CoinJoin#
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:
Wallet Preparation#
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
Creating Denominations
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) |
---|---|---|
0 |
1 |
10.0001 |
1 |
2 |
01.00001 |
2 |
4 |
00.100001 |
3 |
8 |
00.0100001 |
4 |
16 |
00.00100001 |
Protocol version 70213 added a 5th denomination (0.001 DASH).
Example Testnet denomination creation transaction
Creating Collaterals
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.
CoinJoin Processing#
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.
Data Flow
Clients |
Direction |
Masternode |
Description |
|
---|---|---|---|---|
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 with the |
||
9 |
← |
Masternode verifies the signed inputs, creates a |
||
10 |
← |
Masternode broadcasts a |
||
11 |
|
→ |
(Optional) |
Additional notes
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
The
dsa
message contains a collateral transactionThis 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
dsq
messages 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
nPoolMinParticipants
(3) tonPoolMaxParticipants
(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
dsq
message with the ready bit set once it has received validdsa
messages from either:The maximum number of clients (20)
Greater than the minimum number of clients (3) and the timeout has been reached (30 seconds)
🚧
Clients must respond to the Queue ready within 30 seconds or risk forfeiting the collateral they provided in the
dsa
message (Step 1) (Dash Core Reference)
Step 4 - Inputs
The collateral transaction can be the same in the
dsi
message as the one in thedsa
message (Step 1) as long as it has not been spentEach 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
dsi
messages from all clients, it creates the final transaction and sends adsf
messageInputs/outputs are ordered deterministically as defined by BIP-69 to avoid leaking any data (Dash Core Reference)
Clients must sign their inputs to the Final Transaction within 15 seconds or risk forfeiting the collateral they provided in the
dsi
message (Step 4) (Dash Core Reference)
Step 10 - Final Transaction broadcast
Prior to protocol version 70213, masternodes could only send a single un-mined
dstx
message at a time. As of protocol version 70213, up to 5 (MASTERNODE_MAX_MIXING_TXES
) un-mineddstx
messages per masternode are allowed. (Dash Core Reference)
General
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).
Fees#
Processing Fees
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
dsq
messages ordsf
messages within the timeout periods may forfeit their collateral. Dash Core charges the abuse fee in 1/3 cases (Dash Core Reference)
Sending Fees
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)