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[FAB-17135] Private data sharing doc
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Document private data enhancements in v2.0 including
private data sharing patterns and example.

Signed-off-by: David Enyeart <enyeart@us.ibm.com>
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Expand Up @@ -21,9 +21,8 @@ A collection is the combination of two elements:

1. **The actual private data**, sent peer-to-peer [via gossip protocol](../gossip.html)
to only the organization(s) authorized to see it. This data is stored in a
private state database on the peers of authorized organizations (sometimes
called a "side" database, or "SideDB"), which can be accessed from chaincode
on these authorized peers.
private state database on the peers of authorized organizations,
which can be accessed from chaincode on these authorized peers.
The ordering service is not involved here and does not see the
private data. Note that because gossip distributes the private data peer-to-peer
across authorized organizations, it is required to set up anchor peers on the channel,
Expand All @@ -45,6 +44,12 @@ third party can then compute the hash of the private data and see if it matches
state on the channel ledger, proving that the state existed between the collection
members at a certain point in time.

In some cases, you may decide to have a set of collections each comprised of a
single organization. For example an organization may record private data in their own
collection, which could later be shared with other channel members and
referenced in chaincode transactions. We'll see examples of this in the sharing
private data topic below.

### When to use a collection within a channel vs. a separate channel

* Use **channels** when entire transactions (and ledgers) must be kept
Expand Down Expand Up @@ -91,9 +96,7 @@ private data collections **(PDC)** can be defined to share private data between:
Using this example, peers owned by the **Distributor** will have multiple private
databases inside their ledger which includes the private data from the
**Distributor**, **Farmer** and **Shipper** relationship and the
**Distributor** and **Wholesaler** relationship. Because these databases are kept
separate from the database that holds the channel ledger, private data is
sometimes referred to as "SideDB".
**Distributor** and **Wholesaler** relationship.

![private-data.private-data](./PrivateDataConcept-3.png)

Expand Down Expand Up @@ -140,6 +143,210 @@ documentation on [transaction flow](../txflow.html).
their copy of the private state database and private writeset storage. The
private data is then deleted from the `transient data store`.

## Sharing private data

In many scenarios private data keys/values in one collection may need to be shared with
other channel members or with other private data collections, for example when you
need to transact on private data with a channel member or group of channel members
who were not included in the original private data collection. The receiving parties
will typically want to verify the private data against the on-chain hashes
as part of the transaction.

There are several aspects of private data collections that enable the
sharing and verification of private data:

* First, you don't necessarily have to be a member of a collection to write to a key in
a collection, as long as the endorsement policy is satisfied.
Endorsement policy can be defined at the chaincode level, key level (using state-based
endorsement), or collection level (starting in Fabric v2.0).

* Second, starting in v1.4.2 there is a chaincode API GetPrivateDataHash() that allows
chaincode on non-member peers to read the hash value of a private key. This is an
important feature as you will see later, because it allows chaincode to verify private
data against the on-chain hashes that were created from private data in previous transactions.

This ability to share and verify private data should be considered when designing
applications and the associated private data collections.
While you can certainly create sets of multilateral private data collections to share data
among various combinations of channel members, this approach may result in a large
number of collections that need to be defined.
Alternatively, consider using a smaller number of private data collections (e.g.
one collection per organization, or one collection per pair of organizations), and
then sharing private data with other channel members, or with other
collections as the need arises. Starting in Fabric v2.0, implicit organization-specific
collections are available for any chaincode to utilize,
so that you don't even have to define these per-organization collections when
deploying chaincode.

### Private data sharing patterns

When modeling private data collections per organization, multiple patterns become available
for sharing or transferring private data without the overhead of defining many multilateral
collections. Here are some of the sharing patterns that could be leveraged in chaincode
applications:

* **Use a corresponding public key for tracking public state** -
You can optionally have a matching public key for tracking public state (e.g. asset
properties, current ownership. etc), and for every organization that should have access
to the asset's corresponding private data, you can create a private key/value in each
organization's private data collection.

* **Chaincode access control** -
You can implement access control in your chaincode, to specify which clients can
query private data in a collection. For example, store an access control list
for a private data collection key or range of keys, then in the chaincode get the
client submitter's credentials (using GetCreator() chaincode API or CID library API
GetID() or GetMSPID() ), and verify they have access before returning the private
data. Similarly you could require a client to pass a passphrase into chaincode,
which must match a passphrase stored at the key level, in order to access the
private data. Note, this pattern can also be used to restrict client access to public
state data.

* **Sharing private data out of band** -
As an off-chain option, you could share private data out of band with other
organizations, and they can hash the key/value to verify it matches
the on-chain hash by using GetPrivateDataHash() chaincode API. For example,
an organization that wishes to purchase an asset from you may want to verify
an asset's properties and that you are the legitimate owner by checking the
on-chain hash, prior to agreeing to the purchase.

* **Sharing private data with other collections** -
You could 'share' the private data on-chain with chaincode that creates a matching
key/value in the other organization's private data collection. You'd pass the
private data key/value to chaincode via transient field, and the chaincode
could confirm a hash of the passed private data matches the on-chain hash from
your collection using GetPrivateDataHash(), and then write the private data to
the other organization's private data collection.

* **Transferring private data to other collections** -
You could 'transfer' the private data with chaincode that deletes the private data
key in your collection, and creates it in another organization's collection.
Again, use the transient field to pass the private data upon chaincode invoke,
and in the chaincode use GetPrivateDataHash() to confirm that the data exists in
your private data collection, before deleting the key from your collection and
creating the key in another organization's collection. To ensure that a
transaction always deletes from one collection and adds to another collection,
you may want to require endorsements from additional parties, such as a
regulator or auditor.

* **Using private data for transaction approval** -
If you want to get a counterparty's approval for a transaction before it is
completed (e.g. an on-chain record that they agree to purchase an asset for
a certain price), the chaincode can require them to 'pre-approve' the transaction,
by either writing a private key to their private data collection or your collection,
which the chaincode will then check using GetPrivateDataHash(). In fact, this is
exactly the same mechanism that the built-in lifecycle system chaincode uses to
ensure organizations agree to a chaincode definition before it is committed to
a channel. Starting with Fabric v2.0, this pattern
becomes more powerful with collection-level endorsement policies, to ensure
that the chaincode is executed and endorsed on the collection owner's own trusted
peer. Alternatively, a mutually agreed key with a key-level endorsement policy
could be used, that is then updated with the pre-approval terms and endorsed
on peers from the required organizations.

* **Keeping transactors private** -
Variations of the prior pattern can also eliminate leaking the transactors for a given
transaction. For example a buyer indicates agreement to buy on their own collection,
then in a subsequent transaction seller references the buyer's private data in
their own private data collection. The proof of transaction with hashed references
is recorded on-chain, only the buyer and seller know that they are the transactors,
but they can reveal the pre-images if a need-to-know arises, such as in a subsequent
transaction with another party who could verify the hashes.

Coupled with the patterns above, it is worth noting that transactions with private
data can be bound to the same conditions as regular channel state data, specifically:

* **Key level transaction access control** -
You can include ownership credentials in a private data value, so that subsequent
transactions can verify that the submitter has ownership privilege to share or transfer
the data. In this case the chaincode would get the submitter's credentials
(e.g. using GetCreator() chaincode API or CID library API GetID() or GetMSPID() ),
combine it with other private data that gets passed to the chaincode, hash it,
and use GetPrivateDataHash() to verify that it matches the on-chain hash before
proceeding with the transaction.

* **Key level endorsement policies** -
And also as with normal channel state data, you can use state-based endorsement
to specify which organizations must endorse transactions that share or transfer
private data, using SetPrivateDataValidationParameter() chaincode API,
for example to specify that only an owner's organization peer, custodian's organization
peer, or other third party must endorse such transactions.

### Private data sharing example

The private data sharing patterns mentioned above can be combined to enable powerful
chaincode-based applications. For example, consider how an asset transfer scenario
could be implemented using per-organization private data collections:

* An asset may be tracked by a UUID key in public chaincode state. Only the asset's
ownership is recorded, nothing else is known about the asset.

* The chaincode will require that any transfer request must originate from the owning client,
and the key is bound by state-based endorsement requiring that a peer from the
owner's organization and a regulator's organization must endorse any transfer requests.

* The asset owner's private data collection contains the private details about
the asset, keyed by a hash of the UUID. Other organizations and the ordering
service will only see a hash of the asset details.

* Let's assume the regulator is a member of each collection as well, and therefore
persists the private data, although this need not be the case.

A transaction to trade the asset would unfold as follows:

1. Off-chain, the owner and a potential buyer strike a deal to trade the asset
for a certain price.

2. The seller provides proof of their ownership, by either passing the private details
out of band, or by providing the buyer with credentials to query the private
data on their node or the regulator's node.

3. Buyer verifies a hash of the private details matches the on-chain public hash.

4. The buyer invokes chaincode to record their bid details in their own private data collection.
The chaincode is invoked on buyer's peer, and potentially on regulator's peer if required
by the collection endorsement policy.

5. The current owner (seller) invokes chaincode to sell and transfer the asset, passing in the
private details and bid information. The chaincode is invoked on peers of the
seller, buyer, and regulator, in order to meet the endorsement policy of the public
key, as well as the endorsement policies of the buyer and seller private data collections.

6. The chaincode verifies that the submitting client is the owner, verifies the private
details against the hash in the seller's collection, and verifies the bid details
against the hash in the buyer's collection. The chaincode then writes the proposed
updates for the public key (setting ownership to the buyer, and setting endorsement
policy to be the buying organization and regulator), writes the private details to the
buyer's private data collection, and potentially deletes the private details from seller's
collection. Prior to final endorsement, the endorsing peers ensure private data is
disseminated to any other authorized peers of the seller and regulator.

7. The seller submits the transaction with the public data and private data hashes
for ordering, and it is distributed to all channel peers in a block.

8. Each peer's block validation logic will consistently verify the endorsement policy
was met (buyer, seller, regulator all endorsed), and verify that public and private
state that was read in the chaincode has not been modified by any other transaction
since chaincode execution.

9. All peers commit the transaction as valid since it passed validation checks.
Buyer peers and regulator peers retrieve the private data from other authorized
peers if they did not receive it at endorsement time, and persist the private
data in their private data state database (assuming the private data matched
the hashes from the transaction).

10. With the transaction completed, the asset has been transferred, and other
channel members interested in the asset may query the history of the public
key to understand its provenance, but will not have access to any private
details unless an owner shares it on a need-to-know basis.

The basic asset transfer scenario could be extended for other considerations,
for example the transfer chaincode could verify that a payment record is available
to satisfy payment versus delivery requirements, or verify that a bank has
submitted a letter of credit, prior to the execution of the transfer chaincode.
And instead of transactors directly hosting peers, they could transact through
custodian organizations who are running peers.

## Purging private data

For very sensitive data, even the parties sharing the private data might want
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