BIP: 78 source
  Layer: Applications
  Title: A Simple Payjoin Proposal
  Author: Nicolas Dorier <nicolas.dorier@gmail.com>
  Replaces: 79
  Comments-Summary: No comments yet.
  Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-0078
  Status: Draft
  Type: Standards Track
  Created: 2019-05-01
  License: BSD-2-Clause

This document proposes a protocol for two parties to negotiate a coinjoin transaction during a payment between them.

This BIP is licensed under the 2-clause BSD license.

When two parties (later referred to as sender and receiver) want to transact, most of the time, the sender creates a transaction spending their own Unspent Transaction Outputs (UTXOs), signs it and broadcasts it on the network.

This simple model gave birth to several heuristics impacting the privacy of the parties and of the network as a whole.

• Common input ownership heuristic: In most transactions, all the inputs belong to the same party.
• Change identification from scriptPubKey type: If all inputs are spending UTXOs of a certain scriptPubKey type, then the change output is likely to have the same scriptPubKey type, too.
• Change identification from round amount: If an output in the transaction has a round amount, it is likely an output belonging to the receiver.

The problems we aim to solve are:

• For the receiver, there is a missed opportunity to consolidate their own UTXOs or making payment in the sender's transaction.
• For the sender, there are privacy leaks regarding their wallet that happen when someone applies the heuristics detailed above to their transaction.

Note that the existence of this proposal is also improving the privacy of parties who are not using it by making the three heuristics unreliable to the network as a whole.

Another implementation proposal has been written: BIP79 Bustapay.

We decided to deviate from it for several reasons:

• It was not using PSBT, so if the receiver wanted to bump the fee, they would need the full UTXO set.
• Inability to change the payment output to match scriptPubKey type.
• Lack of basic versioning negotiation if the protocol evolves.
• No standardization of error condition for proper feedback to the sender.

In a payjoin payment, the following steps happen:

• The receiver of the payment, presents a BIP 21 URI to the sender with a parameter pj= describing a payjoin endpoint.
• The sender creates a signed, finalized PSBT with witness UTXO or previous transactions of the inputs. We call this PSBT the original.
• The receiver replies back with a signed PSBT containing his own signed inputs/outputs and those of the sender. We call this PSBT Payjoin proposal.
• The sender verifies the proposal, re-signs his inputs and broadcasts the transaction to the Bitcoin network. We call this transaction Payjoin transaction.

+----------+                        +--------+         +-----------------+
| Receiver |                        | Sender |         | Bitcoin Network |
+----+-----+                        +---+----+         +-------+---------+
     |       +-----------------+        |                      |
     +-------+ BIP21 with ?pj= +------->+                      |
     |       +-----------------+        |                      |
     |                                  |                      |
     |        +---------------+         |                      |
     +<-------+ Original PSBT +---------+                      |
     |        +---------------+         |                      |
     |                                  |                      |
     |       +------------------+       |                      |
     |       | Payjoin Proposal |       |                      |
     +-------+      PSBT        +------>+                      |
     |       +------------------+       |                      |
     |                                  |   +--------------+   |
     |                                  |---+ Payjoin      |   |
     |                                  |   | transaction  +-->+
     |                                  |   +--------------+   |
     +                                  +                      +

To ensure compatibility with web-wallets and browser-based-tools, all responses (including errors) must contain the HTTP header Access-Control-Allow-Origin: *.

The sender must ensure that the URL refers to a scheme or protocol using authenticated encryption, for example TLS with certificate validation, or a .onion link to a hidden service whose public key identifier has already been communicated via a TLS connection. Senders SHOULD NOT accept a URL representing an unencrypted or unauthenticated connection.

The original PSBT MUST:

• Have all the witnessUTXO or nonWitnessUTXO information filled in.
• Be finalized.
• Not include fields unneeded for the receiver such as global xpubs or keypath information.
• Be broadcastable.

• Have outputs unrelated to the payment for batching purpose.

• Use all the inputs from the original PSBT.
• Use all the outputs which do not belong to the receiver from the original PSBT.
• Only finalize the inputs added by the receiver. (Referred later as additional inputs)
• Only fill the witnessUTXO or nonWitnessUTXO for the additional inputs.

• Add, remove or modify the outputs belonging to the receiver.

• Shuffle the order of inputs or outputs, the additional outputs or additional inputs must be inserted at a random index.
• Decrease the absolute fee of the original transaction.

This proposal is defining the following new BIP 21 URI parameters:

• pj=: Represents an http(s) endpoint which the sender can POST the original PSBT.
• pjos=0: Signal to the sender that they MUST disallow payment output substitution. (See Unsecured payjoin server)

When the payjoin sender posts the original PSBT to the receiver, he can optionally specify the following HTTP query string parameters:

• v=, the version number of the payjoin protocol that the sender is using. The current version is 1.

If the receiver does not support the version of the sender, they should send an error with the list of supported versions:

{
    "errorCode": "version-unsupported",
    "supported" : [ 2, 3, 4 ],
    "message": "The version is not supported anymore"
}

• additionalfeeoutputindex=, if the sender is willing to pay for increased fee, this indicate output can have its value subtracted to pay for it.

• maxadditionalfeecontribution=, if the sender is willing to pay for increased fee, an integer defining the maximum amount in satoshis that the sender is willing to contribute towards fees for the additional inputs. maxadditionalfeecontribution must be ignored if set to less than zero. See fee output for more information.

• minfeerate=, a decimal in satoshi per vbyte that the sender can use to constraint the receiver to not drop the minimum fee rate too much.
• disableoutputsubstitution=, a boolean indicating if the sender forbids the receiver to substitute the receiver's output, see payment output substitution. (default to false)

If for some reason the receiver is unable to create a payjoin proposal, it will reply with a HTTP code different than 200. The receiver is not constrained to specific set of errors, some are specified in this proposal.

The errors have the following format:

{
    "errorCode": "leaking-data",
    "message": "Key path information or GlobalXPubs should not be included in the original PSBT."
}

The well-known error codes are:

The receiver is allowed to return implementation-specific errors which may assist the sender to diagnose any issue.

However, it is important that error codes that are not well-known and that the message do not appear on the sender's software user interface. Such error codes or messages could be used maliciously to phish a non-technical user. Instead those errors or messages can only appear in debug logs.

It is advised to hard code the description of the well known error codes into the sender's software.

In some situation, the sender might want to pay some additional fee in the payjoin proposal. If such is the case, the sender must use both optional parameters additionalfeeoutputindex= and maxadditionalfeecontribution= to indicate which output and how much the receiver can subtract fee.

There is several cases where a fee output is useful:

• The sender's original transaction's fee rate is at the minimum accepted by the network, aka minimum relay transaction fee rate, which is typically 1 satoshi per vbyte.

• The sender's wallet software is using round fee rate.

• The sender's transaction is time sensitive.

Our recommendation for maxadditionalfeecontribution= is originalPSBTFeeRate * vsize(sender_input_type).

The receiver needs to do some check on the original PSBT before proceeding:

• Non-interactive receivers (like a payment processor) need to check that the original PSBT is broadcastable. *
• If the sender included inputs in the original PSBT owned by the receiver, the receiver must either return error original-psbt-rejected or make sure they do not sign those inputs in the payjoin proposal.
• If the sender's inputs are all from the same scriptPubKey type, the receiver must match the same type. If the receiver can't match the type, they must return error unavailable.
• Make sure that the inputs included in the original transaction have never been seen before.
  • This prevents probing attacks.
  • This prevents reentrant payjoin, where a sender attempts to use payjoin transaction as a new original transaction for a new payjoin.

The sender should check the payjoin proposal before signing it to prevent a malicious receiver from stealing money.

• Verify that the absolute fee of the payjoin proposal is equals or higher than the original PSBT.
• If the receiver's BIP21 signalled pjos=0, disable payment output substitution.
• Verify that the transaction version, and the nLockTime are unchanged.
• Check that the sender's inputs' sequence numbers are unchanged.
• For each input in the proposal:
  • Verify that no keypaths are in the PSBT input
  • Verify that no partial signature has been filled
  • If it is one of the sender's inputs:
    • Verify that input's sequence is unchanged.
    • Verify the PSBT input is not finalized
    • Verify that non_witness_utxo and witness_utxo are not specified.
  • If it is one of the receiver's inputs:
    • Verify the PSBT input is finalized
    • Verify that non_witness_utxo or witness_utxo are filled in.
  • Verify that the payjoin proposal inputs all specify the same sequence value.
  • Verify that the payjoin proposal did not introduce mixed input's type.
  • Verify that all of sender's inputs from the original PSBT are in the proposal.
• For each output in the proposal:
  • Verify that no keypaths are in the PSBT output
  • If the output is the fee output:
    • The amount that was subtracted from the output's value is less than or equal to maxadditionalfeecontribution. Let's call this amount actual contribution.
    • Make sure the actual contribution is only going towards fees: The actual contribution is less than or equals to the difference of absolute fee between the payjoin proposal and the original PSBT.
    • Make sure the actual contribution is only paying for fees incurred by additional inputs: actual contribution is less than or equal to originalPSBTFeeRate * vsize(sender_input_type) * (count(payjoin_proposal_inputs) - count(original_psbt_inputs)). (see Fee output section)
  • If the output is the payment output and payment output substitution is allowed,
    • Do not make any check
  • Else
    • Make sure the output's value did not decrease.
  • Verify that all sender's outputs (ie, all outputs except the output actually paid to the receiver) from the original PSBT are in the proposal.
• Once the proposal is signed, if minfeerate was specified, check that the fee rate of the payjoin transaction is not less than this value.

Note:

• The sender must allow the receiver to add/remove or modify the receiver's own outputs. (if payment output substitution is disabled, the receiver's outputs must not be removed or decreased in value)
• The sender should allow the receiver to not add any inputs. This is useful for the receiver to change the payment output scriptPubKey type.
• If the receiver added no inputs, the sender's wallet implementation should accept the payjoin proposal, but not mark the transaction as an actual payjoin in the user interface.

There is several consequences of our proposal:

• The receiver can bump the fee of the original transaction.
• The receiver can modify the outputs of the original PSBT.
• The sender must provide the UTXO information (Witness or previous transaction) in the PSBT.

To be properly relayed, a Bitcoin transaction needs to pay at least 1 satoshi per virtual byte. When blocks are not full, the original transaction might already at the minimum relay fee rate (currently 1 satoshi per virtual byte), so if the receiver adds their own input, they need to make sure the fee is increased such that the rate does not drop below the minimum relay fee rate. In such case, the sender must set both maxadditionalfeecontribution= and additionalfeeoutputindex=.

See the Fee output section for more information.

We also recommend the sender to set minfeerate=, as the sender's node policy might be different from the receiver's policy.

Most wallets are creating a round fee rate (like 2 sat/b). If the payjoin transaction's fee was not increased by the added size, then those payjoin transactions could easily be identifiable on the blockchain.

Not only would those transactions stand out by not having a round fee (like 1.87 sat/b), but any suspicion of payjoin could be confirmed by checking if removing one input would create a round fee rate. In such case, the sender must set both maxadditionalfeecontribution= and additionalfeeoutputindex=.

The recommended value maxadditionalfeecontribution= is explained in the Fee output section. We also recommend the sender to set minfeerate=, as the sender's node policy might be different from the receiver's policy.

Because the receiver needs to bump the fee to keep the same fee rate as the original PSBT, it needs the input's UTXO information to know what is the original fee rate. Without PSBT, light wallets like Wasabi Wallet would not be able to receive a payjoin transaction.

The validation (policy and consensus) of the original transaction is optional: a receiver without a full node can decide to create the payjoin transaction and automatically broadcast the original transaction after a timeout of 1 minute, and only verify that it has been propagated in the network.

However, non-interactive receivers (like a payment processor) need to verify the transaction to prevent UTXO probing attacks.

This is not a concern for interactive receivers like Wasabi Wallet, because those receivers can just limit the number of original PSBT proposals of a specific address to one. With such wallets, the attacker has no way to generate new deposit addresses to probe the UTXOs.

Small change inside wallets are detrimental to privacy. Mixers like Wasabi wallet, because of its protocol, eventually generate such small change.

A common way to protect your privacy is to donate those spare changes, to deposit them in an exchange or on your favorite merchant's store account. Those kind of transactions can easily be spotted on the blockchain: There is only one output.

However, if you donate via payjoin, it will look like a normal transaction.

On top of this the receiver can poison analysis by randomly faking a round amount of satoshi for the additional output.

Unless disallowed by the sender explicitly via disableoutputsubstitution=true or by the BIP21 URL via the query parameter pjos=0, the receiver is free to decrease the amount, remove, or change the scriptPubKey output paying to himself. Note that if payment output substitution is disallowed, the reveiver can still increase the amount of the output. (See the reference implementation)

For example, if the sender's scriptPubKey type is P2WPKH while the receiver's payment output in the original PSBT is P2SH, then the receiver can substitute the payment output to be P2WPKH to match the sender's scriptPubKey type.

A receiver might run the payment server (generating the BIP21 invoice) on a different server than the payjoin server, which could be less trusted than the payment server.

In such case, the payment server can signal to the sender, via the BIP21 parameter pjos=0, that they MUST disallow payment output substitution. A compromised payjoin server could steal the hot wallet outputs of the receiver, but would not be able to re-route payment to himself.

Our proposal of payjoin breaks the following blockchain heuristics:

• Common inputs heuristics.

• Change identification from scriptPubKey type heuristics

See payment output substitution.

• Change identification from round change amount

For this reason, during a spare change case, the receiver may add an output with a rounded amount randomly.

When the receiver creates a payjoin proposal, they expose one or more inputs belonging to them.

An attacker could create multiple original transactions in order to learn the UTXOs of the receiver, while not broadcasting the payjoin proposal.

While we cannot prevent this type of attack entirely, we implemented the following mitigations:

• When the receiver detects an original transaction being broadcast, or if the receiver detects that the original transaction has been double spent, then they will reuse the UTXO that was exposed for the next payjoin.
• While the exposed UTXO will be reused in priority to not leak other UTXOs, there is no strong guarantee about it. This prevents the attacker from detecting with certainty the next payjoin of the merchant to another peer.

For a successful payjoin to happen, the sender needs to sign two transactions double spending each other: The original transaction and the payjoin proposal.

The sender's software wallet can verify that the payjoin proposal is legitimate by the sender's checklist.

However, a hardware wallet can't verify that this is indeed the case. This means that the security guarantee of the hardware wallet is decreased. If the sender's software is compromised, the hardware wallet would sign two valid transactions, thus sending two payments.

Without payjoin, the maximum amount of money that could be lost by a compromised software is equal to one payment (via payment output substitution). Note that the sender can disallow payment output substitution by using the optional parameter disableoutputsubstitution=true.

With payjoin, the maximum amount of money that can be lost is equal to two payments.

Here is pseudo code of a sender implementation. RequestPayjoin takes the BIP21 URI of the payment, the wallet and the signedPSBT.

The signedPSBT represents a PSBT which has been fully signed, but not yet finalized. We then prepare originalPSBT from the signedPSBT via the CreateOriginalPSBT function and get back the proposal.

While we verify the proposal, we also import into it information about our own inputs and outputs from the signedPSBT. At the end of this RequestPayjoin, the proposal is verified and ready to be signed.

We logged the different PSBT involved, and show the result in our test vectors.

public async Task<PSBT> RequestPayjoin(
    BIP21Uri bip21,
    Wallet wallet,
    PSBT signedPSBT,
    PayjoinClientParameters optionalParameters)
{
    Log("Unfinalized signed PSBT" + signedPSBT);
    // Extracting the pj link.
    var endpoint = bip21.ExtractPayjointEndpoint();
    if (signedPSBT.IsAllFinalized())
        throw new InvalidOperationException("The original PSBT should not be finalized.");
    ScriptPubKeyType inputScriptType = wallet.ScriptPubKeyType();
    PSBTOutput feePSBTOutput = null;

    bool allowOutputSubstitution = !optionalParameters.DisableOutputSubstitution;
    if (bip21.Parameters.Contains("pjos") && bip21.Parameters["pjos"] == "0")
        allowOutputSubstitution = false;

    if (optionalParameters.AdditionalFeeOutputIndex != null && optionalParameters.MaxAdditionalFeeContribution != null)
        feePSBTOutput = signedPSBT.Outputs[optionalParameters.AdditionalFeeOutputIndex];
    Script paymentScriptPubKey = bip21.Address == null ? null : bip21.Address.ScriptPubKey;
    decimal originalFee = signedPSBT.GetFee();
    PSBT originalPSBT = CreateOriginalPSBT(signedPSBT);
    Transaction originalGlobalTx = signedPSBT.GetGlobalTransaction();
    TxOut feeOutput = feePSBTOutput == null ? null : originalGlobalTx.Outputs[feePSBTOutput.Index];
    var originalInputs = new Queue<(TxIn OriginalTxIn, PSBTInput SignedPSBTInput)>();
    for (int i = 0; i < originalGlobalTx.Inputs.Count; i++)
    {
        originalInputs.Enqueue((originalGlobalTx.Inputs[i], signedPSBT.Inputs[i]));
    }
    var originalOutputs = new Queue<(TxOut OriginalTxOut, PSBTOutput SignedPSBTOutput)>();
    for (int i = 0; i < originalGlobalTx.Outputs.Count; i++)
    {
      originalOutputs.Enqueue((originalGlobalTx.Outputs[i], signedPSBT.Outputs[i]));
    }
    // Add the client side query string parameters
    endpoint = ApplyOptionalParameters(endpoint, optionalParameters);
    Log("original PSBT" + originalPSBT);
    PSBT proposal = await SendOriginalTransaction(endpoint, originalPSBT, cancellationToken);
    Log("payjoin proposal" + proposal);
    // Checking that the PSBT of the receiver is clean
    if (proposal.GlobalXPubs.Any())
    {
        throw new PayjoinSenderException("GlobalXPubs should not be included in the receiver's PSBT");
    }
    ////////////

    if (proposal.CheckSanity() is List<PSBTError> errors && errors.Count > 0)
        throw new PayjoinSenderException($"The proposal PSBT is not sane ({errors[0]})");

    var proposalGlobalTx = proposal.GetGlobalTransaction();
    // Verify that the transaction version, and nLockTime are unchanged.
    if (proposalGlobalTx.Version != originalGlobalTx.Version)
        throw new PayjoinSenderException($"The proposal PSBT changed the transaction version");
    if (proposalGlobalTx.LockTime != originalGlobalTx.LockTime)
        throw new PayjoinSenderException($"The proposal PSBT changed the nLocktime");

    HashSet<Sequence> sequences = new HashSet<Sequence>();
    // For each inputs in the proposal:
    foreach (PSBTInput proposedPSBTInput in proposal.Inputs)
    {
        if (proposedPSBTInput.HDKeyPaths.Count != 0)
            throw new PayjoinSenderException("The receiver added keypaths to an input");
        if (proposedPSBTInput.PartialSigs.Count != 0)
            throw new PayjoinSenderException("The receiver added partial signatures to an input");
        PSBTInput proposedTxIn = proposalGlobalTx.Inputs.FindIndexedInput(proposedPSBTInput.PrevOut).TxIn;
        bool isOurInput = originalInputs.Count > 0 && originalInputs.Peek().OriginalTxIn.PrevOut == proposedPSBTInput.PrevOut;
        // If it is one of our input
        if (isOurInput)
        {
            OutPoint inputPrevout = ourPrevouts.Dequeue();
            TxIn originalTxin = originalGlobalTx.Inputs.FromOutpoint(inputPrevout);
            PSBTInput originalPSBTInput = originalPSBT.Inputs.FromOutpoint(inputPrevout);
            // Verify that sequence is unchanged.
            if (input.OriginalTxIn.Sequence != proposedTxIn.Sequence)
                throw new PayjoinSenderException("The proposedTxIn modified the sequence of one of our inputs")
            // Verify the PSBT input is not finalized
            if (proposedPSBTInput.IsFinalized())
                throw new PayjoinSenderException("The receiver finalized one of our inputs");
            // Verify that <code>non_witness_utxo</code> and <code>witness_utxo</code> are not specified.
            if (proposedPSBTInput.NonWitnessUtxo != null || proposedPSBTInput.WitnessUtxo != null)
                throw new PayjoinSenderException("The receiver added non_witness_utxo or witness_utxo to one of our inputs");
            sequences.Add(proposedTxIn.Sequence);

            // Fill up the info from the original PSBT input so we can sign and get fees.
            proposedPSBTInput.NonWitnessUtxo = input.SignedPSBTInput.NonWitnessUtxo;
            proposedPSBTInput.WitnessUtxo = input.SignedPSBTInput.WitnessUtxo;
            // We fill up information we had on the signed PSBT, so we can sign it.
            foreach (var hdKey in input.SignedPSBTInput.HDKeyPaths)
            proposedPSBTInput.HDKeyPaths.Add(hdKey.Key, hdKey.Value);
            proposedPSBTInput.RedeemScript = signedPSBTInput.RedeemScript;
            proposedPSBTInput.RedeemScript = input.SignedPSBTInput.RedeemScript;
        }
        else
        {
            // Verify the PSBT input is finalized
            if (!proposedPSBTInput.IsFinalized())
                throw new PayjoinSenderException("The receiver did not finalized one of their input");
            // Verify that non_witness_utxo or witness_utxo are filled in.
            if (proposedPSBTInput.NonWitnessUtxo == null && proposedPSBTInput.WitnessUtxo == null)
                throw new PayjoinSenderException("The receiver did not specify non_witness_utxo or witness_utxo for one of their inputs");
            sequences.Add(proposedTxIn.Sequence);
            // Verify that the payjoin proposal did not introduced mixed inputs' type.
            if (inputScriptType != proposedPSBTInput.GetInputScriptPubKeyType())
                throw new PayjoinSenderException("Mixed input type detected in the proposal");
        }
    }

    // Verify that all of sender's inputs from the original PSBT are in the proposal.
    if (originalInputs.Count != 0)
        throw new PayjoinSenderException("Some of our inputs are not included in the proposal");

    // Verify that the payjoin proposal did not introduced mixed inputs' sequence.
    if (sequences.Count != 1)
        throw new PayjoinSenderException("Mixed sequence detected in the proposal");

    decimal newFee = proposal.GetFee();
    decimal additionalFee = newFee - originalFee;
    if (additionalFee < 0)
      throw new PayjoinSenderException("The receiver decreased absolute fee");
    // For each outputs in the proposal:
    foreach (PSBTOutput proposedPSBTOutput in proposal.Outputs)
    {
        // Verify that no keypaths is in the PSBT output
        if (proposedPSBTOutput.HDKeyPaths.Count != 0)
            throw new PayjoinSenderException("The receiver added keypaths to an output");
        if (originalOutputs.Count == 0)
                continue;
        var originalOutput = originalOutputs.Peek();
        bool isOriginalOutput = originalOutput.OriginalTxOut.ScriptPubKey == proposedPSBTOutput.ScriptPubKey;
        bool substitutedOutput = !isOriginalOutput &&
                                allowOutputSubstitution &&
                                originalOutput.OriginalTxOut.ScriptPubKey == paymentScriptPubKey;
        if (isOriginalOutput || substitutedOutput)
        {
            originalOutputs.Dequeue();
            if (output.OriginalTxOut == feeOutput)
            {
                var actualContribution = feeOutput.Value - proposedPSBTOutput.Value;
                // The amount that was subtracted from the output's value is less than or equal to maxadditionalfeecontribution
                if (actualContribution > optionalParameters.MaxAdditionalFeeContribution)
                    throw new PayjoinSenderException("The actual contribution is more than maxadditionalfeecontribution");
                // Make sure the actual contribution is only paying fee
                if (actualContribution > additionalFee)
                    throw new PayjoinSenderException("The actual contribution is not only paying fee");
                // Make sure the actual contribution is only paying for fee incurred by additional inputs
                int additionalInputsCount = proposalGlobalTx.Inputs.Count - originalGlobalTx.Inputs.Count;
                if (actualContribution > originalFeeRate * GetVirtualSize(inputScriptType) * additionalInputsCount)
                    throw new PayjoinSenderException("The actual contribution is not only paying for additional inputs");
            }
            else if (allowOutputSubstitution && output.OriginalTxOut.ScriptPubKey == paymentScriptPubKey)
            {
                // That's the payment output, the receiver may have changed it.
            }
            else
            {
                if (originalOutput.OriginalTxOut.Value > proposedPSBTOutput.Value)
                    throw new PayjoinSenderException("The receiver decreased the value of one of the outputs");
            }
            // We fill up information we had on the signed PSBT, so we can sign it.
            foreach (var hdKey in output.SignedPSBTOutput.HDKeyPaths)
                proposedPSBTOutput.HDKeyPaths.Add(hdKey.Key, hdKey.Value);
            proposedPSBTOutput.RedeemScript = output.SignedPSBTOutput.RedeemScript;
        }
    }
    // Verify that all of sender's outputs from the original PSBT are in the proposal.
    if (originalOutputs.Count != 0)
    {
        // The payment output may have been substituted
        if (!allowOutputSubstitution ||
            originalOutputs.Count != 1 ||
            originalOutputs.Dequeue().OriginalTxOut.ScriptPubKey != paymentScriptPubKey)
            {
                throw new PayjoinSenderException("Some of our outputs are not included in the proposal");
            }
    }

    // After signing this proposal, we should check if minfeerate is respected.
    Log("payjoin proposal filled with sender's information" + proposal);
    return proposal;
}

int GetVirtualSize(ScriptPubKeyType? scriptPubKeyType)
{
    switch (scriptPubKeyType)
    {
        case ScriptPubKeyType.Legacy:
            return 148;
        case ScriptPubKeyType.Segwit:
            return 68;
        case ScriptPubKeyType.SegwitP2SH:
            return 91;
        default:
            return 110;
    }
}

// Finalize the signedPSBT and remove confidential information
PSBT CreateOriginalPSBT(PSBT signedPSBT)
{
    var original = signedPSBT.Clone();
    original = original.Finalize();
    foreach (var input in original.Inputs)
    {
        input.HDKeyPaths.Clear();
        input.PartialSigs.Clear();
        input.Unknown.Clear();
    }
    foreach (var output in original.Outputs)
    {
        output.Unknown.Clear();
        output.HDKeyPaths.Clear();
    }
    original.GlobalXPubs.Clear();
    return original;
}

A successful exchange with:

Unfinalized signed PSBT

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

Original PSBT

cHNidP8BAHMCAAAAAY8nutGgJdyYGXWiBEb45Hoe9lWGbkxh/6bNiOJdCDuDAAAAAAD+////AtyVuAUAAAAAF6kUHehJ8GnSdBUOOv6ujXLrWmsJRDCHgIQeAAAAAAAXqRR3QJbbz0hnQ8IvQ0fptGn+votneofTAAAAAAEBIKgb1wUAAAAAF6kU3k4ekGHKWRNbA1rV5tR5kEVDVNCHAQcXFgAUx4pFclNVgo1WWAdN1SYNX8tphTABCGsCRzBEAiB8Q+A6dep+Rz92vhy26lT0AjZn4PRLi8Bf9qoB/CMk0wIgP/Rj2PWZ3gEjUkTlhDRNAQ0gXwTO7t9n+V14pZ6oljUBIQMVmsAaoNWHVMS02LfTSe0e388LNitPa1UQZyOihY+FFgABABYAFEb2Giu6c4KO5YW0pfw3lGp9jMUUAAA=

payjoin proposal

cHNidP8BAJwCAAAAAo8nutGgJdyYGXWiBEb45Hoe9lWGbkxh/6bNiOJdCDuDAAAAAAD+////jye60aAl3JgZdaIERvjkeh72VYZuTGH/ps2I4l0IO4MBAAAAAP7///8CJpW4BQAAAAAXqRQd6EnwadJ0FQ46/q6NcutaawlEMIcACT0AAAAAABepFHdAltvPSGdDwi9DR+m0af6+i2d6h9MAAAAAAAEBIICEHgAAAAAAF6kUyPLL+cphRyyI5GTUazV0hF2R2NWHAQcXFgAUX4BmVeWSTJIEwtUb5TlPS/ntohABCGsCRzBEAiBnu3tA3yWlT0WBClsXXS9j69Bt+waCs9JcjWtNjtv7VgIge2VYAaBeLPDB6HGFlpqOENXMldsJezF9Gs5amvDQRDQBIQJl1jz1tBt8hNx2owTm+4Du4isx0pmdKNMNIjjaMHFfrQAAAA==

payjoin proposal filled with sender's information

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

• BlueWallet is in the process of implementing the protocol.
• BTCPay Server has implemented sender and receiver side of this protocol.
• Wasabi Wallet has merged sender's support.
• Join Market has implemented sender and receiver side of this protocol.
• JavaScript sender implementation.

The receivers advertise payjoin capabilities through BIP21's URI Scheme.

Senders not supporting payjoin will just ignore the pj variable and thus, will proceed to normal payment.

Special thanks to Kukks for developing the initial support to BTCPay Server, to junderw, AdamISZ, lukechilds, ncoelho, nopara73, lontivero, yahiheb, SomberNight, andrewkozlik, instagibbs, RHavar for all the feedback we received since our first implementation. Thanks again to RHavar who wrote the BIP79 Bustapay proposal, this gave a good starting point for our proposal.