Crosschain borrow automation

[annamehr]

Protocol specs
UI specs

• Apps can already start working on it
• Analyze fees
• Batch under chopstick not yet correct
• Use call data and add to polkadot.js and analyze why so little money arrives at assethub
• Why are the fees so high?
• Need to hold DOT; it's a requirement
• Min. 2-3 days of work
• Testing with chopsticks
• Decode call https://polkadot.js.org/apps/?rpc=wss%3A%2F%2Ffullnode.centrifuge.io#/extrinsics/decode

Background

Given the merge of remote account derivation via xcm users/pallets/on-chain accounts/etc. are now able to have safe remote accounts on all system parachains. This enables us to make interaction with these chains safer and easer for users

The overall goal is that issuers can on-ramp and off-ramp USDC from Polkadot AssetHub without directly interacting with AssetHub. All interactions MUST be triggered through XCMs send from Centrifuge.

The usual on-ramp flow includes three steps:

• Wire USD to Circle; convert USD -> USDC
• Send USDC to account on AssetHub
• XCM-Transfer USDC to account on Centrifuge --- tx is signed on AssetHub

The last step will not be triggered on AssetHub but on Centrifuge. We can abstract away all complexity for the end-user, making it much safer than using polkadot.js.

The usual off-ramp flow includes x steps:

• XCM-Transfer USDC to user controlled account on AssetHub --- tx is signed on Centrifuge
• Local-Transfer USDC to off-ramp account on AssetHub --- tx is signed on AssetHub

:::info
💡 We can XCM-Transfer directly into the off-ramp account on AssetHub as Circle is rejecting those transfers, leading to a loss of funds.
:::

Requirements

• Withdraw assets from a remote accounts holdings and transfer them to Centrifuge --- by dispatching the appropriate XCM from Centrifuge
• Transact a call from a remote account on another chain --- by dispatching the appropriate XCM from Centrifuge
• Transfer an assets from Centrifuge to another chain and transact a call from a remote account on said chain afterwards --- by dispatching the appropriate XCM from Centrifuge

Foreseable Obstacles

:::danger
⚠️ Testing is only possible via Fudge, Altair and on Centrifuge. Dev and Demo do not work well here.
:::

💻 Technical Spec

Option A - Preferred

Adapting the existing runtime configuration to allow a batch(xtokens::transfer(..), pallet_xcm::send(..)). Where the first generates an XCM that transfers the funds to the remote controlled account, and the second creates the actual transaction to send tokens.

:::info
💡 Generating the derived account on other chains is trivial by using our AccountConversionApi.conversionOf runtime api with the multilocation that the receiving chain will see.
:::

Adapt Runtime Configuration

• Fix LocalOriginToLocation

  Current definition is:

  pub type LocalOriginToLocation = SignedToAccountId32<RuntimeOrigin, AccountId, RelayNetwork>;

  but rather must be

  struct ByGenesis<T>(PhandomtData<T>)
  
  impl<T> Get<Option<NetworkId>> for ByGenesis<T> {
      fn get() -> OptionM<NetworkId> {
          // retrieve genesis hash here.
          let genesis_hash = todo!();
          Some(NetoworkId::ByGensis(genesis_hash))
      }
  }
  
  pub type LocalOriginToLocation = SignedToAccountId32<RuntimeOrigin, AccountId, ByGenesis>;

  Note: Only possible if Expose Genesis Hash to Runtime paritytech/polkadot-sdk#3344 is solved. Else use None.

• Fix BasicCallFilter to allow pallet_xcm::send{..}

Option B

Creating correct XCM messages is hard - the orml-xtokens provides a good insight into how tedious it is to transfer assets with XCM. Unfortunately, most of the pallets method are private and not well suitable for composition.

:::success
📓 Proposal for...

• copy-pasta the transfer XCM generation logic from the orml-xtokens pallet will be the fastest to get this live. Waiting for orml to adapt our changes and us being able to use them will take too long.

:::

pallet-restricted-xtokens Extension

Types

RemoteTransact

A type that allows to easily create the right Instruction.

pub struct RemoteTransact<T: Config> {
	call: BoundedVec<u8, T::MaxTransact>,
	require_weight_at_most: Weight,
}

impl<T: Config> Into<Instruction<()>> for RemoteTransact<T> {
	fn into(self) -> Instruction<()> {
		Instruction::Transact {
			origin_kind: OriginKind::SovereignAccount,
			require_weight_at_most: self.require_weight_at_most,
			call: From::<Vec<u8>>::from(self.call.into()),
		}
	}
}

Calls

The idea is to extend our pallet-restricted-xtokens with three additional calls:

---

fn remote_transact

:::info
💡 An extrinsics allows to create an XCM that is send to another chain and executes a transaction on behalf of the sender on Centrifuge via its remote account.
:::

• Logical Requirements
  • Callable by all signed origins
  • MUST use Instruction::DescendOrigin(who) before the Instruction::Transact
• Method Parameters

  origin: OriginFor<T>,

  remote_transact: RemoteTransact<T>,

  // NOTE: Type used for defining how much the user wants 
  //       to pay at most on the receiving side for the execution.
  dest_weight_limit: WeightLimit,

---

fn remote_deposit_transact

:::info
💡 An extrinsics allows to send assets to another account on another chain and in the same XCM processing to also execute a transaction on behalf of the sender on Centrifuge via its remote account.
:::
:::info
💡 NOTE: Needed for doing off-ramping in one step. Receiver of funds in this case will also be the remote account.
:::

• Logical Requirements
  • Callable by all signed origins
  • MUST use Instruction::DescendOrigin(who) before the Instruction::Transact
• Method Parameters

  origin: OriginFor<T>,

  // NOTE: Currency taken from out chain and send to the other chain
  currency_id: T::CurrencyId,

  amount: T::Balance,

  // NOTE: Receiver of the tokens
  remote_dest: Box<VersionedMultiLocation>,

  remote_transact: RemoteTransact<T>,

  // NOTE: Type used for defining how much the user wants 
  //       to pay at most on the receiving side for the execution.
  dest_weight_limit: WeightLimit,

---

fn remote_withdraw

:::info
💡 An extrinsics allows to withdraw funds from the origins remote account on another chain and sends it to Centrifuge/or possibly any other chain
:::

• Logical Requirements
  • Callable by all signed origins
  • MUST use Instruction::DescendOrigin(who) before the XCM Transfer in order to take tokens out of the remote account holdings.
• Method Parameters

  origin: OriginFor<T>,

  // NOTE: Currency taken from out chain and send to the other chain
  currency_id: T::CurrencyId,

  amount: T::Balance,

  // NOTE: Receiver of the tokens
  remote_dest: Box<VersionedMultiLocation>,

  // NOTE: Type used for defining how much the user wants 
  //       to pay at most on the receiving side for the execution.
  dest_weight_limit: WeightLimit,

---

🔗 Links

• UI Usage Description
• Protocol Testing