Dash Core's mixing feature provides a way to improve privacy by performing non-custodial CoinJoin. For additional details, reference this Official Documentation page.
The following video provides an overview with a good introduction to the details:
The wallet completes two operations in this phase:
Split value into inputs matching the CoinJoin denominations by sending transactions to itself
Split value into inputs to use for collateral by sending transactions to itself
Note: Both these operations incur the standard transaction fee like any other transaction
The CoinJoin denominations include a bit mapping to easily differentiate them. The
dsa message and
dsq message use this bit shifted integer instead of sending the actual denomination. The table below lists the bit, its associated integer value used in P2P messages, and the actual Dash value.
|Bit||Denom. (Integer)||Denomination (DASH)|
Protocol version 70213 added a 5th denomination (0.001 DASH).
Collaterals are used to pay CoinJoin fees, but are kept separate from the denominations to maximize privacy. Since protocol version 70213, the minimum collateral fee is 1/10 of the smallest denomination for all sessions regardless of denomination. In Dash Core, collaterals are created with enough value to pay 4 collateral fees (4 x 0.001 DASH). (Dash Core Reference)
In protocol version 70208, collateral inputs can be anything from 2x the minimum collateral amount to the maximum collateral amount (currently defined as 4x the minimum collateral). In protocol versions > 70208, Dash Core can use any input from 1x the minimum collateral amount to the maximum collateral amount.
This phase involves exchanging a sequence of messages with a masternode so it can construct a denominate transaction with inputs from the clients in its pool.
|0||Client determines whether to join an existing pool or create a new one|
|1||→||Client asks to join pool or have the masternode create a new one|
|2||←||Masternode provides a pool status update (Typical - State: |
|3||←||Masternode notifies clients when it is ready to receive inputs|
|4||→||Upon receiving a |
|5||←||Masternode provides a pool status update (typical - State: |
|6||←||Masternode sends the final transaction containing all clients inputs (unsigned) and all client outputs to each client for verification|
|7||←||Masternode provides a pool status update (Typical - State: |
|8||→||After verifying the final transaction, clients each sign their own inputs and send them back|
|9||←||Masternode verifies the signed inputs, creates a |
|10||←||Masternode broadcasts a |
Step 0 - Pool Selection
- Existing pool information is derived from the Queue messages seen by the client
- Dash Core attempts to join an existing pool and only requests creation of a new one if that fails, although this is not a requirement that alternative implementations would be required to follow
Step 1 - Pool Request
dsamessage contains a collateral transaction
- This transaction uses a collateral input created in the Wallet Preparation phase
- The collateral is a signed transaction that pays the collateral back to a client address minus a fee of 0.001 DASH
Step 3 - Queue
- A masternode broadcasts
dsqmessages when it starts a new queue. These message are relayed by all peers.
- As of protocol version 70214, sessions have a variable number of participants defined by the range
nPoolMaxParticipants(5). Prior protocol version sessions always contained exactly 3 participants. Spork 22 introduced in Dash Core 0.16.0 expanded the maximum number of participants to 20 and also reduced the minimum number of participants to 2 for testnet/devnet/regtest networks. The spork was removed in Dash Core 0.17.0 which made the change permanent.
- The masternode sends a
dsqmessage with the ready bit set once it has received valid
dsamessages from either:
- The maximum number of clients (20)
- Greater than the minimum number of clients (3) and the timeout has been reached (30 seconds)
Step 4 - Inputs
- The collateral transaction can be the same in the
dsimessage as the one in the
dsamessage (Step 1) as long as it has not been spent
- Each client can provide up to 9 (
COINJOIN_ENTRY_MAX_SIZE) inputs (and an equal number of outputs) to be used (Dash Core Reference)
- This is the only message in the process that contains enough information to link a wallet's CoinJoin inputs with its outputs
- This message is sent directly between a client and the masternode processing the session (not relayed across the Dash network) so no other clients see it
Step 6 - Final Transaction (unsigned)
- Once the masternode has received valid
dsimessages from all clients, it creates the final transaction and sends a
Step 10 - Final Transaction broadcast
- Prior to protocol version 70213, masternodes could only send a single un-mined
dstxmessage at a time. As of protocol version 70213, up to 5 (
dstxmessages per masternode are allowed. (Dash Core Reference)
With the exception of the
dsq message and the
dstx message (which need to be public and do not expose any private information), all CoinJoin P2P messages are sent directly between the masternode processing the transaction and the relevant client(s).
- If processing completes successfully, Dash Core charges the collateral randomly in 1/10 denominate transactions to pay miners (Dash Core Reference)
- Clients that abuse the system by failing to respond to
dsfmessages within the timeout periods may forfeit their collateral. Dash Core charges the abuse fee in 1/3 cases (Dash Core Reference)
To maintain privacy when using CoinJoin funds, transactions must fully spend all inputs to a single output (with the remainder becoming the fee - i.e. no change output). This can result in large fees depending on the value being sent.
For example, an extreme case is sending the minimum non-dust value (546 duffs). This results in an extremely large transaction fee because the minimum denomination (0.00100001 DASH or 100,001 duffs) must be fully spent with no change. This results in a fee of 0.00999464 DASH and a sent value of only 0.00000546 DASH as shown by the calculation below.
100001 duffs (smallest CoinJoin denomination) - 546 duffs (value to send) = 99455 duffs (fee)
Updated 3 days ago