Check if invoices contain unknown required features

This adds the final missing BOLT 11 failure test, checking for
unknown required feature flags before accepting an invoice.

Author: TheBlueMatt

lightning-invoice/src/lib.rs

@@ -1052,7 +1052,9 @@ impl Invoice {
 			None if has_payment_secret => Err(SemanticError::InvalidFeatures),
 			None => Ok(()),
 			Some(TaggedField::Features(features)) => {
-				if features.supports_payment_secret() && has_payment_secret {
+				if features.requires_unknown_bits() {
+					Err(SemanticError::InvalidFeatures)
+				} else if features.supports_payment_secret() && has_payment_secret {
 					Ok(())
 				} else if has_payment_secret {
 					Err(SemanticError::InvalidFeatures)

lightning-invoice/tests/ser_de.rs

@@ -154,6 +154,9 @@ fn deserialize() {
 #[test]
 fn test_bolt_invaoid_invoices() {
 	// Tests the BOLT 11 invalid invoice test vectors
+	assert_eq!(Invoice::from_str(
+		"lnbc25m1pvjluezpp5qqqsyqcyq5rqwzqfqqqsyqcyq5rqwzqfqqqsyqcyq5rqwzqfqypqdq5vdhkven9v5sxyetpdeessp5zyg3zyg3zyg3zyg3zyg3zyg3zyg3zyg3zyg3zyg3zyg3zyg3zygs9q4psqqqqqqqqqqqqqqqpqsqq40wa3khl49yue3zsgm26jrepqr2eghqlx86rttutve3ugd05em86nsefzh4pfurpd9ek9w2vp95zxqnfe2u7ckudyahsa52q66tgzcp6t2dyk"
+		), Err(ParseOrSemanticError::SemanticError(SemanticError::InvalidFeatures)));
 	assert_eq!(Invoice::from_str(
 		"lnbc2500u1pvjluezpp5qqqsyqcyq5rqwzqfqqqsyqcyq5rqwzqfqqqsyqcyq5rqwzqfqypqdpquwpc4curk03c9wlrswe78q4eyqc7d8d0xqzpuyk0sg5g70me25alkluzd2x62aysf2pyy8edtjeevuv4p2d5p76r4zkmneet7uvyakky2zr4cusd45tftc9c5fh0nnqpnl2jfll544esqchsrnt"
 		), Err(ParseOrSemanticError::ParseError(ParseError::Bech32Error(bech32::Error::InvalidChecksum))));

lightning/src/ln/features.rs

@@ -548,7 +548,9 @@ impl<T: sealed::Context> Features<T> {
 		&self.flags
 	}
 
-	pub(crate) fn requires_unknown_bits(&self) -> bool {
+	/// Returns true if this `Features` object contains unknown feature flags which are set as
+	/// "required".
+	pub fn requires_unknown_bits(&self) -> bool {
 		// Bitwise AND-ing with all even bits set except for known features will select required
 		// unknown features.
 		let byte_count = T::KNOWN_FEATURE_MASK.len();