Computed Origin Accounts¶

Introduction¶

The Computed Origin, previously referred to as the multilocation-derivative account, is an account computed when executing remote calls via XCM.

Computed origins are keyless (the private key is unknown). Consequently, Computed Origins can only be accessed through XCM extrinsics from the origin account. In other words, the origin account is the only account that can initiate transactions on your Computed Origin account, and if you lose access to your origin account, you’ll also lose access to your Computed Origin account.

The Computed Origin is calculated from the origin that is being used to execute the XCM in the destination chain. By default, this is the Sovereign account of the source chain in the destination chain. This origin can be mutated by the DescendOrigin XCM instruction. However, the destination chain can decide whether or not to use the newly mutated origin for the execution of the XCM. On Moonbeam, the Computed Origin account is used to execute the XCM.

Moonbeam-based networks follow the Computed Origins standard set by Polkadot, that is, through a blake2 hash of a data structure that depends on the origin of the XCM message. However, because Moonbeam uses Ethereum-styled accounts, Computed Origins are truncated to 20 bytes.

The Origin Conversion¶

The origin conversion for a remote call happens when the Transact instruction gets executed. The new origin on the target chain is the one that pays for the fees for XCM execution on the target chain.

For example, from the relay chain, the DescendOrigin instruction is natively injected by the XCM Pallet. In the case of Moonbase Alpha's relay chain (based on Westend), it has the following format (a multilocation junction):

{
  DescendOrigin: {
    X1: {
      AccountId32: {
        network: { westend: null },
        id: decodedAddress,
      },
    },
  },
}

Where the decodedAddress corresponds to the address of the account who signed the transaction on the relay chain (in a decoded 32-byte format). You can make sure that your address is properly decoded by using the following snippet, which will decode an address if needed and ignore it if not:

import { decodeAddress } from '@polkadot/util-crypto';
const decodedAddress = decodeAddress('INSERT_ADDRESS');

When the XCM instruction gets executed in Moonbeam (Moonbase Alpha in this example), the origin will have mutated to the following multilocation:

{
  DescendOrigin: {
    parents: 1,
    interior: {
      X1: {
        AccountId32: {
          network: { westend: null },
          id: decodedAddress,
        },
      },
    },
  },
}

How to Calculate the Computed Origin¶

You can easily calculate the Computed Origin account through the calculate-multilocation-derivative-account or the calculate-remote-origin script in the xcm-tools repository.

The script accepts the following inputs:

• --ws-provider or -w - corresponds to the endpoint to use to fetch the Computed Origin. This should be the endpoint for the target chain
• --address or --a - specifies the source chain address that is sending the XCM message
• --para-id or --p - (optional) specifies the parachain ID of the origin chain of the XCM message. It is optional, as the XCM message might come from the relay chain (no parachain ID). Or parachains can act as relay chains for other parachains
• --parents - (optional) corresponds to the parents value of the source chain in relation to the target chain. If you're calculating the Computed Origin account for an account on the relay chain, this value would be 1. If left out, the parents value defaults to 0

To use the script, you can take the following steps:

1. Clone the xcm-tools repo
2. Run yarn to install the necessary packages

3. Run the script

   yarn calculate-multilocation-derivative-account \
   --ws-provider INSERT_RPC_ENDPOINT \
   --address INSERT_ORIGIN_ACCOUNT \
   --para-id INSERT_ORIGIN_PARACHAIN_ID_IF_APPLIES \
   --parents INSERT_PARENTS_VALUE_IF_APPLIES
   

You can also calculate the Computed Origin account using the multilocationToAddress function of the XCM Utilities Precompile.

Calculate the Computed Origin on a Moonbeam-based Network¶

For example, to calculate the Computed Origin on Moonbase Alpha for Alice's relay chain account, which is 5DV1dYwnQ27gKCKwhikaw1rz1bYdvZZUuFkuduB4hEK3FgDT, you would use the following command to run the script:

yarn calculate-multilocation-derivative-account \
--ws-provider wss://wss.api.moonbase.moonbeam.network \
--address 5DV1dYwnQ27gKCKwhikaw1rz1bYdvZZUuFkuduB4hEK3FgDT \
--parents 1

The returned output includes the following values:

Consequently, for this example, Alice's Computed Origin account on Moonbase Alpha is 0xdd2399f3b5ca0fc584c4637283cda4d73f6f87c0. Note that Alice is the only person who can access this account through a remote transact from the relay chain, as she is the owner of its private keys and the Computed Origin account is keyless.