# Private data

## What is private data?

In cases where a group of organizations on a channel need to keep data private from
other organizations on that channel, they have the option to create a new channel
comprising just the organizations who need access to the data. However, creating
separate channels in each of these cases creates additional administrative overhead
(maintaining chaincode versions, policies, MSPs, etc), and doesn't allow for use
cases in which you want all channel participants to see a transaction while keeping
a portion of the data private.

That's why Fabric offers the ability to create
**private data collections**, which allow a defined subset of organizations on a
channel the ability to endorse, commit, or query private data without having to
create a separate channel.

## What is a private data collection?

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,
   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,
   and configure CORE_PEER_GOSSIP_EXTERNALENDPOINT on each peer,
   in order to bootstrap cross-organization communication.

2. **A hash of that data**, which is endorsed, ordered, and written to the ledgers
   of every peer on the channel. The hash serves as evidence of the transaction and
   is used for state validation and can be used for audit purposes.

The following diagram illustrates the ledger contents of a peer authorized to have
private data and one which is not.

![private-data.private-data](./PrivateDataConcept-2.png)

Collection members may decide to share the private data with other parties if they
get into a dispute or if they want to transfer the asset to a third party. The
third party can then compute the hash of the private data and see if it matches the
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
  confidential within a set of organizations that are members of the channel.

* Use **collections** when transactions (and ledgers) must be shared among a set
  of organizations, but when only a subset of those organizations should have
  access to some (or all) of the data within a transaction.  Additionally,
  since private data is disseminated peer-to-peer rather than via blocks,
  use private data collections when transaction data must be kept confidential
  from ordering service nodes.

## A use case to explain collections

Consider a group of five organizations on a channel who trade produce:

* **A Farmer** selling his goods abroad
* **A Distributor** moving goods abroad
* **A Shipper** moving goods between parties
* **A Wholesaler** purchasing goods from distributors
* **A Retailer** purchasing goods from shippers and wholesalers

The **Distributor** might want to make private transactions with the
**Farmer** and **Shipper** to keep the terms of the trades confidential from
the **Wholesaler** and the **Retailer** (so as not to expose the markup they're
charging).

The **Distributor** may also want to have a separate private data relationship
with the **Wholesaler** because it charges them a lower price than it does the
**Retailer**.

The **Wholesaler** may also want to have a private data relationship with the
**Retailer** and the **Shipper**.

Rather than defining many small channels for each of these relationships, multiple
private data collections **(PDC)** can be defined to share private data between:

1. PDC1: **Distributor**, **Farmer** and **Shipper**
2. PDC2: **Distributor** and **Wholesaler**
3. PDC3: **Wholesaler**, **Retailer** and **Shipper**

![private-data.private-data](./PrivateDataConcept-1.png)

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.

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

## Transaction flow with private data

When private data collections are referenced in chaincode, the transaction flow
is slightly different in order to protect the confidentiality of the private
data as transactions are proposed, endorsed, and committed to the ledger.

For details on transaction flows that don't use private data refer to our
documentation on [transaction flow](../txflow.html).

1. The client application submits a proposal request to invoke a chaincode
   function (reading or writing private data) to endorsing peers which are
   part of authorized organizations of the collection. The private data, or
   data used to generate private data in chaincode, is sent in a `transient`
   field of the proposal.

2. The endorsing peers simulate the transaction and store the private data in
   a `transient data store` (a temporary storage local to the peer). They
   distribute the private data, based on the collection policy, to authorized peers
   via [gossip](../gossip.html).

3. The endorsing peer sends the proposal response back to the client. The proposal
   response includes the endorsed read/write set, which includes public
   data, as well as a hash of any private data keys and values. *No private data is
   sent back to the client*. For more information on how endorsement works with
   private data, click [here](../private-data-arch.html#endorsement).

4. The client application submits the transaction (which includes the proposal
   response with the private data hashes) to the ordering service. The transactions
   with the private data hashes get included in blocks as normal.
   The block with the private data hashes is distributed to all the peers. In this way,
   all peers on the channel can validate transactions with the hashes of the private
   data in a consistent way, without knowing the actual private data.

5. At block commit time, authorized peers use the collection policy to
   determine if they are authorized to have access to the private data. If they do,
   they will first check their local `transient data store` to determine if they
   have already received the private data at chaincode endorsement time. If not,
   they will attempt to pull the private data from another authorized peer. Then they
   will validate the private data against the hashes in the public block and commit the
   transaction and the block. Upon validation/commit, the private data is moved to
   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.

### Example scenario: Asset transfer using private data collections

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
--- or might be required by government regulations --- to periodically "purge" the data
on their peers, leaving behind a hash of the data on the blockchain
to serve as immutable evidence of the private data.

In some of these cases, the private data only needs to exist on the peer's private
database until it can be replicated into a database external to the peer's
blockchain. The data might also only need to exist on the peers until a chaincode business
process is done with it (trade settled, contract fulfilled, etc).

To support these use cases, private data can be purged if it has not been modified
for a configurable number of blocks. Purged private data cannot be queried from chaincode,
and is not available to other requesting peers.

## How a private data collection is defined

For more details on collection definitions, and other low level information about
private data and collections, refer to the [private data reference topic](../private-data-arch.html).

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