corda/docs/source/tutorial-tear-offs.rst

Transaction tear-offs

Example of usage

Let’s focus on a code example. We want to construct a transaction with commands containing interest rate fix data as in: oracles. After construction of a partial transaction, with included Fix commands in it, we want to send it to the Oracle for checking and signing. To do so we need to specify which parts of the transaction are going to be revealed. That can be done by constructing filtering function over fields of WireTransaction of type (Any) -> Boolean.

val partialTx = ...
val oracle: Party = ...
fun filtering(elem: Any): Boolean {
        return when (elem) {
            is Command -> oracleParty.owningKey in elem.signers && elem.value is Fix
            else -> false
        }
}

Assuming that we already assembled partialTx with some commands and know the identity of Oracle service, we construct filtering function over commands - filtering. It performs type checking and filters only Fix commands as in IRSDemo example. Then we can construct FilteredTransaction:

val wtx: WireTransaction = partialTx.toWireTransaction()
val ftx: FilteredTransaction = wtx.buildFilteredTransaction(filtering)

In the Oracle example this step takes place in RatesFixFlow by overriding filtering function, see: filtering_ref.

FilteredTransaction holds filteredLeaves (data that we wanted to reveal) and Merkle branch for them.

// Direct accsess to included commands, inputs, outputs, attachments etc.
val cmds: List<Command> = ftx.filteredLeaves.commands
val ins: List<StateRef> = ftx.filteredLeaves.inputs
val timestamp: Timestamp? = ftx.filteredLeaves.timestamp
...

../../samples/irs-demo/src/main/kotlin/net/corda/irs/api/NodeInterestRates.kt

Above code snippet is taken from NodeInterestRates.kt file and implements a signing part of an Oracle. You can check only leaves using leaves.checkWithFun { check(it) } and then verify obtained FilteredTransaction to see if data from PartialMerkleTree belongs to WireTransaction with provided id. All you need is the root hash of the full transaction:

if (!ftx.verify(merkleRoot)){
        throw MerkleTreeException("Rate Fix Oracle: Couldn't verify partial Merkle tree.")
}

Or combine the two steps together:

ftx.verifyWithFunction(merkleRoot, ::check)

Note

The way the FilteredTransaction is constructed ensures that after signing of the root hash it's impossible to add or remove leaves. However, it can happen that having transaction with multiple commands one party reveals only subset of them to the Oracle. As signing is done now over the Merkle root hash, the service signs all commands of given type, even though it didn't see all of them. This issue will be handled after implementing partial signatures.