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81 lines
3.3 KiB
ReStructuredText
81 lines
3.3 KiB
ReStructuredText
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Transaction tear-offs
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=====================
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Example of usage
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----------------
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Let’s focus on a code example. We want to construct a transaction with commands containing interest rate fix data as in:
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:doc:`oracles`. After construction of a partial transaction, with included ``Fix`` commands in it, we want to send it
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to the Oracle for checking and signing. To do so we need to specify which parts of the transaction are going to be
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revealed. That can be done by constructing filtering function over fields of ``WireTransaction`` of type ``(Any) ->
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Boolean``.
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.. container:: codeset
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.. sourcecode:: kotlin
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val partialTx = ...
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val oracle: Party = ...
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fun filtering(elem: Any): Boolean {
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return when (elem) {
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is Command -> oracleParty.owningKey in elem.signers && elem.value is Fix
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else -> false
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}
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}
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Assuming that we already assembled partialTx with some commands and know the identity of Oracle service, we construct
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filtering function over commands - ``filtering``. It performs type checking and filters only ``Fix`` commands as in
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IRSDemo example. Then we can construct ``FilteredTransaction``:
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.. container:: codeset
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.. sourcecode:: kotlin
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val wtx: WireTransaction = partialTx.toWireTransaction()
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val ftx: FilteredTransaction = wtx.buildFilteredTransaction(filtering)
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In the Oracle example this step takes place in ``RatesFixFlow`` by overriding ``filtering`` function, see:
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:ref:`filtering_ref`.
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``FilteredTransaction`` holds ``filteredLeaves`` (data that we wanted to reveal) and Merkle branch for them.
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.. container:: codeset
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.. sourcecode:: kotlin
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// Direct accsess to included commands, inputs, outputs, attachments etc.
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val cmds: List<Command> = ftx.filteredLeaves.commands
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val ins: List<StateRef> = ftx.filteredLeaves.inputs
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val timestamp: Timestamp? = ftx.filteredLeaves.timestamp
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...
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.. literalinclude:: ../../samples/irs-demo/src/main/kotlin/net/corda/irs/api/NodeInterestRates.kt
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:language: kotlin
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:start-after: DOCSTART 1
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:end-before: DOCEND 1
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Above code snippet is taken from ``NodeInterestRates.kt`` file and implements a signing part of an Oracle. You can
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check only leaves using ``leaves.checkWithFun { check(it) }`` and then verify obtained ``FilteredTransaction`` to see
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if data from ``PartialMerkleTree`` belongs to ``WireTransaction`` with provided ``id``. All you need is the root hash
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of the full transaction:
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.. container:: codeset
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.. sourcecode:: kotlin
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if (!ftx.verify(merkleRoot)){
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throw MerkleTreeException("Rate Fix Oracle: Couldn't verify partial Merkle tree.")
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}
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Or combine the two steps together:
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.. container:: codeset
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.. sourcecode:: kotlin
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ftx.verifyWithFunction(merkleRoot, ::check)
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.. note:: The way the ``FilteredTransaction`` is constructed ensures that after signing of the root hash it's impossible to add or remove
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leaves. However, it can happen that having transaction with multiple commands one party reveals only subset of them to the Oracle.
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As signing is done now over the Merkle root hash, the service signs all commands of given type, even though it didn't see
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all of them. This issue will be handled after implementing partial signatures.
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