Adding `aggregateTx_InnerTx.toAggregate()` for single signer

[rg911]

Enhancement feature - For discussion

• Raised by Aleix on slack

AggregatedCompleteTransaction

At the moment when issuing an Aggregated Complete Transaction, PublicAccount is used as a mandatory parameter on innerTx.toAggregate(..) for every inner transactions. Catapult server will validate cosignatories if multiple accounts (signers) provided in the inner transactions to make sure the AggregateCompleteTransaction isComplete. There could be potentially an enhancement to be created by making this parameter optional. So that the ACT Can/Only be signed by the issuer if other signers are unknown.

example:

const transferTransaction = TransferTransaction.create(
                Deadline.create(),
                recipient.address,
                [NetworkCurrencyMosaic.createAbsolute(1)],
                PlainMessage.create('test-message'),
                NetworkType.MIJIN_TEST,
            );
const aggregateTransaction = AggregateTransaction.createComplete(Deadline.create(),
                [transferTransaction.toAggregate()],
                NetworkType.MIJIN_TEST,
                [],
            );
const signedTransaction = aggregateTransaction.signWith(issuer, generationHash);

• Node that the inner transaction's toAggregate method is taking zero parameter, so the only the issuer is needed to sign the ACT

[rg911]

My view: this enhancement is Optional as when constructing the ACT, the toAggregate() method can take the issuer's Public Account if other signer is unknown, so that only the issuer is required to sign.

const transferTransaction = TransferTransaction.create(
                Deadline.create(),
                account2.address,
                [NetworkCurrencyMosaic.createAbsolute(1)],
                PlainMessage.create('test-message'),
                NetworkType.MIJIN_TEST,
            );
const aggregateTransaction = AggregateTransaction.createComplete(Deadline.create(),
                [transferTransaction.toAggregate(issuer.publicAccount)],
                NetworkType.MIJIN_TEST,
                [],
            );
const signedTransaction = aggregateTransaction.signWith(issuer, generationHash);

• Above code is currently working in the SDK. As can be seen, if we use the issuer's public account in the toAggregate(..) method, it is effectively doing the same job as taking no param

[evias]

This feature in fact delegates signature validation to an external entity and can be useful for application that implement aggregate transactions without knowing exactly who will be signing.

What can be done on the side of the SDK is to have the parameter set to be optional, and delegate the verification of "correct signer" to the broadcaster where, in case the signer is not set, the issuer is used. In those cases, transactions will most likely be passed over as payloads, and wallets intercept the payload and fills it with the correct issuer public key before it get broadcast.

The AggregateTransaction.signWith method could stay as is and the toAggregate method should store undefined in the publicAccount field such that signWith will recognize and fill with issuer when needed.

[rg911]

Just to confirm that this logic only applies on AggregateComplete transaction!