Date: Thu, 10 Aug 2023 14:57:02 +0000
To: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
From: AdamISZ <AdamISZ@protonmail.com>
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Subject: [bitcoin-dev] Fw: Re:  BIP for Serverless Payjoin
Precedence: list

Sorry forgot to include list on this:

> Hi Dan,
> Thanks for this! I look forward to reading it in detail, these ideas are =
for sure very interesting.
>=20
> I wanted to immediately "nit" a point I saw as I was reading:
>=20
> > Because BIP 78 messages are neither authenticated nor encrypted a malic=
ious unsecured payjoin server is able to modify the Payjoin PSBT in flight,
>=20
>=20
> Taken as is - i.e. out of context! - this is just wrong. The BIP explicit=
ly states:
>=20
> "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 b=
een communicated via a TLS connection. Senders SHOULD NOT accept a url repr=
esenting an unencrypted or unauthenticated connection. "
>=20
> (I remember this clearly as it was a point I was keen on including; where=
as the unsecured payjoin server idea, I honestly didn't even know it was in=
 the BIP!)
>=20
> Now, back to your BIP draft: in context, this sentence makes sense, but I=
 guarantee 99% of readers won't notice, because the sentence itself does no=
t make a qualification, and will think that BIP78 ignores the fact that mes=
sages can be altered in flight, which would have been a pretty appalling er=
ror - exactly as discussed (pjos etc etc) - and, it definitely does not!
>=20
> Sent with Proton Mail secure email.
>=20
>=20
> ------- Original Message -------
> On Wednesday, August 9th, 2023 at 11:32, Dan Gould via bitcoin-dev bitcoi=
n-dev@lists.linuxfoundation.org wrote:
>=20
>=20
>=20
> > Hi all,
> >=20
> > The Serverless Payjoin idea has come a long way toward formal specifica=
tion of a Payjoin version 2. In the spirit of BIP 2, I=E2=80=99m sharing an=
 intermediate draft of the BIP here before opening a draft on GitHub for th=
e BIP editors, and before this exact specification has a complete reference=
 implementation. The draft does reference two proof of concept payjoin impl=
ementations, one demonstrating use of symmetric cryptography, and the other=
 asynchronous messaging and backwards compatibility.
> >=20
> > I=E2=80=99ve updated the Serverless Payjoin gist to reflect this draft =
specification https://gist.github.com/DanGould/243e418752fff760c9f6b23bba8a=
32f9 in order to preserve the edit history before opening a bips PR.
> >=20
> > The specifics have changed significantly compared to the first mailing =
list 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
> > =3D=3DAbstract=3D=3D
> >=20
> > This document proposes a backwards-compatible second version of the pay=
join protocol described in [[bip-0078.mediawiki|BIP 78]], allowing complete=
 payjoin receiver functionality including payment output substitution witho=
ut requiring them to host a secure public endpoint. This requirement is rep=
laced with an untrusted third-party relay and streaming clients which commu=
nicate 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,=
 that transactions with multiple inputs "necessarily reveal that their inpu=
ts were owned by the same owner." Breaking that common-input ownership assu=
mption and others requires input from multiple owners. Cooperative transact=
ion construction also increases transaction throughput by providing new opp=
ortunity for payment batching and transaction cut-through.
> >=20
> > Version 1 coordinates payjoins over a public server endpoint secured by=
 either TLS or Tor onion hidden service hosted by the receiver. Version 1 i=
s synchronous, so both sender and reciever must be online simultaneously to=
 payjoin. Both requirements present significant barriers for all but sophis=
ticated server operators or those wallets with complex Tor integration. The=
se barriers are [[https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2=
021-January/018358.html|regarded]] as limits to payjoin adoption.
> >=20
> > The primary goal of this proposal is to provide a practical coordinatio=
n mechanism to be adopted in a vast majority of wallet environments. This i=
s realized as a simple protocol built on bitcoin URI requests, web standard=
s, 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 whi=
le being encapsulated in authenticated encryption, forgoing HTTP messaging =
for WebTransport streaming of asynchronus interactions, and leveraging PSBT=
 version 2.
> >=20
> > The BIP 78 standard allows for an [[https://github.com/bitcoin/bips/blo=
b/master/bip-0078.mediawiki#unsecured-payjoin-server|unsecured payjoin serv=
er|]] to operate separately from the so-called "payment server" responsible=
 for generating [[https://github.com/bitcoin/bips/blob/master/bip-0021.medi=
awiki|BIP 21]] request URIs. Because BIP 78 messages are neither authentica=
ted nor encrypted a malicious unsecured payjoin server is able to modify th=
e Payjoin PSBT in flight, thus requiring [[payment output substitition]] to=
 be disabled. Output substitition is useful for a number of block space opt=
imizations, including payment batching and transaction cut-through. This pr=
oposal introduces authentication and encryption to secure output substition=
 while using a relay without compromising sender or receiver privacy.
> >=20
> > Although unsecured payjoin server separation is mentioned in BIP 78, no=
 known specification or implementation of one exists. This document specifi=
es one to be backwards compatible with version 1 senders. Receivers respond=
ing to version 1 senders must disable output substitution their payloads ar=
e plaintext so they may payjoin without the risk of the relay stealing fund=
s.
> >=20
> > The protocols in this document reuse BIP 78's BIP 21 URI parameters. A =
Fallback PSBT timeout parameter is introduced which may also help coordinat=
e 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/ST=
OWAWAY.md|Stowaway]] is a payjoin coordination mechanism which depends on T=
or, 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. T=
he payjoin version 2 protocol uses one-time symmetric keys for relay subdir=
ectory identification, authentication, and encryption instead of BIP 47 pub=
lic keys derived from the wallet. Payjoin version 2 also supports asynchron=
ous messaging, in contrast to online Stowaway's synchronous HTTP-based mess=
aging. Offline stowaway may depends on manual message passing rather than a=
n asynchronous network protocol. Successful Stowaway execution results in 2=
-output transactions, while BIP 79, 78, and this work may produce batched t=
ransactions 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 publ=
ic HTTP endpoint, this scheme allows a WebTransport client to enroll with a=
 relay server to receive payjoin. Relays may optionally require an authoriz=
ation credential before allocating resources in order to prevent DoS attack=
s. Sender and receiver payloads are buffered at the relay to support asynch=
ronous interaction. Symmetric authenticated encryption (ChaCha20-Poly1305 A=
EAD) prevents the relay from snooping on message contents or forging messag=
es. Aside from a pre-shared secret and relayed asynchronus networking, the =
version 2 messaging takes much the same form as the existing BIP 78 specifi=
cation.
> >=20
> > =3D=3D=3DBasic scheme=3D=3D=3D
> >=20
> > The recipient first generates a 256-bit key <code>psk</code>. This pre-=
shared key will be the basis of end-to-end authenticated encryption and ide=
ntification of a particular payjoin over the relay.
> >=20
> > Rather than hosting a public server, they start a streaming session to =
receive messages and allocate a subdirectory from which to relay messages. =
The first message must include the first 4 bytes of the Sha256 hash of thei=
r <code>psk</code> to be enrolled as a subdirectory identifier. The next me=
ssage streamed from the relay to sender includes the enrolled subdirectory =
payjoin endpoint. After enrollment, they await a payjoin request on a sessi=
on identified by the subdirectory. Out of band, the receiver shares a [[htt=
ps://github.com/bitcoin/bips/blob/master/bip-0021.mediawiki|BIP 21]] payjoi=
n uri including the relay endpoint in the <code>pj=3D</code> query paramete=
r and the pre-shared key in a new <code>psk=3D</code> query parameter.
> >=20
> > The sender constructs an encrypted and authenticated payload containing=
 a PSBT and optional parameters similar to BIP 78. The resulting ciphertext=
 ensures message secrecy and integrity when streamed to the recipient by th=
e relay-hosted subdirectory <code>pj=3D</code> endpoint.
> >=20
> > The sender's request is relayed to the receiver over a streaming sessio=
n at the subdirectory identified by the hash of <code>psk</code>. Messages =
are secured by symmetric cipher rather than TLS or Onion routing session ke=
y. Sender and receiver may experience network interruption and proceed with=
 the protocol since their request and response are buffered at the Payjoin =
relay subdirectory.
> >=20
> > =3D=3D=3DPayjoin version 2 messaging=3D=3D=3D
> >=20
> > Payjoin v2 messages use [[https://github.com/bitcoin/bips/blob/master/b=
ip-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 opti=
onal authentication credential according to [[#receiver-relay-enrollment|re=
ceiver relay enrollment]] protocol. It may go offline and replay enrollment=
 to come back online.
> >=20
> > * Out of band, the receiver of the payment, shares a bitcoin URI with t=
he sender including a <code>pj=3D</code> query parameter describing the rel=
ay subdirectory endpoint and <code>psk=3D</code> parameter with base64 enco=
ded 256-bit secret key. To support version 1 senders the relay acts as an u=
nsecured payjoin server so <code>pjos=3D0</code> must be specified in the U=
RI. Version 2 senders may safely allow output substitution regardless.
> >=20
> > * The sender creates a valid PSBT according to [[https://github.com/bit=
coin/bips/blob/master/bip-0078#receivers-original-psbt-checklist|the receiv=
er checklist]] formatted as PSBTv2. We call this the <code>Fallback PSBT</c=
ode>. This Fallback PSBT and optional sender parameters are encrypted and a=
uthenticated 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 encr=
ypted <code>Payjoin PSBT</code>. It can replay the <code>Fallback PSBT</cod=
e> 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 fr=
om the relay. The receiver decrypts aund authenticates the payload then che=
cks it according to [[https://github.com/bitcoin/bips/blob/master/bip-0078#=
receivers-original-psbt-checklist|the receiver checklist]]. It updates it t=
o include new signed inputs and outputs invalidating sender signatures, and=
 may adjust the fee. We call this the <code>Payjoin PSBT</code>.
> >=20
> > * It responds with the <code>Payjoin PSBT</code> encrypted then authent=
icated under <code>psk</code> using ChaCha20Poly1305.
> >=20
> > * The relay awaits a connection from the sender if it goes offline. Upo=
n connection, it relays the encrypted <code>Payjoin PSBT</code> to the send=
er.
> >=20
> > * The sender validates the <code>Payjoin PSBT</code> according to [[#se=
nders-payjoin-psbt-checklist|the sender checklist]], signs its inputs and b=
roadcasts the transaction to the Bitcoin network.
> >=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 informatio=
n. <!-- 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 batchin=
g 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 Fall=
back PSBT.
> > * Include complete UTXO data.
> >=20
> > The Payjoin PSBT sender MAY:
> >=20
> > * Add, remove or modify Fallback PSBT outputs under the control of the =
receiver (i.e. not sender change).
> >=20
> > The Payjoin PSBT MUST NOT:
> >=20
> > * Shuffle the order of inputs or outputs; the additional outputs or add=
itional 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 [=
[https://github.com/bitcoin/bips/blob/master/bip-0078.mediawiki#receivers-o=
riginal-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://g=
ithub.com/bitcoin/bips/blob/master/bip-0078#senders-payjoin-proposal-checkl=
ist|the BIP 78 checklist]] with the exception that it expects ALL utxo data=
 to be filled in. BIP 78 required sender inputs UTXO data to be excluded fr=
om the PSBT which has caused many headaches since it required the sender to=
 add them back to the Payjoin proposal PSBT. Version 2 has no such requirem=
ent.
> >=20
> > =3D=3D=3DRelay interactions=3D=3D=3D
> >=20
> > The Payjoin Relay provides a rendezvous point for sender and receiver t=
o meet. It stores Payjoin payloads to support asynchronous communication. I=
t is available on the open internet over HTTPS to accept both WebTransport =
for 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 =
may begin by having a receiver send the first 4 bytes of the Sha256 hash of=
 their <code>psk</code> to the relay. The receiver returns the subdirectory=
 endpoint url. Enrollment may be replayed in case the receiver goes offline=
.
> >=20
> > Optionally, before returning the uri the receiver may request an authen=
tication token by presenting a message containing only the word <code>Authe=
nticate: <description></code> after which the receiver is required to submi=
t an <code>Authenticate: <token></code> including the token from the Relay =
out of band. If authentication fails an error is returned.
> >=20
> > In the case a relay is operated by an exchange, it may give out authent=
ication 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 t=
he parameters of their authorization. Specific credentialing is out of the =
scope 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 Ch=
aCha20Poly1305 under <code>psk</code>.
> >=20
> > =3D=3D=3DSender interactions=3D=3D=3D
> >=20
> > The sender starts a WebTransport session with the relay at the Payjoin =
endpoint URI provided by the receiver. It sends the following payload and a=
waits 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=
 encrypted as follows.
> >=20
> > <pre>
> >=20
> > {
> > "psbt": "<fallback_psbt_data_base64>",
> >=20
> > "params": {
> > "param1": "<value1>",
> >=20
> > "param2": "<value1>",
> >=20
> > ...
> > }
> > }
> > </pre>
> >=20
> > The payload must be encrypted using ChaCha20Poly1305 by the sender usin=
g the <code>psk</code>.
> >=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 Rel=
ay should convert the PSBT to PSBTv2 and construct the JSON payload from th=
e HTTP request body and optional query parameters. Upon receiving an unencr=
ypted PSBTv2 response from a receiver, it should convert it to PSBTv0 for c=
ompatibility with BIP 78.
> >=20
> > =3D=3D=3DAsynchronous relay buffers=3D=3D=3D
> >=20
> > Each receiver subdirectory on the relay server has a buffer for request=
s and one for responses. Each buffer updates listeners through awaitable ev=
ents 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=
 its compatibility with the universal BIP 21 bitcoin URI standard.
> >=20
> > This proposal defines the following new [[https://github.com/bitcoin/bi=
ps/blob/master/bip-0021.mediawiki|BIP 21 URI]] parameters:
> >=20
> > * <code>psk</code>: the pre-shared symmetric key for encryption and aut=
hentication with ChaCha20-Poly1305
> >=20
> > * <code>exp</code>: represents a request expiration after which the rec=
eiver reserves the right to broadcast the Fallback and ignore requests.
> >=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=
 optionally specify the following HTTP query string parameters:
> >=20
> > * <code>v</code>: represents the version number of the payjoin protocol=
 that the sender is using. This version is <code>2</code>.
> >=20
> > BIP 78's optional query parameters are also valid as version 2 paramete=
rs.
> >=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 pee=
r comes online. However, the BIP 78 spec recommends broadcasting request PS=
BTs in the case of an offline counterparty. Doing so exposes a na=C3=AFve, =
surveillance-vulnerable transaction which payjoin intends to avoid.
> >=20
> > The existing BIP 78 protocol has to be synchronous only for automated e=
ndpoints which may be vulnerable to probing attacks. It can cover this trad=
eoff by demanding a fallback transaction that would not preserve privacy th=
e same way as a payjoin. BIP 21 URI can communicate a request expiration to=
 alleviate both of these problems. Receivers may specify a deadline after w=
hich they will broadcast this fallback with a new expiration parameter <cod=
e>exp=3D</code>. <!-- I also like to for timeout, but it's hard to coordina=
te in an asynchronous way -->
> >=20
> > =3D=3D=3DWebTransport=3D=3D=3D
> >=20
> > Many transport protocols are good candidates for Serverless Payjoin fun=
ctionality, but WebTransport stands out in its ability to stream and take a=
dvantage of QUIC's performance in mobile environments. In developing this B=
IP, serverless payjoin proofs of concept using TURN, HTTP/1.1 long polling,=
 WebSockets, Magic Wormhole, and Nostr have been made. Streaming allows the=
 relay to have more granular and asynchronous understanding of the state of=
 the peers, and this protcol is designed specifically to address the shortc=
omings of an HTTP protocol's requirement to receive from a reliable, always=
-online connection.
> >=20
> > While WebTransport and HTTP/3 it is built on are relatively new, widesp=
read support across browsers assures me that it is being accepted as a stan=
dard and even has a fallback to HTTP/2 environments. Being built on top of =
QUIC allows it to multiplex connections from a relay to multiple peers whic=
h may prove advantageous for later payjoin protocols between more than two =
participants contributing inputs, such as those used to fund a lightning no=
de with channels from multiple sources in one transaction, or those with th=
reat models more similar to ZeroLink CoinJoin.
> >=20
> > While Nostr is fascinating from the perspective of censorship resistanc=
e, the backwards compatibility with Payjoin v1 would mean only custom Nostr=
 Payjoin relays exposing an https endpoint would be suitable. Nostr transpo=
rt is also limited by the performance of WebSockets, being an abstraction o=
n top of that protocol. If Nostr authentication were used instead of a symm=
etric <code>psk</code> then those keys would also need to be communicated o=
ut of band and complicate the protocol. There is nothing stopping a new ver=
sion of this protocol or a NIP making Payjoin version 2 possible over Nostr=
 should Payjoin censorship become a bottleneck in the way of adoption.
> >=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-webt=
ransport/|a working draft for full P2P WebTransport]] without any relay, wh=
ich 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.wikiped=
ia.org/wiki/ChaCha20-Poly1305|algorithm]] is standardized in RFC 8439 and h=
as high performance. ChaCha20Poly1305 AEAD seems to be making its way into =
bitcoin by way of [[https://github.com/bitcoin/bips/blob/master/bip-0324.me=
diawiki|BIP 324]] as well. The protocol has widespread support in browsers,=
 OpenSSL and libsodium. AES-GCM is more widespread but is both older, slowe=
r, and not a dependency in bitcoin software.
> >=20
> > secp256k1 asymmetric cryptography could be used, but symmetric encrypti=
on allows for many fewer messages to be sent, a single ephemeral key, and s=
eems suitable given the one time use of BIP 21 URIs for Payjoin. Payjoin al=
ready requires base64 encoding for PSBTs, so we have it available to encode=
 the 256-bit <code>psk</code> in the BIP 21 parameter.
> >=20
> > =3D=3D=3DPSBT Version 2=3D=3D=3D
> >=20
> > The PSBT version 1 protocol was replaced because it was not designed to=
 have inputs and outputs be mutated. Payjoin mutates the PSBT, so BIP 78 us=
es a hack where a new PSBT is created by the receiver instead of mutating i=
t. 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 m=
akes mutating a PSBT's inputs and outputs trivial. It also eliminates the t=
ransaction finalization step. Receivers who do not understand PSBT version =
1 may choose to reject Payjoin version 1 requests and only support PSBT ver=
sion 2.
> >=20
> > =3D=3D=3DAttack vectors=3D=3D=3D
> >=20
> > Since relays store arbitrary encrypted payloads to the tragedy of the c=
ommons and denial of service attacks. Relay operators may impose an authent=
ication requirement before they provide relay service to receivers to mitig=
ate such attacks.
> >=20
> > Since <code>psk</code> is a symmetric key, the first message containing=
 the sender's original PSBT does not have forward secrecy. Since relay buff=
ers are associated with a single ephemeral relay directory, to support requ=
est-response simplicity of version 1, this seems appropriate.
> >=20
> > Since the Fallback PSBT is valid, even where <code>exp=3D</code> is spe=
cified, the receiver may broadcast it and lose out on ambiguous privacy pro=
tection from payjoin at any time. Though unfortunate, this is the typical b=
itcoin transaction flow today anyhow.
> >=20
> > =3D=3D=3DNetwork privacy=3D=3D=3D
> >=20
> > Unlike BIP 78 implementations, sender and receiver peers will only see =
the IP address of the relay, not their peer's. Relays may be made available=
 via Tor hidden service or Oblivious HTTP in addition to IP / DNS to allow =
either of the peers to protect their IP from the relay with without requiri=
ng both peers to use additional network security dependencies.
> >=20
> > =3D=3DBackwards compatibility=3D=3D
> >=20
> > The receivers advertise payjoin capabilities through [[https://github.c=
om/bitcoin/bips/blob/master/bip-0021.mediawiki|BIP21's URI Scheme]].
> >=20
> > Senders not supporting payjoin will just ignore the <code>pj=3D</code> =
parameter and proceed to typical address-based transaction flows. <code>req=
-pj=3D</code> may be used to compel payjoin.
> >=20
> > Receivers may choose to support version 1 payloads. Version 2 payjoin U=
RIs should enable <code>pjos=3D0</code> so that these v1 senders disable ou=
tput substitution since the v1 messages are neither encrypted nor authentic=
ated, putting them at risk for man-in-the-middle attacks otherwise. The rel=
ay protocol should carry on as normal, validating based on HTTP headers and=
 constructing an unencrypted Version 2 payload from optional query paramete=
rs, and PSBT in the body.
> >=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 a=
t https://github.com/payjoin/rust-payjoin/pull/78. It implements an asynchr=
onous payment flow using WebSockets using PSBTv1 without encryption. Anothe=
r reference can be found at https://github.com/payjoin/rust-payjoin/pull/21=
 which uses HTTP long polling for transport and Noise NNpsk0 for crypto. Re=
cently, I've come to realize the rationale for WebTransport, PSBTv2, and Ch=
aCha20-Poly1305 AEAD substitutions and am working on an implementation incl=
uding this exact specification, but wanted to get early feedback on this de=
sign in the spirit of BIP 2.
> >=20
> > =3D=3DAcknowledgements=3D=3D
> >=20
> > Thank you to OpenSats for funding this pursuit, to Human Rights Foundat=
ion for putting a bounty on it and funding invaluable BOB Space space suppo=
rt, who I owe a thank you to as well. Thank you to Ethan Heilman, Nicolas D=
orier, Kukks, nopara73, Kristaps Kaupe, Kixunil, /dev/fd0/, Craig Raw, Mike=
 Schmidt, Murch, D=C3=A1vid Moln=C3=A1r, Lucas Ontiviero, and uncountable t=
witter 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 writi=
ng the "All About BIPS" zine which I've referenced a number of times in the=
 drafting process. Thanks to Armin Sabouri, Ron Stoner, and Johns Beharry f=
or hacking on the first iOS Payjoin receiver and uncovering the problem tha=
t this solves in the first place.
> > _______________________________________________
> > bitcoin-dev mailing list
> > bitcoin-dev@lists.linuxfoundation.org
> > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev