fix(receive): Enforce BIP-352 K_MAX per-group recipient limit in scan_txouts

silentpayments/src/receive/error.rs

@@ -8,6 +8,8 @@ pub enum SpReceiveError {
     SliceError(bitcoin::key::FromSliceError),
     /// Cannot derive silent payment output without input prevout outpoints
     NoOutpoints(crate::LexMinError),
+    /// The number of matched tweaks exceeded the BIP-352 per-group recipient limit
+    ScanLimitExceeded(u32),
 }
 
 impl From<crate::LexMinError> for SpReceiveError {
@@ -41,6 +43,9 @@ impl std::fmt::Display for SpReceiveError {
             SpReceiveError::Secp256k1Error(e) => write!(f, "Silent payment receive error: {e}"),
             SpReceiveError::SliceError(e) => write!(f, "Silent payment receive error: {e}"),
             Self::NoOutpoints(e) => write!(f, "Silent payment sending error: {e}"),
+            SpReceiveError::ScanLimitExceeded(k) => {
+                write!(f, "Silent payment receive error: Scan limit exceeded ({k})")
+            }
         }
     }
 }

silentpayments/src/receive/mod.rs

@@ -93,6 +93,9 @@ pub fn extract_pubkey(txin: TxIn, script_pubkey: &ScriptBuf) -> Option<(SpInputs
     })
 }
 
+/// BIP-352 per-group recipient limit.
+pub const K_MAX: u32 = 2323;
+
 pub fn scan_txouts(
     spend_pk: PublicKey,
     label_lookup: &BTreeMap<PublicKey, (Scalar, u32)>,
@@ -123,6 +126,9 @@ pub fn scan_txouts(
     ) {
         spouts_found.push(spout);
         matched_tweaks += 1;
+        if matched_tweaks > K_MAX {
+            return Err(SpReceiveError::ScanLimitExceeded(K_MAX));
+        }
     }
 
     Ok(spouts_found)

silentpayments/tests/functional_tests/bip352_vectors/receive.rs

@@ -274,7 +274,6 @@ fn input_keys_intermediate_sum_is_zero_but_final_sum_is_non_zero() {
 }
 
 #[test]
-#[ignore = "limit k-max not implemented"]
 fn maximum_per_group_recipient_limit_k_max_is_exceeded() {
     check_cases(27);
 }