1 files changed, 49 insertions, 2 deletions

diff --git a/src/lib/libssl/ssl_tlsext.c b/src/lib/libssl/ssl_tlsext.c
index dcd9a31205..d879b3304e 100644
--- a/src/lib/libssl/ssl_tlsext.c
+++ b/src/lib/libssl/ssl_tlsext.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ssl_tlsext.c,v 1.158 2025/12/04 21:03:42 beck Exp $ */
+/* $OpenBSD: ssl_tlsext.c,v 1.159 2025/12/04 21:16:17 beck Exp $ */
 /*
  * Copyright (c) 2016, 2017, 2019 Joel Sing <jsing@openbsd.org>
  * Copyright (c) 2017 Doug Hogan <doug@openbsd.org>
@@ -1445,7 +1445,7 @@ tlsext_keyshare_client_needs(SSL *s, uint16_t msg_type)
 static int
 tlsext_keyshare_client_build(SSL *s, uint16_t msg_type, CBB *cbb)
 {
-        CBB client_shares, key_exchange;
+        CBB client_shares, key_exchange, key_exchange2;
 
         if (!CBB_add_u16_length_prefixed(cbb, &client_shares))
                 return 0;
@@ -1458,6 +1458,31 @@ tlsext_keyshare_client_build(SSL *s, uint16_t msg_type, CBB *cbb)
         if (!tls_key_share_public(s->s3->hs.key_share, &key_exchange))
                 return 0;
 
+        /*
+         * We wish to include a second key share prediction in a TLS 1.3 client
+         * hello if we have more than one preferred group. We never wish to do
+         * this in response to a server selected group (Either from a TLS 1.2
+         * server, or from a hello retry request after having negotiated TLS
+         * 1.3).
+         *
+         * Therefore we only do this if we have not yet negotiated
+         * a version, and our max version could negotiate TLS 1.3.
+         */
+        if (s->s3->hs.negotiated_tls_version == 0 &&
+            s->s3->hs.our_max_tls_version >= TLS1_3_VERSION) {
+                if (s->s3->hs.tls13.key_share != NULL) {
+                        if (!CBB_add_u16(&client_shares,
+                            tls_key_share_group(s->s3->hs.tls13.key_share)))
+                                return 0;
+                        if (!CBB_add_u16_length_prefixed(&client_shares,
+                            &key_exchange2))
+                                return 0;
+                        if (!tls_key_share_public(s->s3->hs.tls13.key_share,
+                            &key_exchange2))
+                                return 0;
+                }
+        }
+
         if (!CBB_flush(cbb))
                 return 0;
 
@@ -1687,10 +1712,32 @@ tlsext_keyshare_client_process(SSL *s, uint16_t msg_type, CBS *cbs, int *alert)
                 *alert = SSL_AD_INTERNAL_ERROR;
                 return 0;
         }
+
+        if (s->s3->hs.tls13.server_version >= TLS1_3_VERSION &&
+            tls_key_share_group(s->s3->hs.key_share) != group &&
+            s->s3->hs.tls13.key_share != NULL &&
+            tls_key_share_group(s->s3->hs.tls13.key_share) == group) {
+                /*
+                 * Server chose our second key share prediction, switch to it,
+                 * and discard the first one.
+                 */
+                tls_key_share_free(s->s3->hs.key_share);
+                s->s3->hs.key_share = s->s3->hs.tls13.key_share;
+                s->s3->hs.tls13.key_share = NULL;
+        }
+
         if (tls_key_share_group(s->s3->hs.key_share) != group) {
                 *alert = SSL_AD_INTERNAL_ERROR;
                 return 0;
         }
+
+        /*
+         * Discard our now unused second key share prediction if we had made one
+         * with our initial 1.3 client hello
+         */
+        tls_key_share_free(s->s3->hs.tls13.key_share);
+        s->s3->hs.tls13.key_share = NULL;
+
         if (!tls_key_share_client_peer_public(s->s3->hs.key_share,
             &key_exchange, &decode_error, NULL)) {
                 if (!decode_error)