How to use as_derivative function found in the Utility pallet?

Question

I found the as_derivative() function call in the FRAME Utility pallet which suggests that I can use it to call a function from address_A on behalf of address_B, provided that address_B was made as a derivative account from address_A.

Using the polkadot.js extension I created a derivative account from address_A and funded it with some test tokens. Then using the polkadot.js/apps extrinsics tab I wrapped a basic transfer_keep_alive() in a call to as_derivative() so that I would be able to submit and pay for transaction fees using address_A, but the transfer amount would be sent from address_B.

However, when I submit the transaction I receive a Balance.InsufficientBalance error. I have double-checked that the derivative address I made with the polkadot.js extension does in fact have a sufficient amount of tokens, so I am not sure what I am missing.

Answer 1

It turns out this is a bit of a naming collision. The derived account that I created using the polkadot.js extension (known as /soft-path or //hard-path derivation) is different from the type of derived address the Utility pallet requires for the as_derivative() function.

In order to create a derived address that works with as_derivative(), I needed to use a different function found in the polkdot-js/util-crypto library called encodeDerivedAddress()

By passing in the raw byte array along with an index number, I was able to generate a usable derivative address. Once funded, the as_derivative() function call I attempted above worked as expected.