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XLS-66: specify the counterparty signing prefixes #634

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XLS-66: specify the counterparty signing prefixes #634

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11 changes: 11 additions & 0 deletions XLS-0066-lending-protocol/README.md
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Expand Up @@ -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:

@Tapanito Tapanito Sep 16, 2026

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Hey @hliosone this is a nice PR! Could you address the Copilot comment please.


| 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`.

@Tapanito Tapanito Sep 16, 2026

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Could you please remove this sentence as well. It can cause the specs to go stale whenever we'll update the code.


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.
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