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To: Dan Gould <d@ngould.dev>,
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Subject: Re: [bitcoin-dev] BIP for Serverless Payjoin
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Hi Dan,
A couple more more thoughts:

> Out of band, the receiver of the payment, shares a bitcoin URI with the s=
ender including a <code>pj=3D</code> query parameter describing the relay s=
ubdirectory endpoint and <code>psk=3D</code> parameter with base64 encoded =
256-bit secret key.

You're sending the symmetric secret key out of band; but isn't this obscuri=
ng the question of securely sharing the secret key? Did you consider DH-ing=
 this as other protocols do? At the very least I would claim that it's like=
ly that implementers might be sloppy here; at the most I would claim this i=
s just insecure full stop.

About attack vectors:

> Since relays store arbitrary encrypted payloads to the tragedy of the com=
mons and denial of service attacks. Relay operators may impose an authentic=
ation requirement before they provide relay service to receivers to mitigat=
e such attacks.

Isn't the most obvious concern with this architecture, that the relays have=
 metadata - most obviously, they can time correlate messages, with bitcoin =
network events, so at the least they could tie transactions to clients. *If=
* both parties use anonymised network connections then this is ameliorated =
(though not removed) as a vector, but then we'd need to be clear that we *r=
equire* those (e.g. Tor). Not sure if it's palatable to do this if otherwis=
e, i.e. if we think the relays can tie network addresses to transactions? W=
ell, not sure, but I'd expect it to be mentioned?

Cheers,
AdamISZ/waxwing


Sent with Proton Mail secure email.

------- Original Message -------
On Wednesday, August 9th, 2023 at 11:32, Dan Gould via bitcoin-dev <bitcoin=
-dev@lists.linuxfoundation.org> wrote:


> Hi all,
>=20
> The Serverless Payjoin idea has come a long way toward formal specificati=
on of a Payjoin version 2. In the spirit of BIP 2, I=E2=80=99m sharing an i=
ntermediate draft of the BIP here before opening a draft on GitHub for the =
BIP editors, and before this exact specification has a complete reference i=
mplementation. The draft does reference two proof of concept payjoin implem=
entations, one demonstrating use of symmetric cryptography, and the other a=
synchronous messaging and backwards compatibility.
>=20
> I=E2=80=99ve updated the Serverless Payjoin gist to reflect this draft sp=
ecification https://gist.github.com/DanGould/243e418752fff760c9f6b23bba8a32=
f9 in order to preserve the edit history before opening a bips PR.
>=20
> The specifics have changed significantly compared to the first mailing li=
st post to reflect feedback. Looking forward to hear your thoughts.
>=20
> Dan
>=20
> <pre>
>=20
> BIP: ???
> Layer: Applications
> Title: Payjoin Version 2: Serverless Payjoin
> Author: Dan Gould d@ngould.dev
>=20
> Status: Draft
> Replaces: 78
> Type: Standards Track
> Created: 2023-08-08
> License: BSD-2-Clause
> </pre>
>=20
>=20
> =3D=3DAbstract=3D=3D
>=20
> This document proposes a backwards-compatible second version of the payjo=
in protocol described in [[bip-0078.mediawiki|BIP 78]], allowing complete p=
ayjoin receiver functionality including payment output substitution without=
 requiring them to host a secure public endpoint. This requirement is repla=
ced with an untrusted third-party relay and streaming clients which communi=
cate using an asynchronous protocol and authenticated encrypted payloads.
>=20
> =3D=3DCopyright=3D=3D
>=20
> This BIP is licensed under the 2-clause BSD license.
>=20
> =3D=3DMotivation=3D=3D
>=20
> Payjoin solves the sole privacy problem left open in the bitcoin paper, t=
hat transactions with multiple inputs "necessarily reveal that their inputs=
 were owned by the same owner." Breaking that common-input ownership assump=
tion and others requires input from multiple owners. Cooperative transactio=
n construction also increases transaction throughput by providing new oppor=
tunity for payment batching and transaction cut-through.
>=20
> Version 1 coordinates payjoins over a public server endpoint secured by e=
ither TLS or Tor onion hidden service hosted by the receiver. Version 1 is =
synchronous, so both sender and reciever must be online simultaneously to p=
ayjoin. Both requirements present significant barriers for all but sophisti=
cated server operators or those wallets with complex Tor integration. These=
 barriers are [[https://lists.linuxfoundation.org/pipermail/bitcoin-dev/202=
1-January/018358.html|regarded]] as limits to payjoin adoption.
>=20
> The primary goal of this proposal is to provide a practical coordination =
mechanism to be adopted in a vast majority of wallet environments. This is =
realized as a simple protocol built on bitcoin URI requests, web standards,=
 common crypto, and minimal dependencies.
>=20
> =3D=3D=3DRelation to BIP 78 (Payjoin version 1)=3D=3D=3D
>=20
> The message payloads in this version parrallel those used in BIP 78 while=
 being encapsulated in authenticated encryption, forgoing HTTP messaging fo=
r WebTransport streaming of asynchronus interactions, and leveraging PSBT v=
ersion 2.
>=20
> The BIP 78 standard allows for an [[https://github.com/bitcoin/bips/blob/=
master/bip-0078.mediawiki#unsecured-payjoin-server|unsecured payjoin server=
|]] to operate separately from the so-called "payment server" responsible f=
or generating [[https://github.com/bitcoin/bips/blob/master/bip-0021.mediaw=
iki|BIP 21]] request URIs. Because BIP 78 messages are neither authenticate=
d nor encrypted a malicious unsecured payjoin server is able to modify the =
Payjoin PSBT in flight, thus requiring [[payment output substitition]] to b=
e disabled. Output substitition is useful for a number of block space optim=
izations, including payment batching and transaction cut-through. This prop=
osal introduces authentication and encryption to secure output substition w=
hile using a relay without compromising sender or receiver privacy.
>=20
> Although unsecured payjoin server separation is mentioned in BIP 78, no k=
nown specification or implementation of one exists. This document specifies=
 one to be backwards compatible with version 1 senders. Receivers respondin=
g to version 1 senders must disable output substitution their payloads are =
plaintext so they may payjoin without the risk of the relay stealing funds.
>=20
> The protocols in this document reuse BIP 78's BIP 21 URI parameters. A Fa=
llback PSBT timeout parameter is introduced which may also help coordinate =
the synchronous version 1 protocol.
>=20
> =3D=3D=3DRelation to Stowaway=3D=3D=3D
>=20
> [[https://code.samourai.io/wallet/ExtLibJ/-/blob/develop/doc/cahoots/STOW=
AWAY.md|Stowaway]] is a payjoin coordination mechanism which depends on Tor=
, a third-party relay, and the [[https://samouraiwallet.com/paynym|PayNym]]=
 [[https://github.com/bitcoin/bips/blob/master/bip-0047.mediawiki|BIP 47]] =
Payment codes directory for subdirectory identification and encryption. The=
 payjoin version 2 protocol uses one-time symmetric keys for relay subdirec=
tory identification, authentication, and encryption instead of BIP 47 publi=
c keys derived from the wallet. Payjoin version 2 also supports asynchronou=
s messaging, in contrast to online Stowaway's synchronous HTTP-based messag=
ing. Offline stowaway may depends on manual message passing rather than an =
asynchronous network protocol. Successful Stowaway execution results in 2-o=
utput transactions, while BIP 79, 78, and this work may produce batched tra=
nsactions with many outputs.
>=20
> =3D=3DSpecification=3D=3D
>=20
> =3D=3D=3DOverview=3D=3D=3D
>=20
> Payjoin requests are made using familiar BIP 21 URIs. Instead of a public=
 HTTP endpoint, this scheme allows a WebTransport client to enroll with a r=
elay server to receive payjoin. Relays may optionally require an authorizat=
ion credential before allocating resources in order to prevent DoS attacks.=
 Sender and receiver payloads are buffered at the relay to support asynchro=
nous interaction. Symmetric authenticated encryption (ChaCha20-Poly1305 AEA=
D) prevents the relay from snooping on message contents or forging messages=
. Aside from a pre-shared secret and relayed asynchronus networking, the ve=
rsion 2 messaging takes much the same form as the existing BIP 78 specifica=
tion.
>=20
> =3D=3D=3DBasic scheme=3D=3D=3D
>=20
> The recipient first generates a 256-bit key <code>psk</code>. This pre-sh=
ared key will be the basis of end-to-end authenticated encryption and ident=
ification of a particular payjoin over the relay.
>=20
>=20
> Rather than hosting a public server, they start a streaming session to re=
ceive messages and allocate a subdirectory from which to relay messages. Th=
e first message must include the first 4 bytes of the Sha256 hash of their =
<code>psk</code> to be enrolled as a subdirectory identifier. The next mess=
age streamed from the relay to sender includes the enrolled subdirectory pa=
yjoin endpoint. After enrollment, they await a payjoin request on a session=
 identified by the subdirectory. Out of band, the receiver shares a [[https=
://github.com/bitcoin/bips/blob/master/bip-0021.mediawiki|BIP 21]] payjoin =
uri including the relay endpoint in the <code>pj=3D</code> query parameter =
and the pre-shared key in a new <code>psk=3D</code> query parameter.
>=20
>=20
> The sender constructs an encrypted and authenticated payload containing a=
 PSBT and optional parameters similar to BIP 78. The resulting ciphertext e=
nsures message secrecy and integrity when streamed to the recipient by the =
relay-hosted subdirectory <code>pj=3D</code> endpoint.
>=20
>=20
> The sender's request is relayed to the receiver over a streaming session =
at the subdirectory identified by the hash of <code>psk</code>. Messages ar=
e secured by symmetric cipher rather than TLS or Onion routing session key.=
 Sender and receiver may experience network interruption and proceed with t=
he protocol since their request and response are buffered at the Payjoin re=
lay subdirectory.
>=20
>=20
> =3D=3D=3DPayjoin version 2 messaging=3D=3D=3D
>=20
> Payjoin v2 messages use [[https://github.com/bitcoin/bips/blob/master/bip=
-0370.mediawiki|BIP 370 PSBT v2]] format to fascilitate PSBT mutation.
>=20
> The payjoin version 2 protocol takes the following steps:
>=20
> * The recipient sends the first 4 bytes of <code>H(psk)</code> and option=
al authentication credential according to [[#receiver-relay-enrollment|rece=
iver relay enrollment]] protocol. It may go offline and replay enrollment t=
o come back online.
>=20
> * Out of band, the receiver of the payment, shares a bitcoin URI with the=
 sender including a <code>pj=3D</code> query parameter describing the relay=
 subdirectory endpoint and <code>psk=3D</code> parameter with base64 encode=
d 256-bit secret key. To support version 1 senders the relay acts as an uns=
ecured payjoin server so <code>pjos=3D0</code> must be specified in the URI=
. Version 2 senders may safely allow output substitution regardless.
>=20
> * The sender creates a valid PSBT according to [[https://github.com/bitco=
in/bips/blob/master/bip-0078#receivers-original-psbt-checklist|the receiver=
 checklist]] formatted as PSBTv2. We call this the <code>Fallback PSBT</cod=
e>. This Fallback PSBT and optional sender parameters are encrypted and aut=
henticated with the <code>psk</code> using ChaCha20Poly1305 and streamed to=
 the relay subdirectory endpoint.
>=20
> * The sender awaits a response from the relay stream containing an encryp=
ted <code>Payjoin PSBT</code>. It can replay the <code>Fallback PSBT</code>=
 to request a response if it goes offline.
>=20
> * The request is stored in the receiver's subdirectory buffer.
> * Once the receiver is online, it awaits a stream of request updates from=
 the relay. The receiver decrypts aund authenticates the payload then check=
s it according to [[https://github.com/bitcoin/bips/blob/master/bip-0078#re=
ceivers-original-psbt-checklist|the receiver checklist]]. It updates it to =
include new signed inputs and outputs invalidating sender signatures, and m=
ay adjust the fee. We call this the <code>Payjoin PSBT</code>.
>=20
> * It responds with the <code>Payjoin PSBT</code> encrypted then authentic=
ated under <code>psk</code> using ChaCha20Poly1305.
>=20
> * The relay awaits a connection from the sender if it goes offline. Upon =
connection, it relays the encrypted <code>Payjoin PSBT</code> to the sender=
.
>=20
> * The sender validates the <code>Payjoin PSBT</code> according to [[#send=
ers-payjoin-psbt-checklist|the sender checklist]], signs its inputs and bro=
adcasts the transaction to the Bitcoin network.
>=20
>=20
> The encrypted Fallback PSBT and Payjoin PSBT payloads are sent as bytes.
>=20
> The Fallback PSBT MUST:
>=20
> * Include complete UTXO data.
> * Be signed.
> * Exclude unnecessary fields such as global xpubs or keypath information.=
 <!-- I believe PSBTv2 obviates this requirement -->
>=20
> * Set input and output Transaction Modifiable Flags to 1
> * Be broadcastable.
>=20
> The Fallback PSBT MAY:
>=20
> * Include outputs unrelated to the sender-receiver transfer for batching =
purposes.
> * Set SIGHASH_SINGLE Transaction Modifiable Flags flags to 1
>=20
> The Payjoin PSBT MUST:
>=20
> * Include all inputs from the Fallback PSBT.
> * Include all outputs which do not belong to the receiver from the Fallba=
ck PSBT.
> * Include complete UTXO data.
>=20
> The Payjoin PSBT sender MAY:
>=20
> * Add, remove or modify Fallback PSBT outputs under the control of the re=
ceiver (i.e. not sender change).
>=20
> The Payjoin PSBT MUST NOT:
>=20
> * Shuffle the order of inputs or outputs; the additional outputs or addit=
ional inputs must be inserted at a random index.
> * Decrease the absolute fee of the original transaction.
>=20
> =3D=3D=3DReceiver's Payjoin PSBT checklist=3D=3D=3D
>=20
> Other than requiring PSBTv2 the receiver checklist is the same as the [[h=
ttps://github.com/bitcoin/bips/blob/master/bip-0078.mediawiki#receivers-ori=
ginal-psbt-checklist|the BIP 78 receiver checklist]]
>=20
> =3D=3D=3DSender's Payjoin PSBT checklist=3D=3D=3D
>=20
> The version 2 sender's checklist is largely the same as the [[https://git=
hub.com/bitcoin/bips/blob/master/bip-0078#senders-payjoin-proposal-checklis=
t|the BIP 78 checklist]] with the exception that it expects ALL utxo data t=
o be filled in. BIP 78 required sender inputs UTXO data to be excluded from=
 the PSBT which has caused many headaches since it required the sender to a=
dd them back to the Payjoin proposal PSBT. Version 2 has no such requiremen=
t.
>=20
> =3D=3D=3DRelay interactions=3D=3D=3D
>=20
> The Payjoin Relay provides a rendezvous point for sender and receiver to =
meet. It stores Payjoin payloads to support asynchronous communication. It =
is available on the open internet over HTTPS to accept both WebTransport fo=
r Payjoin version 2, accepting encrypted payloads, and optionally HTTP/1.1 =
to support backwards compatible Payjoin version 1 requests.
>=20
> =3D=3D=3DReceiver interactions=3D=3D=3D
>=20
> =3D=3D=3D=3DRelay enrollment=3D=3D=3D=3D
>=20
> Receivers must enroll to have resources allocated on a relay. Sessions ma=
y begin by having a receiver send the first 4 bytes of the Sha256 hash of t=
heir <code>psk</code> to the relay. The receiver returns the subdirectory e=
ndpoint url. Enrollment may be replayed in case the receiver goes offline.
>=20
>=20
> Optionally, before returning the uri the receiver may request an authenti=
cation token by presenting a message containing only the word <code>Authent=
icate: <description></code> after which the receiver is required to submit =
an <code>Authenticate: <token></code> including the token from the Relay ou=
t of band. If authentication fails an error is returned.
>=20
>=20
> In the case a relay is operated by an exchange, it may give out authentic=
ation tokens for users of its app, or may require some proof of work out of=
 band. Tokens should be anonymous credentials from the relay describing the=
 parameters of their authorization. Specific credentialing is out of the sc=
ope of this proposal.
>=20
> =3D=3D=3D=3DReceiver Payjoin PSBT response=3D=3D=3D=3D
>=20
> The receiver streams the base64 Payjoin PSBT as encrypted bytes from ChaC=
ha20Poly1305 under <code>psk</code>.
>=20
>=20
> =3D=3D=3DSender interactions=3D=3D=3D
>=20
> The sender starts a WebTransport session with the relay at the Payjoin en=
dpoint URI provided by the receiver. It sends the following payload and awa=
its a relayed response payload from the receiver.
>=20
> =3D=3D=3D=3DVersion 2 Fallback PSBT request=3D=3D=3D=3D
>=20
> The version 2 Fallback PSBT Payload is constructed in JSON before being e=
ncrypted as follows.
>=20
> <pre>
>=20
> {
> "psbt": "<fallback_psbt_data_base64>",
>=20
> "params": {
> "param1": "<value1>",
>=20
> "param2": "<value1>",
>=20
> ...
> }
> }
> </pre>
>=20
>=20
> The payload must be encrypted using ChaCha20Poly1305 by the sender using =
the <code>psk</code>.
>=20
>=20
> =3D=3D=3D=3DVersion 1 Fallback PSBT request=3D=3D=3D=3D
>=20
> The message should be the same as version 2 but unencrypted, as version 1=
 is unaware of encryption when using an unsecured payjoin server. The Relay=
 should convert the PSBT to PSBTv2 and construct the JSON payload from the =
HTTP request body and optional query parameters. Upon receiving an unencryp=
ted PSBTv2 response from a receiver, it should convert it to PSBTv0 for com=
patibility with BIP 78.
>=20
> =3D=3D=3DAsynchronous relay buffers=3D=3D=3D
>=20
> Each receiver subdirectory on the relay server has a buffer for requests =
and one for responses. Each buffer updates listeners through awaitable even=
ts so that updates are immediately apparent to relevant client sessions.
>=20
> =3D=3D=3DBIP 21 receiver parameters=3D=3D=3D
>=20
> A major benefit of BIP 78 payjoin over other coordination mechanisms is i=
ts compatibility with the universal BIP 21 bitcoin URI standard.
>=20
> This proposal defines the following new [[https://github.com/bitcoin/bips=
/blob/master/bip-0021.mediawiki|BIP 21 URI]] parameters:
>=20
> * <code>psk</code>: the pre-shared symmetric key for encryption and authe=
ntication with ChaCha20-Poly1305
>=20
> * <code>exp</code>: represents a request expiration after which the recei=
ver reserves the right to broadcast the Fallback and ignore requests.
>=20
>=20
> BIP 78's BIP 21 payjoin parameters are also valid for version 2.
>=20
> =3D=3D=3DOptional sender parameters=3D=3D=3D
>=20
> When the payjoin sender posts the original PSBT to the receiver, it can o=
ptionally specify the following HTTP query string parameters:
>=20
> * <code>v</code>: represents the version number of the payjoin protocol t=
hat the sender is using. This version is <code>2</code>.
>=20
>=20
> BIP 78's optional query parameters are also valid as version 2 parameters=
.
>=20
> =3D=3DRationale=3D=3D
>=20
> =3D=3D=3DRequest expiration & fallback=3D=3D=3D
>=20
> The relay may hold a request for an offline payjoin peer until that peer =
comes online. However, the BIP 78 spec recommends broadcasting request PSBT=
s in the case of an offline counterparty. Doing so exposes a na=C3=AFve, su=
rveillance-vulnerable transaction which payjoin intends to avoid.
>=20
> The existing BIP 78 protocol has to be synchronous only for automated end=
points which may be vulnerable to probing attacks. It can cover this tradeo=
ff by demanding a fallback transaction that would not preserve privacy the =
same way as a payjoin. BIP 21 URI can communicate a request expiration to a=
lleviate both of these problems. Receivers may specify a deadline after whi=
ch they will broadcast this fallback with a new expiration parameter <code>=
exp=3D</code>. <!-- I also like to for timeout, but it's hard to coordinate=
 in an asynchronous way -->
>=20
>=20
> =3D=3D=3DWebTransport=3D=3D=3D
>=20
> Many transport protocols are good candidates for Serverless Payjoin funct=
ionality, but WebTransport stands out in its ability to stream and take adv=
antage of QUIC's performance in mobile environments. In developing this BIP=
, serverless payjoin proofs of concept using TURN, HTTP/1.1 long polling, W=
ebSockets, Magic Wormhole, and Nostr have been made. Streaming allows the r=
elay to have more granular and asynchronous understanding of the state of t=
he peers, and this protcol is designed specifically to address the shortcom=
ings of an HTTP protocol's requirement to receive from a reliable, always-o=
nline connection.
>=20
> While WebTransport and HTTP/3 it is built on are relatively new, widespre=
ad support across browsers assures me that it is being accepted as a standa=
rd and even has a fallback to HTTP/2 environments. Being built on top of QU=
IC allows it to multiplex connections from a relay to multiple peers which =
may prove advantageous for later payjoin protocols between more than two pa=
rticipants contributing inputs, such as those used to fund a lightning node=
 with channels from multiple sources in one transaction, or those with thre=
at models more similar to ZeroLink CoinJoin.
>=20
> While Nostr is fascinating from the perspective of censorship resistance,=
 the backwards compatibility with Payjoin v1 would mean only custom Nostr P=
ayjoin relays exposing an https endpoint would be suitable. Nostr transport=
 is also limited by the performance of WebSockets, being an abstraction on =
top of that protocol. If Nostr authentication were used instead of a symmet=
ric <code>psk</code> then those keys would also need to be communicated out=
 of band and complicate the protocol. There is nothing stopping a new versi=
on of this protocol or a NIP making Payjoin version 2 possible over Nostr s=
hould Payjoin censorship become a bottleneck in the way of adoption.
>=20
>=20
> WebTransport is already shipped in both Firefox, Chrome, h3 in Rust, Go, =
and all popular languages. There is also [[https://w3c.github.io/p2p-webtra=
nsport/|a working draft for full P2P WebTransport]] without any relay, whic=
h a future payjoin protocol may make use of.
>=20
> =3D=3D=3DChaCha20Poly1305 AEAD=3D=3D=3D
>=20
> This authenticated encryption with additional data [[https://en.wikipedia=
.org/wiki/ChaCha20-Poly1305|algorithm]] is standardized in RFC 8439 and has=
 high performance. ChaCha20Poly1305 AEAD seems to be making its way into bi=
tcoin by way of [[https://github.com/bitcoin/bips/blob/master/bip-0324.medi=
awiki|BIP 324]] as well. The protocol has widespread support in browsers, O=
penSSL and libsodium. AES-GCM is more widespread but is both older, slower,=
 and not a dependency in bitcoin software.
>=20
> secp256k1 asymmetric cryptography could be used, but symmetric encryption=
 allows for many fewer messages to be sent, a single ephemeral key, and see=
ms suitable given the one time use of BIP 21 URIs for Payjoin. Payjoin alre=
ady requires base64 encoding for PSBTs, so we have it available to encode t=
he 256-bit <code>psk</code> in the BIP 21 parameter.
>=20
>=20
> =3D=3D=3DPSBT Version 2=3D=3D=3D
>=20
> The PSBT version 1 protocol was replaced because it was not designed to h=
ave inputs and outputs be mutated. Payjoin mutates the PSBT, so BIP 78 uses=
 a hack where a new PSBT is created by the receiver instead of mutating it.=
 This can cause some strange behaviors from signers who don't know where to=
 look to find the scripts that they are accountable for. PSBT version 2 mak=
es mutating a PSBT's inputs and outputs trivial. It also eliminates the tra=
nsaction finalization step. Receivers who do not understand PSBT version 1 =
may choose to reject Payjoin version 1 requests and only support PSBT versi=
on 2.
>=20
> =3D=3D=3DAttack vectors=3D=3D=3D
>=20
> Since relays store arbitrary encrypted payloads to the tragedy of the com=
mons and denial of service attacks. Relay operators may impose an authentic=
ation requirement before they provide relay service to receivers to mitigat=
e such attacks.
>=20
> Since <code>psk</code> is a symmetric key, the first message containing t=
he sender's original PSBT does not have forward secrecy. Since relay buffer=
s are associated with a single ephemeral relay directory, to support reques=
t-response simplicity of version 1, this seems appropriate.
>=20
>=20
> Since the Fallback PSBT is valid, even where <code>exp=3D</code> is speci=
fied, the receiver may broadcast it and lose out on ambiguous privacy prote=
ction from payjoin at any time. Though unfortunate, this is the typical bit=
coin transaction flow today anyhow.
>=20
>=20
> =3D=3D=3DNetwork privacy=3D=3D=3D
>=20
> Unlike BIP 78 implementations, sender and receiver peers will only see th=
e IP address of the relay, not their peer's. Relays may be made available v=
ia Tor hidden service or Oblivious HTTP in addition to IP / DNS to allow ei=
ther of the peers to protect their IP from the relay with without requiring=
 both peers to use additional network security dependencies.
>=20
> =3D=3DBackwards compatibility=3D=3D
>=20
> The receivers advertise payjoin capabilities through [[https://github.com=
/bitcoin/bips/blob/master/bip-0021.mediawiki|BIP21's URI Scheme]].
>=20
> Senders not supporting payjoin will just ignore the <code>pj=3D</code> pa=
rameter and proceed to typical address-based transaction flows. <code>req-p=
j=3D</code> may be used to compel payjoin.
>=20
>=20
> Receivers may choose to support version 1 payloads. Version 2 payjoin URI=
s should enable <code>pjos=3D0</code> so that these v1 senders disable outp=
ut substitution since the v1 messages are neither encrypted nor authenticat=
ed, putting them at risk for man-in-the-middle attacks otherwise. The relay=
 protocol should carry on as normal, validating based on HTTP headers and c=
onstructing an unencrypted Version 2 payload from optional query parameters=
, and PSBT in the body.
>=20
>=20
> The BIP 78 error messages are already JSON formatted, so it made sense to=
 rely on the same dependency for these payloads and error messages.
>=20
> =3D=3DReference implementation=3D=3D
>=20
> An early proof of concept draft reference implementation can be found at =
https://github.com/payjoin/rust-payjoin/pull/78. It implements an asynchron=
ous payment flow using WebSockets using PSBTv1 without encryption. Another =
reference can be found at https://github.com/payjoin/rust-payjoin/pull/21 w=
hich uses HTTP long polling for transport and Noise NNpsk0 for crypto. Rece=
ntly, I've come to realize the rationale for WebTransport, PSBTv2, and ChaC=
ha20-Poly1305 AEAD substitutions and am working on an implementation includ=
ing this exact specification, but wanted to get early feedback on this desi=
gn in the spirit of BIP 2.
>=20
> =3D=3DAcknowledgements=3D=3D
>=20
> Thank you to OpenSats for funding this pursuit, to Human Rights Foundatio=
n for putting a bounty on it and funding invaluable BOB Space space support=
, who I owe a thank you to as well. Thank you to Ethan Heilman, Nicolas Dor=
ier, Kukks, nopara73, Kristaps Kaupe, Kixunil, /dev/fd0/, Craig Raw, Mike S=
chmidt, Murch, D=C3=A1vid Moln=C3=A1r, Lucas Ontiviero, and uncountable twi=
tter plebs for feedback that has turned this idea from concept into draft, =
to Mike Jarmuz for suggesting that I write a BIP, and to Satsie for writing=
 the "All About BIPS" zine which I've referenced a number of times in the d=
rafting process. Thanks to Armin Sabouri, Ron Stoner, and Johns Beharry for=
 hacking on the first iOS Payjoin receiver and uncovering the problem that =
this solves in the first place.
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