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<div class="section" id="transaction-tear-offs">
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<h1>Transaction tear-offs<a class="headerlink" href="#transaction-tear-offs" title="Permalink to this headline">¶</a></h1>
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<p>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 achive 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?</p>
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<p>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 <a class="reference external" href="https://en.wikipedia.org/wiki/Merkle_tree">here</a>.</p>
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<div class="section" id="merkle-trees-in-corda">
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<h2>Merkle trees in Corda<a class="headerlink" href="#merkle-trees-in-corda" title="Permalink to this headline">¶</a></h2>
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<p>Transactions are split into leaves, each of them contains either input, output, command or attachment. Other fields like
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timestamp or signers are not used in the calculation.
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Next, the Merkle tree is built in the normal way by hashing the concatenation
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of nodes’ hashes below the current one together. It’s visible on the example image below, where <code class="docutils literal"><span class="pre">H</span></code> denotes sha256 function,
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“+” - concatenation.</p>
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<img alt="_images/merkleTree.png" src="_images/merkleTree.png" />
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<p>The transaction has one input state, one output and three commands. If a tree is not a full binary tree, the rightmost nodes are
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duplicated in hash calculation (dotted lines).</p>
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<p>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.</p>
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</div>
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<div class="section" id="hiding-data">
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<h2>Hiding data<a class="headerlink" href="#hiding-data" title="Permalink to this headline">¶</a></h2>
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<p>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.</p>
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<img alt="_images/partialMerkle.png" src="_images/partialMerkle.png" />
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<p>In the example above, the red node is the one holding data for signing Oracle service. Blue nodes’ hashes form the Partial Merkle
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Tree, dotted ones are not included. Having the command that should be in a red 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 belongs to this transaction.</p>
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</div>
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<div class="section" id="example-of-usage">
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<h2>Example of usage<a class="headerlink" href="#example-of-usage" title="Permalink to this headline">¶</a></h2>
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<p>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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<a class="reference internal" href="oracles.html"><span class="doc">Writing oracle services</span></a>.
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After construction of a partial transaction, with included <code class="docutils literal"><span class="pre">Fix</span></code> 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 functions for inputs, outputs, attachments and commands separately. If a function is not provided by default none
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of the elements from this group will be included in a Partial Merkle Tree.</p>
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<div class="codeset container">
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<div class="highlight-kotlin"><div class="highlight"><pre><span></span><span class="k">val</span> <span class="py">partialTx</span> <span class="p">=</span> <span class="p">...</span>
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<span class="k">val</span> <span class="py">oracle</span><span class="p">:</span> <span class="n">Party</span> <span class="p">=</span> <span class="p">...</span>
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<span class="k">fun</span> <span class="nf">filterCommands</span><span class="p">(</span><span class="n">c</span><span class="p">:</span> <span class="n">Command</span><span class="p">)</span> <span class="p">=</span> <span class="n">oracle</span><span class="p">.</span><span class="n">owningKey</span> <span class="k">in</span> <span class="n">c</span><span class="p">.</span><span class="n">signers</span> <span class="p">&&</span> <span class="n">c</span><span class="p">.</span><span class="n">value</span> <span class="k">is</span> <span class="n">Fix</span>
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<span class="k">val</span> <span class="py">filterFuns</span> <span class="p">=</span> <span class="n">FilterFuns</span><span class="p">(</span><span class="n">filterCommands</span> <span class="p">=</span> <span class="o">::</span><span class="n">filterCommands</span><span class="p">)</span>
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</pre></div>
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</div>
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</div>
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<p>Assuming that we already assembled partialTx with some commands and know the identity of Oracle service,
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we pass filtering function over commands - <code class="docutils literal"><span class="pre">filterCommands</span></code> to <code class="docutils literal"><span class="pre">FilterFuns</span></code>. It filters only
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commands of type <code class="docutils literal"><span class="pre">Fix</span></code> as in IRSDemo example. Then we can construct <code class="docutils literal"><span class="pre">FilteredTransaction</span></code>:</p>
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<div class="codeset container">
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<div class="highlight-kotlin"><div class="highlight"><pre><span></span><span class="k">val</span> <span class="py">wtx</span><span class="p">:</span> <span class="n">WireTransaction</span> <span class="p">=</span> <span class="n">partialTx</span><span class="p">.</span><span class="n">toWireTransaction</span><span class="p">()</span>
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<span class="k">val</span> <span class="py">ftx</span> <span class="p">=</span> <span class="n">FilteredTransaction</span><span class="p">.</span><span class="n">buildMerkleTransaction</span><span class="p">(</span><span class="n">wtx</span><span class="p">,</span> <span class="n">filterFuns</span><span class="p">)</span>
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</pre></div>
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</div>
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</div>
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<p>In the Oracle example this step takes place in <code class="docutils literal"><span class="pre">RatesFixFlow</span></code>:</p>
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<div class="codeset container">
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<div class="highlight-kotlin"><div class="highlight"><pre><span></span><span class="k">val</span> <span class="py">flow</span> <span class="p">=</span> <span class="n">RatesFixFlow</span><span class="p">(</span><span class="n">partialTx</span><span class="p">,</span> <span class="n">filterFuns</span><span class="p">,</span> <span class="n">oracle</span><span class="p">,</span> <span class="n">fixOf</span><span class="p">,</span> <span class="s">"0.675"</span><span class="p">.</span><span class="n">bd</span><span class="p">,</span> <span class="s">"0.1"</span><span class="p">.</span><span class="n">bd</span><span class="p">)</span>
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</pre></div>
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</div>
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</div>
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<p><code class="docutils literal"><span class="pre">FilteredTransaction</span></code> holds <code class="docutils literal"><span class="pre">filteredLeaves</span></code> (data that we wanted to reveal) and Merkle branch for them.</p>
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<div class="codeset container">
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<div class="highlight-kotlin"><div class="highlight"><pre><span></span><span class="c1">// Getting included commands, inputs, outputs, attachments.</span>
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<span class="k">val</span> <span class="py">cmds</span><span class="p">:</span> <span class="n">List</span><span class="p"><</span><span class="n">Command</span><span class="p">></span> <span class="p">=</span> <span class="n">ftx</span><span class="p">.</span><span class="n">filteredLeaves</span><span class="p">.</span><span class="n">commands</span>
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<span class="k">val</span> <span class="py">ins</span><span class="p">:</span> <span class="n">List</span><span class="p"><</span><span class="n">StateRef</span><span class="p">></span> <span class="p">=</span> <span class="n">ftx</span><span class="p">.</span><span class="n">filteredLeaves</span><span class="p">.</span><span class="n">inputs</span>
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<span class="k">val</span> <span class="py">outs</span><span class="p">:</span> <span class="n">List</span><span class="p"><</span><span class="n">TransactionState</span><span class="p"><</span><span class="n">ContractState</span><span class="p">>></span> <span class="p">=</span> <span class="n">ftx</span><span class="p">.</span><span class="n">filteredLeaves</span><span class="p">.</span><span class="n">outputs</span>
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<span class="k">val</span> <span class="py">attchs</span><span class="p">:</span> <span class="n">List</span><span class="p"><</span><span class="n">SecureHash</span><span class="p">></span> <span class="p">=</span> <span class="n">ftx</span><span class="p">.</span><span class="n">filteredLeaves</span><span class="p">.</span><span class="n">attachments</span>
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</pre></div>
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</div>
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</div>
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<p>If you want to verify obtained <code class="docutils literal"><span class="pre">FilteredTransaction</span></code> all you need is the root hash of the full transaction:</p>
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<div class="codeset container">
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<div class="highlight-kotlin"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="p">(!</span><span class="n">ftx</span><span class="p">.</span><span class="n">verify</span><span class="p">(</span><span class="n">merkleRoot</span><span class="p">)){</span>
|
||
<span class="k">throw</span> <span class="n">MerkleTreeException</span><span class="p">(</span><span class="s">"Rate Fix Oracle: Couldn't verify partial Merkle tree."</span><span class="p">)</span>
|
||
<span class="p">}</span>
|
||
</pre></div>
|
||
</div>
|
||
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|
||
<div class="admonition note">
|
||
<p class="first admonition-title">Note</p>
|
||
<p class="last">The way the <code class="docutils literal"><span class="pre">FilteredTransaction</span></code> 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.</p>
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