Confidential identities

Confidential identities are key pairs where the corresponding X.509 certificate (and path) are not made public, so that parties who are not involved in the transaction cannot identify the owner. They are owned by a well-known identity, which must sign the X.509 certificate. Before constructing a new transaction the involved parties must generate and exchange new confidential identities, a process which is managed using SwapIdentitiesFlow (discussed below). The public keys of these confidential identities are then used when generating output states and commands for the transaction.

Where using outputs from a previous transaction in a new transaction, counterparties may need to know who the involved parties are. One example is the TwoPartyTradeFlow, where an existing asset is exchanged for cash. If confidential identities are being used, the buyer will want to ensure that the asset being transferred is owned by the seller, and the seller will likewise want to ensure that the cash being transferred is owned by the buyer. Verifying this requires both nodes to have a copy of the confidential identities for the asset and cash input states. IdentitySyncFlow manages this process. It takes as inputs a transaction and a counterparty, and for every confidential identity involved in that transaction for which the calling node holds the certificate path, it sends this certificate path to the counterparty.

SwapIdentitiesFlow

SwapIdentitiesFlow is typically run as a subflow of another flow. It takes as its sole constructor argument the counterparty we want to exchange confidential identities with. It returns a mapping from the identities of the caller and the counterparty to their new confidential identities. In the future, this flow will be extended to handle swapping identities with multiple parties at once.

You can see an example of using SwapIdentitiesFlow in TwoPartyDealFlow.kt .

SwapIdentitiesFlow goes through the following key steps:

• Generate a new confidential identity from our well-known identity
• Create a CertificateOwnershipAssertion object containing the new confidential identity (X500 name, public key)
• Sign this object with the confidential identity’s private key
• Send the confidential identity and aforementioned signature to counterparties, while receiving theirs
• Verify the signatures to ensure that identities were generated by the involved set of parties
• Verify the confidential identities are owned by the expected well known identities
• Store the confidential identities and return them to the calling flow

This ensures not only that the confidential identity X.509 certificates are signed by the correct well-known identities, but also that the confidential identity private key is held by the counterparty, and that a party cannot claim ownership of another party’s confidential identities.

IdentitySyncFlow

When constructing a transaction whose input states reference confidential identities, it is common for counterparties to require knowledge of which well-known identity each confidential identity maps to. IdentitySyncFlow handles this process. You can see an example of its use in TwoPartyTradeFlow.kt.

IdentitySyncFlow is divided into two parts:

• IdentitySyncFlow.Send
• IdentitySyncFlow.Receive

IdentitySyncFlow.Send is invoked by the party initiating the identity synchronization:

The identity synchronization flow goes through the following key steps:

• Extract participant identities from all input and output states and remove any well known identities. Required signers on commands are currently ignored as they are presumed to be included in the participants on states, or to be well-known identities of services (such as an oracle service)
• For each counterparty node, send a list of the public keys of the confidential identities, and receive back a list of those the counterparty needs the certificate path for
• Verify the requested list of identities contains only confidential identities in the offered list, and abort otherwise
• Send the requested confidential identities as PartyAndCertificate instances to the counterparty

IdentitySyncFlow will serve all confidential identities in the provided transaction, irrespective of well-known identity. This is important for more complex transaction cases with 3 or more parties, for example:

• Alice is building the transaction, and provides some input state x owned by a confidential identity of Alice
• Bob provides some input state y owned by a confidential identity of Bob
• Charlie provides some input state z owned by a confidential identity of Charlie

Alice may know all of the confidential identities ahead of time, but Bob not know about Charlie’s and vice-versa. The assembled transaction therefore has three input states x, y and z, for which only Alice possesses certificates for all confidential identities. IdentitySyncFlow must send not just Alice’s confidential identity but also any other identities in the transaction to the Bob and Charlie.