Enhance HumanReadableName validation

lightning/src/onion_message/dns_resolution.rs

@@ -191,8 +191,8 @@ const REQUIRED_EXTRA_LEN: usize = ".user._bitcoin-payment.".len() + 1;
 
 /// A struct containing the two parts of a BIP 353 Human Readable Name - the user and domain parts.
 ///
-/// The `user` and `domain` parts, together, cannot exceed 231 bytes in length, and both must be
-/// non-empty.
+/// The `user` and `domain` parts combined cannot exceed 231 bytes in length;
+/// each DNS label within them must be non-empty and no longer than 63 bytes.
 ///
 /// If you intend to handle non-ASCII `user` or `domain` parts, you must handle [Homograph Attacks]
 /// and do punycode en-/de-coding yourself. This struct will always handle only plain ASCII `user`
@@ -211,16 +211,21 @@ pub struct HumanReadableName {
 impl HumanReadableName {
 	/// Constructs a new [`HumanReadableName`] from the `user` and `domain` parts. See the
 	/// struct-level documentation for more on the requirements on each.
-	pub fn new(user: &str, mut domain: &str) -> Result<HumanReadableName, ()> {
+	pub fn new(user: &str, domain: &str) -> Result<HumanReadableName, ()> {
 		// First normalize domain and remove the optional trailing `.`
-		if domain.ends_with('.') {
-			domain = &domain[..domain.len() - 1];
-		}
+		let domain = domain.strip_suffix(".").unwrap_or(domain);
 		if user.len() + domain.len() + REQUIRED_EXTRA_LEN > 255 {
 			return Err(());
 		}
-		if user.is_empty() || domain.is_empty() {
-			return Err(());
+		for label in user.split('.') {
+			if label.is_empty() || label.len() > 63 {
+				return Err(());
+			}
+		}
+		for label in domain.split('.') {
+			if label.is_empty() || label.len() > 63 {
+				return Err(());
+			}
 		}
 		if !Hostname::str_is_valid_hostname(&user) || !Hostname::str_is_valid_hostname(&domain) {
 			return Err(());
@@ -558,6 +563,28 @@ mod tests {
 		);
 	}
 
+	#[test]
+	fn test_hrn_validation() {
+		assert!(HumanReadableName::new("user", "example.com").is_ok());
+		assert!(HumanReadableName::new("user", "example.com.").is_ok());
+
+		assert!(HumanReadableName::new("user!", "example.com").is_err());
+		assert!(HumanReadableName::new("user.", "example.com").is_err());
+		assert!(HumanReadableName::new("user", "exa mple.com").is_err());
+		assert!(HumanReadableName::new("", "example.com").is_err());
+		assert!(HumanReadableName::new("user", "").is_err());
+
+		let max_label = "a".repeat(63);
+		assert!(HumanReadableName::new(&max_label, "example.com").is_ok());
+
+		let long_label = "a".repeat(64);
+		assert!(HumanReadableName::new(&long_label, "example.com").is_err());
+		let domain_with_long_label = format!("{long_label}.com");
+		assert!(HumanReadableName::new("user", &domain_with_long_label).is_err());
+		let huge_domain = format!("{max_label}.{max_label}.{max_label}.{max_label}");
+		assert!(HumanReadableName::new("user", &huge_domain).is_err());
+	}
+
 	#[test]
 	#[cfg(feature = "dnssec")]
 	fn test_expiry() {