Defining transaction tear-offs

This tutorial will take you through the steps involved in defining a transaction tear-off.

Introduction

A transaction tear-off is a form of filtered transaction, in which the transaction proposer(s) uses a nested Merkle tree approach to “tear off” any parts of the transaction that the oracle/notary doesn’t need to see before presenting it to them for signing. Transaction tear-offs are used to hide transaction components for privacy purposes. With a transaction tear-off, oracles and non-validating notaries can only see their “related” transaction components, but not the full transaction details.

Filtering the transaction fields

Suppose we want to construct a transaction that includes commands containing interest rate fix data as in Writing oracle services. Before sending the transaction to the oracle to obtain its signature, we need to filter out every part of the transaction except for the Fix commands.

To do so, we need to create a filtering function that specifies which fields of the transaction should be included. Each field will only be included if the filtering function returns true when the field is passed in as input.

val filtering = Predicate<Any> {
    when (it) {
        is Command<*> -> oracle.owningKey in it.signers && it.value is Fix
        else -> false
    }
}

Constructing a filtered transaction

We can now use our filtering function to construct a FilteredTransaction:

val ftx: FilteredTransaction = stx.buildFilteredTransaction(filtering)

In the oracle example, this step takes place in RatesFixFlow by overriding the filtering function. See Using an oracle.

Both WireTransaction and FilteredTransaction inherit from TraversableTransaction, so access to the transaction components is exactly the same. Note that unlike WireTransaction, FilteredTransaction only holds data that we wanted to reveal (after filtering).

// Direct access to included commands, inputs, outputs, attachments etc.
val cmds: List<Command<*>> = ftx.commands
val ins: List<StateRef> = ftx.inputs
val timeWindow: TimeWindow? = ftx.timeWindow
// ...

Implementing transaction signing

The following code snippet is taken from NodeInterestRates.kt and implements a signing part of an Oracle.

fun sign(ftx: FilteredTransaction): TransactionSignature {
    ftx.verify()
    // Performing validation of obtained filtered components.
    fun commandValidator(elem: Command<*>): Boolean {
        require(services.myInfo.legalIdentities.first().owningKey in elem.signers && elem.value is Fix) {
            "Oracle received unknown command (not in signers or not Fix)."
        }
        val fix = elem.value as Fix
        val known = knownFixes[fix.of]
        if (known == null || known != fix)
            throw UnknownFix(fix.of)
        return true
    }

    fun check(elem: Any): Boolean {
        return when (elem) {
            is Command<*> -> commandValidator(elem)
            else -> throw IllegalArgumentException("Oracle received data of different type than expected.")
        }
    }

    require(ftx.checkWithFun(::check))
    ftx.checkCommandVisibility(services.myInfo.legalIdentities.first().owningKey)
    // It all checks out, so we can return a signature.
    //
    // Note that we will happily sign an invalid transaction, as we are only being presented with a filtered
    // version so we can't resolve or check it ourselves. However, that doesn't matter much, as if we sign
    // an invalid transaction the signature is worthless.
    return services.createSignature(ftx, services.myInfo.legalIdentities.first().owningKey)
}

NodeInterestRates.kt