XSC002 Permit

• An adaptation of EIP-2612 for Xian.
• Allows for a user to grant a spender a certain amount of tokens to spend on their behalf, without having to submit a transaction to the network.
• Once permit is successfully called, the spender is granted an allowance, and can spend the users tokens as normal.

How it works :

The user constructs a message to sign, and the contract verifies the signature.

• The message follows consists of the following fields:

  • owner: The address of the owner of the permit
  • spender: The address of the spender
  • value: The amount of tokens to spend
  • deadline: The deadline for the permit
  • signature: The signature of the message
  • contract: The name of the contract to which the permit is granted

• The user / frontend dapp will construct the message like so :

  • msg = f"{owner}:{spender}:{value}:{deadline}:{contract}"
  • signature = wallet.sign_msg(msg)

• The frontend dapp will then call permit(owner, spender, value, deadline, signature) on the contract

• permit() will :

  • construct the message like so :
    • msg = f"{owner}:{spender}:{value}:{deadline}:{ctx.this}"
  • assert the deadline is greater than the current time.
  • construct a permit_hash using SHA3 hash of msg.
  • assert permit_hash is None, if not, revert
  • store permit_hash in permits[permit_hash], to prevent replays
  • call verify(msg, signature)
  • if valid:
    • will add the permit_hash to permits
    • add the allowance to the spender

How to test :

• Setup testing harness by following the instructions in the contract dev environment
• Clone this repo to contracts
• Run pytest in the root directory of the repo