From 8adae56ed932312352090de57b34a454f388d879 Mon Sep 17 00:00:00 2001
From: Jeff Hanson <jshanson7@gmail.com>
Date: Wed, 24 Jun 2026 10:23:56 -0700
Subject: [PATCH 1/2] Omit empty DTLS 1.3 certificate authorities

---
 src/dtls13/message/wrapped.rs | 10 ++++---
 src/dtls13/server.rs          | 54 +++++++++++++++++++----------------
 2 files changed, 35 insertions(+), 29 deletions(-)

diff --git a/src/dtls13/message/wrapped.rs b/src/dtls13/message/wrapped.rs
index 6d39103a..6221d5ba 100644
--- a/src/dtls13/message/wrapped.rs
+++ b/src/dtls13/message/wrapped.rs
@@ -28,13 +28,15 @@ macro_rules! wrapped_slice {
 
                 Ok((input, $name(start..end)))
             }
-
-            pub fn as_slice<'a>(&self, buf: &'a [u8]) -> &'a [u8] {
-                &buf[self.0.clone()]
-            }
         }
     };
 }
 
 wrapped_slice!(Asn1Cert, be_u24, 0);
 wrapped_slice!(DistinguishedName, be_u16, 1);
+
+impl Asn1Cert {
+    pub fn as_slice<'a>(&self, buf: &'a [u8]) -> &'a [u8] {
+        &buf[self.0.clone()]
+    }
+}
diff --git a/src/dtls13/server.rs b/src/dtls13/server.rs
index 36aa5987..2fd1a4b5 100644
--- a/src/dtls13/server.rs
+++ b/src/dtls13/server.rs
@@ -45,7 +45,6 @@ use crate::dtls13::engine::Engine;
 use crate::dtls13::message::Body;
 use crate::dtls13::message::CompressionMethod;
 use crate::dtls13::message::ContentType;
-use crate::dtls13::message::DistinguishedName;
 use crate::dtls13::message::Dtls13CipherSuite;
 use crate::dtls13::message::Extension;
 use crate::dtls13::message::ExtensionType;
@@ -1460,16 +1459,6 @@ fn handshake_create_certificate_request(body: &mut Buf) -> Result<(), Error> {
         extension_data_range: sa_start..sa_end,
     });
 
-    // certificate_authorities extension (empty list)
-    let ca_start = ext_buf.len();
-    let cas: ArrayVec<DistinguishedName, 32> = ArrayVec::new();
-    serialize_certificate_authorities(&cas, &[], &mut ext_buf);
-    let ca_end = ext_buf.len();
-    extensions.push(Extension {
-        extension_type: ExtensionType::CertificateAuthorities,
-        extension_data_range: ca_start..ca_end,
-    });
-
     // Calculate total extensions length
     let mut extensions_len = 0usize;
     for ext in &extensions {
@@ -1486,19 +1475,34 @@ fn handshake_create_certificate_request(body: &mut Buf) -> Result<(), Error> {
     Ok(())
 }
 
-/// Serialize a list of DistinguishedNames for the certificate_authorities extension.
-///
-/// Format: DistinguishedName<3..2^16-1> (outer length + entries)
-fn serialize_certificate_authorities(
-    cas: &ArrayVec<DistinguishedName, 32>,
-    buf: &[u8],
-    output: &mut Buf,
-) {
-    let total_len: usize = cas.iter().map(|dn| 2 + dn.as_slice(buf).len()).sum();
-    output.extend_from_slice(&(total_len as u16).to_be_bytes());
-    for dn in cas {
-        let data = dn.as_slice(buf);
-        output.extend_from_slice(&(data.len() as u16).to_be_bytes());
-        output.extend_from_slice(data);
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn certificate_request_omits_empty_certificate_authorities() {
+        let mut body = Buf::new();
+        handshake_create_certificate_request(&mut body).expect("create CertificateRequest");
+
+        assert_eq!(body[0], 0, "certificate_request_context");
+        let ext_len = u16::from_be_bytes([body[1], body[2]]) as usize;
+        assert_eq!(body.len(), 3 + ext_len);
+
+        let mut extension_types = Vec::new();
+        let mut pos = 3;
+        while pos < body.len() {
+            assert!(pos + 4 <= body.len());
+            let extension_type =
+                ExtensionType::from_u16(u16::from_be_bytes([body[pos], body[pos + 1]]));
+            let extension_data_len = u16::from_be_bytes([body[pos + 2], body[pos + 3]]) as usize;
+            pos += 4;
+            assert!(pos + extension_data_len <= body.len());
+
+            extension_types.push(extension_type);
+            pos += extension_data_len;
+        }
+
+        assert!(extension_types.contains(&ExtensionType::SignatureAlgorithms));
+        assert!(!extension_types.contains(&ExtensionType::CertificateAuthorities));
     }
 }

From b24d07429fdaad90fca6fc8625a73e61aac09d77 Mon Sep 17 00:00:00 2001
From: Martin Algesten <martin@algesten.se>
Date: Fri, 26 Jun 2026 15:59:56 +0200
Subject: [PATCH 2/2] Add changelog entry for certificate authorities fix

---
 CHANGELOG.md | 1 +
 1 file changed, 1 insertion(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 44fda056..aa1d7d04 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,6 @@
 # Unreleased
 
+  * Omit empty DTLS 1.3 CertificateRequest certificate authorities #153
   * Fix DTLS handshake defragmentation across message sequences #151
   * Return `BufferTooSmall` instead of panicking on undersized poll buffers #150
   * Fix DTLS 1.3 RFC 9147 conformance issues #147