Ownership Predicate Specification¶

Overview¶

The ownership predicate is the one of the simplest useful predicates. It gives an address, specified as the state.data.owner , the ability to execute a state transition which changes any part of the owned subrange to a new StateObject by signing an approval transaction.

Predicate API¶

getOwner¶

{
      name: "getOwner",
      constant: true,
      inputs: [],
      outputs: [
          {
              name: "stateOwner",
              type: "address"
          }
      ]
}

Description¶

The getOwner API method allows the client or operator to query the current owner of the state.

Inputs¶

N/A

Outputs¶

stateOwner - address: the current owner based on the state object. Will equal data.owner .

Justification¶

This function allows developers to get the owner without directly dissecting the state object.

send¶

{
      name: "send",
      constant: false,
      inputs: [
          {
              name: "newState",
              type: "StateObject"
          },
          {
              name: "originBlock",
              type: "uint"
          },
          {
              name: "maxBlock",
              type: "uint"
          }
      ],
      outputs: []
}

Description¶

The send method is used to set the state to a new arbitrary state object, given a signature.

Inputs¶

newState - StateObject : the state object that the owner desires to mutate to.
originBlock - uint : the maximum plasma blocknumber of the ownership StateUpdate s from which you are spending.
maxBlock - uint : the maximum plasma block number for which the send is valid.

Outputs¶

N/A

Justification¶

Being able to spend to any new state is the base property of ownership. The targetBlock may be used to produce replay protection while allowing some level of asynchronicity between the client and operator.

State Object Specification¶

struct ownershipStateData:
  owner: address

Fields¶

owner - address: The Ethereum public address of the person who may mutate the state.

Additional Predicate Contract Logic¶

N/A

Predicate Interface¶

Transition Execution¶

def verifyStateTransition(preState: StateUpdate, input: StandardTransaction, witness: bytes postState: StateUpdate)

Requirements¶

MUST ensure that the input.witness is a signature by the preState.stateObject.owner .
MUST ensure that the preState.plasmaBlockNumber is less thana the input.parameters.originBlock .
MUST ensure that the postState.range is the same as input.start and input.end .
MUST ensure that the input.parameters.newState is the same as the postState.state .
MUST ensure that the input.parameters.targetBlock is greater than or equal to the postState.plasmaBlockNumber .

Rationale¶

The addition of limbo exits has removed the need to always specify a target block number which is one more than the client’s currently verified block. Thus, we can allow transactions to be in flight for multiple blocks with this predicate.

Exit Finalization¶

def onExitGameFinalized(exit: Checkpoint, witness: myExitabilityWitness)

Requirements¶

MUST Send the total balance of the subrange to the exit.stateUpdate.state.owner .

Rationale¶

Exit Permission¶

def canStartExitGame(exit: Checkpoint, witness: myExitabilityWitness)

Requirements¶

MUST require via the witness or tx.sender that the person exiting is the same as exit.stateUpdate.state.data.owner .

Rationale¶

Exit Lockup¶

def getAdditionalExitPeriod(exit: Checkpoint, witness: myExitabilityWitness) -> uint256

Requirements¶

Return 0.

Rationale¶

No additonal lockup is required for safety.

Verification Plugin¶

State Transitions¶

def executeStateTransition(preState: StateUpdate, transaction: StandardTransaction)

Requirements¶

MUST ensure that the transaction.witness is a signature by the preState.stateObject.owner .
MUST ensure that the preState.plasmaBlockNumber is less thana the input.parameters.originBlock .
MUST return a StateUpdate with a range the same as transaction.start and transaction.end .
MUST return a StateUpdate with state is the same as the transaction.parameters.newState .
MUST ensure that the transaction.parameters.targetBlock is greater than or equal to the pending plasma block number .

Rationale¶

These steps always produce a StateUpdate which passes the predicate contract’s verifyStateTransition step.

Guarding Plugin¶

TODO