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BIP 77: Specify v1-fallback response mechanism

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The receiver-to-sender response section presented HPKE encryption and
POST as the only path, when v2 receivers servicing v1 (BIP 78) senders
in the wild send the cleartext base64 PSBT via PUT to their own mailbox.
Add a forward-reference from the v2 path to Backwards compatibility,
and specify the v1-fallback response (no HPKE; PUT method; UTF-8 body;
receiver's own mailbox as target) normatively in that section.

Addresses payjoin/rust-payjoin#1487. Partially addresses
payjoin/rust-payjoin#844 (PUT/POST gap).
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DanGould
2026-06-01 16:13:54 +08:00
parent e85e7ff2a7
commit 9997e00576
1 changed files with 17 additions and 0 deletions
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bip-0077.md
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@@ -405,6 +405,13 @@ Oblivious HTTP to the directory's OHTTP Gateway. OHTTP serializes the
inner request as BHTTP, and provides another layer of HPKE encryption,
between the client and directory.
The above describes the receiver's reply path when the sender used
Version 2. When the sender used the BIP 78 backwards-compatible path
(the Original PSBT was a BIP 78 cleartext payload rather than an
HPKE-encrypted v2 message), the receiver's response MUST instead follow
[Backwards compatibility](#backwards-compatibility): the *Proposal PSBT*
is not HPKE-encrypted and the request method is `PUT`.
Once the receiver makes this request, they wait for either transaction from the
Original PSBT or Proposal PSBT to be broadcast to the Bitcoin network.
@@ -744,6 +751,16 @@ seconds or else the directory should respond with an `unavailable` JSON
error code as [defined in BIP
78](https://github.com/bitcoin/bips/blob/master/bip-0078.mediawiki#receivers-well-known-errors).
When responding to a BIP 78 sender, the receiver MUST NOT HPKE-encrypt
the *Proposal PSBT*. The receiver MUST send the *Proposal PSBT* as the
body of a PUT request, with the body being the base64-encoded PSBT,
encoded as ASCII bytes. The target mailbox endpoint MUST be the
receiver's own mailbox (the same mailbox at which the sender posted the
*Original PSBT*), because a BIP 78 sender provides no reply key from which
a sender-side reply mailbox could be derived. The PUT request is then
OHTTP-encapsulated to the directory's OHTTP Gateway as for any other
Payjoin Directory interaction.
## Reference implementation
A production reference implementation client can be found at