Compress the CA extension

For now - we should decide what to do when ECH is used.

Bug: 399937371
Change-Id: Ifb094ba3a19e936c832832dbe818fd9765045590
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/76867
Auto-Submit: Bob Beck <bbe@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

ssl/extensions.cc

diff --git a/ssl/extensions.cc b/ssl/extensions.cc
index 4bd5c5d..bf609f1 100644
--- a/ssl/extensions.cc
+++ b/ssl/extensions.cc
@@ -2510,12 +2510,14 @@
 static bool ext_certificate_authorities_add_clienthello(
     const SSL_HANDSHAKE *hs, CBB *out, CBB *out_compressible,
     ssl_client_hello_type_t type) {
+  // TODO(crbug.com/399937371) Decide what to do with this for ECH.
   if (ssl_has_CA_names(hs->config)) {
     CBB ca_contents;
-    if (!CBB_add_u16(out, TLSEXT_TYPE_certificate_authorities) ||  //
-        !CBB_add_u16_length_prefixed(out, &ca_contents) ||         //
-        !ssl_add_CA_names(hs, &ca_contents) ||                     //
-        !CBB_flush(out)) {
+    if (!CBB_add_u16(out_compressible,
+                     TLSEXT_TYPE_certificate_authorities) ||  //
+        !CBB_add_u16_length_prefixed(out_compressible, &ca_contents) ||    //
+        !ssl_add_CA_names(hs, &ca_contents) ||                //
+        !CBB_flush(out_compressible)) {
       return false;
     }
   }