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Scripts

In this repo there are a few scripts to interact with the contract. It is not just as simple as calling a method in the contract as we will have to do manage the subscriber's MPC public key.

Key derivation​

You'll see in each of these scripts we are using a file called derive-mpc-key.js, as it's name suggests this is used to derive the public key of the MPC key being used to sign the transactions on behalf of the subscriber. Feel free to look into this code but you don't particularly need to understand it.

However, there are some things worth noting. NEAR accounts usually use ed25519 keys but they also support secp256k1 keys. As mentioned previously we are using secp256k1 keys here since this is the only key type currently supported by the MPC contract. You can also see that the full path is a combination of the predecessor to the MPC contract (the subscription contract) along with the derivation path given as the argument. The inclusion of the predecessor means that only this contract is able to sign transactions for the given key.

Subscribe​

To start a new subscription we simply derive the MPC key for the subscriber by inputting the contractAccountId as the predecessorId and the subscriberAccountId as the derviationPath and add it to the account. We then call the subscribe method in the contract.

Charge​

In the charge script, we call the charge_subscription method from the admin account to charge a payment from the subscriber account. To call this method we need to supply the off-chain data to construct the transaction as mentioned before. We need:

  • The subscriber's MPC controlled public_key, so the MPC signs a transaction for the correct public key.
  • The next nonce of the key, so the transaction is unique.
  • A block hash within the last 24 hours. This is used to ensure that the transaction was recently signed. If the signed transaction is not relayed within 24 hours of the block hash supplied then the network will reject the transaction.

These details are available via RPC calls.

The admin then calls the charge_subscription method with the input details and the account Id of the subscriber being charged. We are attaching 0.1 NEAR as deposit for the MPC contract which - in most cases - will be more than enough (the required deposit is variable depending on traffic to the MPC) and we will be refunded any excess. The admin is attaching a deposit and not just the contract because refunds are given to the original signer of the call to the MPC.

We then fetch the result (which is the signed transaction) from the transaction outcome, convert it to a Uint8Array, serialize it to base64, and then broadcast it to the network.

This will execute the method call to the contract from the subscriber charging the subscriber 5 NEAR tokens, provided the subscriber has enough funds in their account.

Unsubscribe​

To unsubscribe from the service the subscriber calls the unsubscribe method in the contract. While, with the deployed code, the contract will no longer have access to the path to sign transactions for the user it is best practice to remove the MPC key from the account just in case the contract is compromised.

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