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* Move merkle building extension functions on wire tx to WireTransaction class. * Add timestamp, notary, transaction type and signers to wire transaction id calculation. * Change construction of MerkleTree from duplicating last node on a given level to padding leaves' list with zero hash to size of the nearest power of 2 - so we always have a full binary tree. The problem was that it was possible to construct 2 different transactions with the same ids. Trick worked for txs having number of leaves that were not power of 2. * Update tear-offs documentation and diagrams to reflect changes in construction of Merkle trees - padding with zero hashes and including all WireTransaction fields in id computation. * Change in filtering API of WireTransaction for partial Merkle trees calculation. Instead of many filtering functions over a transaction only one needs to be provided. Additional change to check and verification of FilteredTransaction. * IRS demo change. Make filtering function a protected method of RatesFixFlow class. Comment on situation when capturing too much scope and connected problems with checkpointing. Change oracle and tear-offs documentation.
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6.3 KiB
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126 lines
6.3 KiB
ReStructuredText
Transaction tear-offs
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=====================
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One of the basic data structures in our platform is a transaction. It can be passed around to be signed and verified,
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also by third parties. The construction of transactions assumes that they form a whole entity with input and output states,
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commands and attachments inside. However all sensitive data shouldn’t be revealed to other nodes that take part in
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the creation of transaction on validation level (a good example of this situation is the Oracle which validates only
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embedded commands). How to achieve it in a way that convinces the other party the data they got for signing really did form
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a part of the transaction?
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We decided to use well known and described cryptographic scheme to provide proofs of inclusion and data integrity.
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Merkle trees are widely used in peer-to-peer networks, blockchain systems and git.
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You can read more on the concept `here <https://en.wikipedia.org/wiki/Merkle_tree>`_.
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Merkle trees in Corda
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---------------------
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Transactions are split into leaves, each of them contains either input, output, command or attachment. Additionally, in
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transaction id calculation we use other fields of ``WireTransaction`` like timestamp, notary, type and signers.
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Next, the Merkle tree is built in the normal way by hashing the concatenation of nodes’ hashes below the current one together.
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It’s visible on the example image below, where ``H`` denotes sha256 function, "+" - concatenation.
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.. image:: resources/merkleTree.png
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The transaction has two input states, one of output, attachment and command each and timestamp. For brevity we didn't
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include all leaves on the diagram (type, notary and signers are presented as one leaf labelled Rest - in reality
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they are separate leaves). Notice that if a tree is not a full binary tree, leaves are padded to the nearest power
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of 2 with zero hash (since finding a pre-image of sha256(x) == 0 is hard computational task) - marked light green above.
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Finally, the hash of the root is the identifier of the transaction, it's also used for signing and verification of data integrity.
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Every change in transaction on a leaf level will change its identifier.
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Hiding data
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-----------
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Hiding data and providing the proof that it formed a part of a transaction is done by constructing Partial Merkle Trees
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(or Merkle branches). A Merkle branch is a set of hashes, that given the leaves’ data, is used to calculate the root’s hash.
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Then that hash is compared with the hash of a whole transaction and if they match it means that data we obtained belongs
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to that particular transaction.
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.. image:: resources/partialMerkle.png
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In the example above, the node ``H(f)`` is the one holding command data for signing by Oracle service. Blue leaf ``H(g)`` is also
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included since it's holding timestamp information. Nodes labelled ``Provided`` form the Partial Merkle Tree, black ones
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are omitted. Having timestamp with the command that should be in a violet node place and branch we are able to calculate
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root of this tree and compare it with original transaction identifier - we have a proof that this command and timestamp
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belong to this transaction.
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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`.
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After construction of a partial transaction, with included ``Fix`` commands in it, we want to send it to the Oracle for checking
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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
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filtering function over fields of ``WireTransaction`` of type ``(Any) -> 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,
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we construct filtering function over commands - ``filtering``. It performs type checking and filters only ``Fix`` commands
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as in 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: :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.
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You can check only leaves using ``leaves.checkWithFun { check(it) }`` and then verify obtained ``FilteredTransaction``
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to see 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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