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Oracles
Summary
- A fact can be included in a transaction as part of a command
- An oracle is a service that will only sign the transaction if the included fact is true
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Overview
In many cases, a transaction's contractual validity depends on some external piece of data, such as the current exchange rate. However, if we were to let each participant evaluate the transaction's validity based on their own view of the current exchange rate, the contract's execution would be non-deterministic: some signers would consider the transaction valid, while others would consider it invalid. As a result, disagreements would arise over the true state of the ledger.
Corda addresses this issue using oracles. Oracles are network services that, upon request, provide commands that encapsulate a specific fact (e.g. the exchange rate at time x) and list the oracle as a required signer.
If a node wishes to use a given fact in a transaction, they request a command asserting this fact from the oracle. If the oracle considers the fact to be true, they send back the required command. The node then includes the command in their transaction, and the oracle will sign the transaction to assert that the fact is true.
If they wish to monetize their services, oracles can choose to only sign a transaction and attest to the validity of the fact it contains for a fee.
Transaction tear-offs
To sign a transaction, the only information the oracle needs to see is their embedded command. Providing any additional transaction data to the oracle would constitute a privacy leak. Similarly, a non-validating notary only needs to see a transaction's input states.
To combat this, the transaction proposer(s) uses a Merkle tree to "tear off" any parts of the transaction that the oracle/notary doesn't need to see before presenting it to them for signing. A Merkle tree is a well-known cryptographic scheme that is commonly used to provide proofs of inclusion and data integrity. Merkle trees are widely used in peer-to-peer networks, blockchain systems and git.
The advantage of a Merkle tree is that the parts of the transaction that were torn off when presenting the transaction to the oracle cannot later be changed without also invalidating the oracle's digital signature.
Transaction Merkle trees
A Merkle tree is constructed from a transaction by splitting the transaction into leaves, where each leaf contains either an input, an output, a command, or an attachment. The Merkle tree also contains the other fields of the WireTransaction, such as the time-window, the notary, the type and the signers.
Next, the Merkle tree is built in the normal way by hashing the concatenation of nodes’ hashes below the current one together. It’s visible on the example image below, where H denotes sha256 function, "+" - concatenation.
The transaction has two input states, one output state, one attachment, one command and a time-window. For brevity we didn't include all leaves on the diagram (type, notary and signers are presented as one leaf labelled Rest - in reality they are separate leaves). Notice that if a tree is not a full binary tree, leaves are padded to the nearest power of 2 with zero hash (since finding a pre-image of sha256(x) == 0 is hard computational task) - marked light green above. Finally, the hash of the root is the identifier of the transaction, it's also used for signing and verification of data integrity. Every change in transaction on a leaf level will change its identifier.
Hiding data
Hiding data and providing the proof that it formed a part of a transaction is done by constructing Partial Merkle Trees (or Merkle branches). A Merkle branch is a set of hashes, that given the leaves’ data, is used to calculate the root’s hash. Then that hash is compared with the hash of a whole transaction and if they match it means that data we obtained belongs to that particular transaction.
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 included since it's holding time-window information. Nodes labelled Provided form the Partial Merkle Tree, black ones are omitted. Having time-window with the command that should be in a violet node place and branch we are able to calculate root of this tree and compare it with original transaction identifier - we have a proof that this command and time-window belong to this transaction.