BIP352: be explicit for the input_hash corner case

bip-0352.mediawiki

@@ -298,13 +298,14 @@ After the inputs have been selected, the sender can create one or more outputs f
 * Let ''a = a<sub>1</sub> + a<sub>2</sub> + ... + a<sub>n</sub>'', where each ''a<sub>i</sub>'' has been negated if necessary
 ** If ''a = 0'', fail
 * Let ''input_hash = hash<sub>BIP0352/Inputs</sub>(outpoint<sub>L</sub> || A)'', where ''outpoint<sub>L</sub>'' is the smallest ''outpoint'' lexicographically used in the transaction<ref name="why_smallest_outpoint"></ref> and ''A = a·G''
+** If ''input_hash'' is not a valid scalar, i.e., if ''input_hash = 0'' or ''input_hash'' is larger or equal to the secp256k1 group order, fail
 * Group receiver silent payment addresses by ''B<sub>scan</sub>'' (e.g. each group consists of one ''B<sub>scan</sub>'' and one or more ''B<sub>m</sub>'')
 * For each group:
 ** Let ''ecdh_shared_secret = input_hash·a·B<sub>scan</sub>''
 ** Let ''k = 0''
 ** For each ''B<sub>m</sub>'' in the group:
 *** Let ''t<sub>k</sub> = hash<sub>BIP0352/SharedSecret</sub>(ser<sub>P</sub>(ecdh_shared_secret) || ser<sub>32</sub>(k))''
-**** If ''t<sub>k</sub>'' is not valid tweak, i.e., if ''t<sub>k</sub> = 0'' or ''t<sub>k</sub>'' is larger or equal to the secp256k1 group order, fail
+**** If ''t<sub>k</sub>'' is not a valid scalar, i.e., if ''t<sub>k</sub> = 0'' or ''t<sub>k</sub>'' is larger or equal to the secp256k1 group order, fail
 *** Let ''P<sub>mn</sub> = B<sub>m</sub> + t<sub>k</sub>·G''
 *** Encode ''P<sub>mn</sub>'' as a [https://github.com/bitcoin/bips/blob/master/bip-0341.mediawiki BIP341] taproot output
 *** Optionally, repeat with k++ to create additional outputs for the current ''B<sub>m</sub>''
@@ -331,12 +332,13 @@ If each of the checks in ''[[#scanning-silent-payment-eligible-transactions|Scan
 * Let ''A = A<sub>1</sub> + A<sub>2</sub> + ... + A<sub>n</sub>'', where each ''A<sub>i</sub>'' is the public key of an input from the ''[[#inputs-for-shared-secret-derivation|Inputs For Shared Secret Derivation]]'' list
 ** If ''A'' is the point at infinity, skip the transaction
 * Let ''input_hash = hash<sub>BIP0352/Inputs</sub>(outpoint<sub>L</sub> || A)'', where ''outpoint<sub>L</sub>'' is the smallest ''outpoint'' lexicographically used in the transaction<ref name="why_smallest_outpoint"></ref>
+** If ''input_hash'' is not a valid scalar, i.e., if ''input_hash = 0'' or ''input_hash'' is larger or equal to the secp256k1 group order, fail
 * Let ''ecdh_shared_secret = input_hash·b<sub>scan</sub>·A''
 * Check for outputs:
 ** Let ''outputs_to_check'' be the taproot output keys from all taproot outputs in the transaction (spent and unspent).
 ** Starting with ''k = 0'':
 *** Let ''t<sub>k</sub> = hash<sub>BIP0352/SharedSecret</sub>(ser<sub>P</sub>(ecdh_shared_secret) || ser<sub>32</sub>(k))''
-**** If ''t<sub>k</sub>'' is not valid tweak, i.e., if ''t<sub>k</sub> = 0'' or ''t<sub>k</sub>'' is larger or equal to the secp256k1 group order, fail
+**** If ''t<sub>k</sub>'' is not a valid scalar, i.e., if ''t<sub>k</sub> = 0'' or ''t<sub>k</sub>'' is larger or equal to the secp256k1 group order, fail
 *** Compute ''P<sub>k</sub> = B<sub>spend</sub> + t<sub>k</sub>·G''
 *** For each ''output'' in ''outputs_to_check'':
 **** If ''P<sub>k</sub>'' equals ''output'':
@@ -484,6 +486,8 @@ The <code>MAJOR</code> version is incremented if changes to the BIP are introduc
 The <code>MINOR</code> version is incremented whenever the inputs or the output of an algorithm changes in a backward-compatible way or new backward-compatible functionality is added.
 The <code>PATCH</code> version is incremented for other changes that are noteworthy (bug fixes, test vectors, important clarifications, etc.).
 
+* '''1.0.2''' (2025-07-25):
+** Clarify how to handle the improbable corner case where the output of SHA256 is equal to 0 or greater than or equal to the secp256k1 curve order.
 * '''1.0.1''' (2024-06-22):
 ** Add steps to fail if private key sum is zero (for sender) or public key sum is point at infinity (for receiver), add corresponding test vectors.
 * '''1.0.0''' (2024-05-08):