UserOperation PaymasterAndData

When gas fee logic for a User Operation is being abstracted, it must contain a paymasterAndData field.

Creating the paymasterAndData

The User Operation paymasterAndData is simply a hex concatenation of two values:

1. The paymaster address.
2. Arbitrary data required by the Paymaster during validatePaymasterUserOp.

What does the data part mean?

The data portion of paymasterAndData has no enforced structure and can be encoded to include anything the paymaster contract requires to pass the validation phase. In the simplest case of a VerifyingPaymaster.sol implementation, this would include 3 things:

1. validUntil: The latest time the paymaster will sponsor a User Operation at.
2. validAfter: The earliest time the paymaster will sponsor a User Operation at.
3. signature: A signature created by an external service that the paymaster trusts.

This data will be parsed and verified during validatePaymasterUserOp. If the timestamps and signatures are valid, then they paymaster will accept to pay the User Operation gas. Further gas fee logic can be built on-top of this. For example, the external Paymaster service could have checked that the user has a valid subscription or payment method on file before signing the User Operation.

Note that this is only one example of how the paymasterAndData could work. By not forcing a structure on the data we can enable more flexibility in use cases for gas fee abstraction.