From 1d13b5d87c6b261179d689f37a706b0308bb9313 Mon Sep 17 00:00:00 2001
From: Steffen Jaeckel <s@jaeckel.eu>
Date: Wed, 29 Jul 2026 09:08:41 +0200
Subject: [PATCH] Check whether the TLS stack supports channel binding.

Add a runtime check per TLS stack which returns whether channel binding is
supported or not.

This fixes #270

Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>
---
 src/auth.c         |  6 ++++--
 src/tls.h          |  1 +
 src/tls_dummy.c    |  6 ++++++
 src/tls_gnutls.c   | 14 ++++++++++++++
 src/tls_openssl.c  | 16 ++++++++++++++++
 src/tls_schannel.c |  6 ++++++
 6 files changed, 47 insertions(+), 2 deletions(-)

diff --git a/src/auth.c b/src/auth.c
index a1bc3b5..c0fbb5d 100644
--- a/src/auth.c
+++ b/src/auth.c
@@ -602,8 +602,10 @@ static int _make_scram_init_msg(struct scram_user_data *scram)
         l = strophe_snprintf(message, message_len, "p=%s,,n=%s,r=%s",
                              binding_type, node, buf);
     } else {
-        l = strophe_snprintf(message, message_len, "%c,,n=%s,r=%s",
-                             is_secured ? 'y' : 'n', node, buf);
+        l = strophe_snprintf(
+            message, message_len, "%c,,n=%s,r=%s",
+            is_secured && tls_supports_channel_binding(conn->tls) ? 'y' : 'n',
+            node, buf);
     }
     if (l < 0 || (size_t)l >= message_len) {
         goto err_msg;
diff --git a/src/tls.h b/src/tls.h
index 79f1e54..ea15b90 100644
--- a/src/tls.h
+++ b/src/tls.h
@@ -45,6 +45,7 @@ unsigned int tls_id_on_xmppaddr_num(xmpp_conn_t *conn);
 
 xmpp_tlscert_t *tls_peer_cert(xmpp_conn_t *conn);
 int tls_set_credentials(tls_t *tls, const char *cafilename);
+int tls_supports_channel_binding(tls_t *tls);
 int tls_init_channel_binding(tls_t *tls,
                              const char **binding_prefix,
                              size_t *binding_prefix_len);
diff --git a/src/tls_dummy.c b/src/tls_dummy.c
index bb5e36d..418f9f9 100644
--- a/src/tls_dummy.c
+++ b/src/tls_dummy.c
@@ -75,6 +75,12 @@ int tls_set_credentials(tls_t *tls, const char *cafilename)
     return -1;
 }
 
+int tls_supports_channel_binding(tls_t *tls)
+{
+    UNUSED(tls);
+    return 0;
+}
+
 int tls_init_channel_binding(tls_t *tls,
                              const char **binding_prefix,
                              size_t *binding_prefix_len)
diff --git a/src/tls_gnutls.c b/src/tls_gnutls.c
index 0aa7c28..9adff5f 100644
--- a/src/tls_gnutls.c
+++ b/src/tls_gnutls.c
@@ -577,6 +577,20 @@ int tls_set_credentials(tls_t *tls, const char *cafilename)
     return err == GNUTLS_E_SUCCESS;
 }
 
+int tls_supports_channel_binding(tls_t *tls)
+{
+    switch (gnutls_protocol_get_version(tls->session)) {
+    case GNUTLS_SSL3:
+    case GNUTLS_TLS1_0:
+    case GNUTLS_TLS1_1:
+    case GNUTLS_TLS1_2:
+    case GNUTLS_TLS1_3:
+        return 1;
+    default:
+        return 0;
+    }
+}
+
 int tls_init_channel_binding(tls_t *tls,
                              const char **binding_prefix,
                              size_t *binding_prefix_len)
diff --git a/src/tls_openssl.c b/src/tls_openssl.c
index 67d2c8a..e23b4cc 100644
--- a/src/tls_openssl.c
+++ b/src/tls_openssl.c
@@ -793,6 +793,22 @@ int tls_set_credentials(tls_t *tls, const char *cafilename)
     return -1;
 }
 
+int tls_supports_channel_binding(tls_t *tls)
+{
+    switch (SSL_version(tls->ssl)) {
+    case SSL3_VERSION:
+    case TLS1_VERSION:
+    case TLS1_1_VERSION:
+    case TLS1_2_VERSION:
+#ifdef TLS1_3_VERSION
+    case TLS1_3_VERSION:
+#endif
+        return 1;
+    default:
+        return 0;
+    }
+}
+
 int tls_init_channel_binding(tls_t *tls,
                              const char **binding_prefix,
                              size_t *binding_prefix_len)
diff --git a/src/tls_schannel.c b/src/tls_schannel.c
index f139085..67cc404 100644
--- a/src/tls_schannel.c
+++ b/src/tls_schannel.c
@@ -237,6 +237,12 @@ int tls_set_credentials(tls_t *tls, const char *cafilename)
     return -1;
 }
 
+int tls_supports_channel_binding(tls_t *tls)
+{
+    UNUSED(tls);
+    return 0;
+}
+
 int tls_init_channel_binding(tls_t *tls,
                              const char **binding_prefix,
                              size_t *binding_prefix_len)