Consensus model¶

Notary¶

We introduce the concept of a Notary, which is an authority responsible for attesting that for a given transaction, it had not signed another transaction consuming any of its input states. The data model is extended so that every state has an appointed notary:

/**
 * A wrapper for [ContractState] containing additional platform-level state information.
 * This is the definitive state that is stored on the ledger and used in transaction outputs
 */
data class TransactionState<out T : ContractState>(
        /** The custom contract state */
        val data: T,
        /** Identity of the notary that ensures the state is not used as an input to a transaction more than once */
        val notary: Party) {
    ...
}

All transactions have to be signed by their input state notary for the output states to be valid (apart from issue transactions, containing no input states).

When the notary is requested to sign a transaction, it either signs over it, attesting that the outputs are the unique successors of the inputs, or provides conflict information for any input state that had been consumed by another transaction it had signed before. In doing so, the notary provides the point of finality in the system. Until the notary signature is obtained, parties cannot be sure that an equally valid, but conflicting transaction, will not be regarded as confirmed. After the signature is obtained, the parties know that the inputs to this transaction have been uniquely consumed by this transaction. Hence it is the point at which we can say finality has occurred.

Multiple notaries¶

More than one notary can exist in the network. This gives the following benefits:

• Custom behaviour. We can have both validating and privacy preserving Notaries – parties can make a choice based on their specific requirements
• Load balancing. Spreading the transaction load over multiple Notaries will allow higher transaction throughput in the platform overall
• Low latency. Latency could be minimised by choosing a notary physically closer the transacting parties

A transaction should only be signed by a notary if all of its input states point to it. In cases where a transaction involves states controlled by multiple notaries, the states first have to be repointed to the same notary. This is achieved by using a special type of transaction that doesn’t modify anything but the notary pointer of the state. Ensuring that all input states point to the same notary is the responsibility of each involved party (it is another condition for an output state of the transaction to be valid)

Validation¶

One of the design decisions for a notary is whether or not to validate a transaction before committing its input states.

If a transaction is not checked for validity, it opens the platform to “denial of state” attacks, where anyone can build an invalid transaction consuming someone else’s states and submit it to the notary to get the states “blocked”. However, validation of a transaction requires the notary to be able to see the full contents of the transaction in question and its dependencies. This is an obvious privacy leak.

Our platform is flexible and we currently support both validating and non-validating notary implementations – a party can select which one to use based on its own privacy requirements.

The conflict information returned by the Notary specifies the consuming transaction id along with the identity of the party that had requested the commit.

If the conflicting transaction is valid, the current one gets aborted; if not – a dispute can be raised and the input states of the conflicting invalid transaction are “un-committed” (to be covered by legal process).

Timestamping¶

In this model the notary also acts as a Timestamping Authority, verifying the transaction timestamp command.

For a timestamp to be meaningful, its implications must be binding on the party requesting it. A party can obtain a timestamp signature in order to prove that some event happened before/on/or after a particular point in time. However, if the party is not also compelled to commit to the associated transaction, it has a choice of whether or not to reveal this fact until some point in the future. As a result, we need to ensure that the notary either has to also sign the transaction within some time tolerance, or perform timestamping and notarisation at the same time, which is the chosen behaviour for this model.

Running a Notary Service¶

At present we have two basic implementations that store committed input states in memory:

• SimpleNotaryService – commits the provided transaction without any validation
• ValidatingNotaryService – retrieves and validates the whole transaction history (including the given transaction) before committing

To run one of these services the node has to simply specify either SimpleNotaryService.Type or ValidatingNotaryService.Type in its advertisedServices set, and the correct type will be initialised.

Obtaining a signature¶

To obtain a signature from a notary use NotaryProtocol.Client, passing in a WireTransaction. The protocol will work out which notary needs to be called based on the input states and the timestamp command. For example, the following snippet can be used when writing a custom protocol:

fun getNotarySignature(wtx: WireTransaction): DigitalSignature.LegallyIdentifiable {
    return subProtocol(NotaryProtocol.Client(wtx))
}

On conflict the NotaryProtocol with throw a NotaryException containing the conflict details:

/** Specifies the consuming transaction for the conflicting input state */
data class Conflict(val stateHistory: Map<StateRef, ConsumingTx>)

/**
 * Specifies the transaction id, the position of the consumed state in the inputs, and
 * the caller identity requesting the commit
 */
data class ConsumingTx(val id: SecureHash, val inputIndex: Int, val requestingParty: Party)

Conflict handling and resolution is currently the responsibility of the protocol author.

Changing notaries¶

To change the notary for an input state, use the NotaryChangeProtocol. For example:

fun changeNotary(originalState: StateAndRef<ContractState>,
                 newNotary: Party): StateAndRef<ContractState> {
    val protocol = NotaryChangeProtocol.Instigator(originalState, newNotary)
    return subProtocol(protocol)
}

The protocol will:

1. Construct a transaction with the old state as the input and the new state as the output
2. Obtain signatures from all participants (a participant is any party that is able to consume this state in a valid transaction, as defined by the state itself)
3. Obtain the old notary signature
4. Record and distribute the final transaction to the participants so that everyone possesses the new state