From c3c46bcd83b559d555640952fb8153731c4d0912 Mon Sep 17 00:00:00 2001
From: hliosone <116442412+hliosone@users.noreply.github.com>
Date: Sun, 13 Sep 2026 11:22:31 +0200
Subject: [PATCH] XLS-66: specify the counterparty signing prefixes

A counterparty signs over a different hash prefix than an ordinary transaction
signature, and the specification does not say so. Document both prefixes, since
section 3.8.1.1 allows a single-key signature and a Signers list, and each uses
a different one.
---
 XLS-0066-lending-protocol/README.md | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/XLS-0066-lending-protocol/README.md b/XLS-0066-lending-protocol/README.md
index 14fe5fc24..9565b654b 100644
--- a/XLS-0066-lending-protocol/README.md
+++ b/XLS-0066-lending-protocol/README.md
@@ -1009,6 +1009,17 @@ The final transaction must include exactly one of
 1. The `SigningPubKey` and `TxnSignature` fields, or
 2. The `Signers` field and, optionally, an empty `SigningPubKey`.
 
+The counterparty signature is computed over the same signing fields as the primary signature, but with a different hash prefix:
+
+| Signature                            | Prefix |    Value     |
+| ------------------------------------ | :----: | :----------: |
+| `CounterpartySignature.TxnSignature` | `CPT`  | `0x43505400` |
+| `CounterpartySignature.Signers`      | `CPM`  | `0x43504D00` |
+
+These differ from the prefixes used for an ordinary transaction signature, `STX` (`0x53545800`) and `SMT` (`0x534D5400`). A counterparty signature computed with an ordinary prefix does not verify.
+
+The values are defined in `HashPrefix.h` as `CounterpartyTxSign` and `CounterpartyTxMultiSign`, and are selected in `Sign.cpp` by `SignatureRole::Counterparty`.
+
 The total fee for the transaction will be increased due to the extra signatures that need to be processed, similar to the additional fees for multisigning. The minimum additional fee will be $(|signatures|) \times base\\_fee$ where $|signatures| == max(1, |tx.CounterPartySignature.Signers|)$
 
 If the `LoanSet` transaction is **not** part of a [`Batch` transaction](../XLS-0056-batch/README.md), the total fee calculation for signatures will now be $(1 + |tx.Signers| + |signatures|) \times base\\_fee$. In other words, even without a `tx.Signers` list, the minimum fee will be $2 \times base\\_fee$. Otherwise, the fee is based on the total number of signatures in the outer transaction. See [Batch Fees](../XLS-0056-batch/README.md#22-transaction-fee) for further details.