BIP 77: Specify v1-fallback response mechanism

bip-0077.md

@@ -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