bip-0375: assign k in output index order

bip-0375.mediawiki

@@ -10,7 +10,7 @@
   Assigned: 2025-01-08
   License: BSD-2-Clause
   Discussion: https://groups.google.com/g/bitcoindev/c/5G5wzqUXyk4
-  Version: 0.1.1
+  Version: 0.1.2
   Requires: 352, 370, 374
 </pre>
 
@@ -237,8 +237,8 @@ Using [https://github.com/bitcoin/bips/blob/master/bip-0374.mediawiki#dleq-proof
 ====Computing the Output Scripts====
 
 Compute the PSBT_OUT_SCRIPT using the procedure in [https://github.com/bitcoin/bips/blob/master/bip-0352.mediawiki#creating-outputs BIP352] but substituting ''a·B<sub>scan</sub>'' with the PSBT_GLOBAL_SP_ECDH_SHARE for that scan key if available, or the sum of all PSBT_IN_SP_ECDH_SHAREs from eligible inputs for that scan key.
-If there are multiple silent payment codes with the same scan key, sort the codes lexicographically in ascending order to determine the ordering of the ''k'' value.
-If there are multiple silent payment codes with both the same scan and spend keys, sort the subgroup by output index in ascending order.
+If there are multiple silent payment codes with the same scan key, the ''k'' values are assigned in ascending output index order: the first of those outputs has ''k'' = 0, the next ''k'' = 1, and so on.
+The counter is per scan key and counts only the outputs sharing that scan key, so it is not the output index itself.
 
 ===Transaction Extractor===
 
@@ -331,7 +331,7 @@ Use the provided [[bip-0375/test_runner.py|test runner]] to validate each test v
 | PSBT_OUT_SCRIPT does not match derived sp output
 |-
 | Output Scripts
-| two sp outputs (same scan / different spend keys) not sorted lexicographically by spend key
+| k values swapped between two sp outputs (same scan key / different spend keys)
 |-
 | Output Scripts
 | k values assigned to wrong output indices with three sp outputs (same scan / spend keys)
@@ -368,7 +368,7 @@ Use the provided [[bip-0375/test_runner.py|test runner]] to validate each test v
 | one input / two sp outputs - output 0 has no label / output 1 uses label=0 convention for sp change
 |-
 | Can Finalize
-| two sp outputs - output 0 uses label=3 / output 1 uses label=1
+| two sp outputs (same scan key) with spend keys in descending order - output 0 uses label=3 / output 1 uses label=1, k follows output index
 |-
 | Can Finalize
 | two inputs using per-input ECDH shares - only eligible inputs contribute shares (P2SH excluded)
@@ -403,6 +403,8 @@ Use the provided [[bip-0375/test_runner.py|test runner]] to validate each test v
 
 == Changelog ==
 
+* '''0.1.2''' (2026-08-14):
+** State that the ''k'' values follow output index order, as the test vectors and the reference validator do.
 * '''0.1.1''' (2026-04-16):
 ** Add test vectors and reference for validating Sending Silent Payments with PSBTs.
 * '''0.1.0''' (2025-01-13):

bip-0375/bip375_test_vectors.json

@@ -477,7 +477,7 @@
       }
     },
     {
-      "description": "output scripts: two sp outputs (same scan / different spend keys) not sorted lexicographically by spend key",
+      "description": "output scripts: k values swapped between two sp outputs (same scan key / different spend keys)",
       "psbt": "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",
       "supplementary": {
         "inputs": [
@@ -966,7 +966,7 @@
       }
     },
     {
-      "description": "can finalize: two sp outputs - output 0 uses label=3 / output 1 uses label=1",
+      "description": "can finalize: two sp outputs (same scan key) with spend keys in descending order - output 0 uses label=3 / output 1 uses label=1, k follows output index",
       "psbt": "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",
       "supplementary": {
         "inputs": [

bip-0375/validator/validate_psbt.py

@@ -304,7 +304,8 @@ def validate_output_scripts(psbt: PSBT) -> Tuple[bool, str]:
             output_index = struct.unpack("<I", output_index_bytes)[0]
             outpoints.append(COutPoint(txid_int, output_index))
 
-    # Track k values per scan key
+    # Track k values per scan key: k is assigned in output index order among
+    # the outputs sharing a scan key (BIP375, Computing the Output Scripts)
     scan_key_k_values = {}
 
     # Validate each SP output