 the runtime receives assetId=16777216.

---

After days of debugging, I was able to reconstruct the whole data-flow:

1. createClass.#signViaSigner receives the correct assetId and then creates a payload in such a way:
   const payload = this.registry.createTypeUnsafe<SignerPayload>('SignerPayload'....
2. it asks the signer to sign the payload:
   if (isFunction(signer.signPayload)) { result = await signer.signPayload(payload.toPayload()); }
3. then stores the payload internally:
   super.addSignature(address, result.signature, payload.toPayload());

if we log the assetId (console.log(payload.toPayload().assetId);) it will print 0x01000000.
That's because it converts all the values to hex here and api.createType('Option<u32>', 1).toHex() returns 0x01000000.

4. Now, having the assetId='0x01000000' signer tries to construct the ExtrinsicPayload:
   const extrinsicPayload = this.#registry.createType('ExtrinsicPayload', payload, { version: payload.version });
   Once I logged the asset id from extrinsicPayload I received 16777216
   console.log(extrinsicPayload.toHuman().assetId.toString());
   I assume this is because api.createType('Option<u32>', '0x01000000').toString() returns 16777216

---

A quick way to reproduce (e.g. put the code below into this spec file):

    const registry = new TypeRegistry();
    const keyring = createTestKeyring({ type: 'ed25519' });
    const aliceEd = keyring.addPair(
      createPair({ toSS58: keyring.encodeAddress, type: 'ed25519' }, {
        publicKey: hexToU8a('0x88dc3417d5058ec4b4503e0c12ea1a0a89be200fe98922423d4334014fa6b0ee'),
        secretKey: hexToU8a('0xabf8e5bdbe30c65656c0a3cbd181ff8a56294a69dfedd27982aace4a7690911588dc3417d5058ec4b4503e0c12ea1a0a89be200fe98922423d4334014fa6b0ee')
      })
    );
    const signer = new SingleAccountSigner(registry, aliceEd);
    const api = await ApiPromise.create({ provider, registry, signer, throwOnConnect: true });

    const receiver = keyring.getPair('0xe659a7a1628cdd93febc04a4e0646ea20e9f5f0ce097d9a05290d4a9e054df4e').address;
    const sendValue = 321564789876512345n;
    const transfer1 = await api.tx.balances.transferAllowDeath(receiver, sendValue).signAsync(aliceEd.address, { assetId: 1 });
    const transfer2 = await api.tx.balances.transferAllowDeath(receiver, sendValue).signAsync(aliceEd, { assetId: 1 });

    console.log(transfer1.inner.signature.assetId.toString()); // 16777216
    console.log(transfer2.inner.signature.assetId.toString()); // 1

[jsidorenko]

TLDR:

api.createType('Option<AssetId>', 1).toHex() returns ‘0x01000000’.
api.createType('Option<AssetId>', '0x01000000').toString() returns 16777216

[jsidorenko]

As a possible solution we can apply a fix here:

      if (!isOption || value.isSome) {
        result[key] = isOption ? value.unwrap().toHex() : value.toHex();
      }

By unwrapping the optional value we'll get the proper value we need:

api.createType('Option<AssetId>', 1).toHex() returns ‘0x01000000’
api.createType('Option<AssetId>', 1).unwrap().toHex() returns ‘0x00000001’

[Robiquet]

Great investigation! We are seeing the same issue at Zeitgeist, we have added ChargeAssetTxPayment to our runtime but are unable to get it to work on our frontend with assetIds other than 0

[polkadot-js-bot]

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