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author | AdamISZ <AdamISZ@protonmail.com> | 2023-08-10 15:46:18 +0000 |
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committer | bitcoindev <bitcoindev@gnusha.org> | 2023-08-10 15:46:34 +0000 |
commit | 54a7a55778ffb5978bbe060a82dcbeb91e92ca75 (patch) | |
tree | 106224b4913fb9ca8a2728e9abe4c1f40bc6a520 | |
parent | cb5db05186e2d078e6e6c4aede3560ed5e91a14a (diff) | |
download | pi-bitcoindev-54a7a55778ffb5978bbe060a82dcbeb91e92ca75.tar.gz pi-bitcoindev-54a7a55778ffb5978bbe060a82dcbeb91e92ca75.zip |
Re: [bitcoin-dev] BIP for Serverless Payjoin
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diff --git a/f9/31ebc80d2ea853007b916fc57642349b03fd13 b/f9/31ebc80d2ea853007b916fc57642349b03fd13 new file mode 100644 index 000000000..36cfb301e --- /dev/null +++ b/f9/31ebc80d2ea853007b916fc57642349b03fd13 @@ -0,0 +1,630 @@ +Return-Path: <AdamISZ@protonmail.com> +Received: from smtp1.osuosl.org (smtp1.osuosl.org [IPv6:2605:bc80:3010::138]) + by lists.linuxfoundation.org (Postfix) with ESMTP id A0B10C0032 + for <bitcoin-dev@lists.linuxfoundation.org>; + Thu, 10 Aug 2023 15:46:34 +0000 (UTC) +Received: from localhost (localhost [127.0.0.1]) + by smtp1.osuosl.org (Postfix) with ESMTP id 7B4CB83F9D + for <bitcoin-dev@lists.linuxfoundation.org>; + Thu, 10 Aug 2023 15:46:34 +0000 (UTC) +DKIM-Filter: OpenDKIM Filter v2.11.0 smtp1.osuosl.org 7B4CB83F9D +Authentication-Results: smtp1.osuosl.org; + dkim=pass (2048-bit key) header.d=protonmail.com header.i=@protonmail.com + header.a=rsa-sha256 header.s=protonmail3 header.b=TVoH4eXy +X-Virus-Scanned: amavisd-new at osuosl.org +X-Spam-Flag: NO +X-Spam-Score: -0.064 +X-Spam-Level: +X-Spam-Status: No, score=-0.064 tagged_above=-999 required=5 + tests=[BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, + DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, FREEMAIL_FROM=0.001, + LONGWORDS=2.035, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, + SPF_HELO_PASS=-0.001, SPF_PASS=-0.001] autolearn=no autolearn_force=no +Received: from smtp1.osuosl.org ([127.0.0.1]) + by localhost (smtp1.osuosl.org [127.0.0.1]) (amavisd-new, port 10024) + with ESMTP id WsZ5xfkWLiNH + for <bitcoin-dev@lists.linuxfoundation.org>; + Thu, 10 Aug 2023 15:46:32 +0000 (UTC) +Received: from mail-40134.protonmail.ch (mail-40134.protonmail.ch + [185.70.40.134]) + by smtp1.osuosl.org (Postfix) with ESMTPS id 5A80683F8A + for <bitcoin-dev@lists.linuxfoundation.org>; + Thu, 10 Aug 2023 15:46:32 +0000 (UTC) +DKIM-Filter: OpenDKIM Filter v2.11.0 smtp1.osuosl.org 5A80683F8A +Date: Thu, 10 Aug 2023 15:46:18 +0000 +DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=protonmail.com; + s=protonmail3; t=1691682388; x=1691941588; + bh=UiFBF72nYB1hns8afdzTTSjmi0V2LRmoJfqAKK3rWak=; + h=Date:To:From:Subject:Message-ID:In-Reply-To:References: + Feedback-ID:From:To:Cc:Date:Subject:Reply-To:Feedback-ID: + Message-ID:BIMI-Selector; + b=TVoH4eXyx4yH11VUKLclAc8ekxSzJ5nbs+ozqwT71yh1yb+SP8g6Ml7i9o5mKMVID + Pz7Oteqvf7dXnYBa9EUzHbgchXJcGO+60cckjlcUjBrZVTBxMHMG0uZN4gfSmj+CRr + qf9UFV+ZAZlB9ZJ5VsUY3F6bVIKT/5glabiraFV8t+b/GmxlhUIjYcggspMJ28zDkN + A7j5nTS5aU4rmWjY3/bk1aXB2PXnX5dnJI1UnygUVEPCxnFndT65afPOT4CUGETKCY + IF4MMLQwK8a5l1yXoRRiZT0t0XZN6/4GHZu0b9VzsTF4B3CRrdhQCjdmN/VMyw3QPD + OKDsyzcq/Xx7g== +To: Dan Gould <d@ngould.dev>, + Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org> +From: AdamISZ <AdamISZ@protonmail.com> +Message-ID: <qLcrxFA7z6NkweC9HhZS7g9dcchQfVpjClR-nrMvjYmBobfYzbRrF37QztsuAVdM6HSZJ8UHl27QKYAWq0zYQMmYnBmg0YE-7HO9S6A1Rxs=@protonmail.com> +In-Reply-To: <qUoIvwrIl8ltj3TkQ7y1ExhjUan6VEpGl7c7TlHNfF1pT-eZWd_mwuNYH13YPRyvMj9OSApLmW-hwrdaHCEapEXr503SlXSywcAGceXcbow=@protonmail.com> +References: <7B11AE34-27A7-46ED-95BF-66CA13BA26F3@ngould.dev> + <qUoIvwrIl8ltj3TkQ7y1ExhjUan6VEpGl7c7TlHNfF1pT-eZWd_mwuNYH13YPRyvMj9OSApLmW-hwrdaHCEapEXr503SlXSywcAGceXcbow=@protonmail.com> +Feedback-ID: 11565511:user:proton +MIME-Version: 1.0 +Content-Type: text/plain; charset=utf-8 +Content-Transfer-Encoding: quoted-printable +X-Mailman-Approved-At: Thu, 10 Aug 2023 16:21:20 +0000 +Subject: Re: [bitcoin-dev] BIP for Serverless Payjoin +X-BeenThere: bitcoin-dev@lists.linuxfoundation.org +X-Mailman-Version: 2.1.15 +Precedence: list +List-Id: Bitcoin Protocol Discussion <bitcoin-dev.lists.linuxfoundation.org> +List-Unsubscribe: <https://lists.linuxfoundation.org/mailman/options/bitcoin-dev>, + <mailto:bitcoin-dev-request@lists.linuxfoundation.org?subject=unsubscribe> +List-Archive: <http://lists.linuxfoundation.org/pipermail/bitcoin-dev/> +List-Post: <mailto:bitcoin-dev@lists.linuxfoundation.org> +List-Help: <mailto:bitcoin-dev-request@lists.linuxfoundation.org?subject=help> +List-Subscribe: <https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev>, + <mailto:bitcoin-dev-request@lists.linuxfoundation.org?subject=subscribe> +X-List-Received-Date: Thu, 10 Aug 2023 15:46:34 -0000 + +Sorry for yet another message but: + +It just occurred to me that while timing correlation itself might not be mu= +ch (in usual circumstances, there are tons of other transactions), it's, as= + usual with metadata, the intersection of more than one thing that could hu= +rt: I know when the tx happens (say within a time window of 10 seconds), bu= +t also I might know the *size* of the message. Perhaps consider random padd= +ing of the Payjoin PSBT message size (iirc chacha is a stream cipher so len= +gths are arbitrary). + +Cheers, +AdamISZ/waxwing + +> Isn't the most obvious concern with this architecture, that the relays ha= +ve metadata - most obviously, they can time correlate messages, with bitcoi= +n network events, so at the least they could tie transactions to clients. I= +f 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 re= +quire those (e.g. Tor). Not sure if it's palatable to do this if otherwise,= + i.e. if we think the relays can tie network addresses to transactions? Wel= +l, not sure, but I'd expect it to be mentioned? +>=20 +> Cheers, +> AdamISZ/waxwing +>=20 +>=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 + |