net/sock/dtls: allow to set PSK Identity hint

pkg/tinydtls/contrib/sock_dtls.c

@@ -182,8 +182,16 @@ static int _get_psk_info(struct dtls_context_t *ctx, const session_t *session,
     switch (type) {
     case DTLS_PSK_HINT:
         DEBUG("sock_dtls: psk hint request\n");
-        /* Ignored. See https://tools.ietf.org/html/rfc4279#section-5.2 */
-        return 0;
+        /* return a hint to the client if set */
+        c_len = strlen(sock->psk_hint);
+        if (c_len) {
+            c = sock->psk_hint;
+            break;
+        }
+        else {
+            DEBUG("sock_dtls: no hint provided\n");
+            return 0;
+        }
     case DTLS_PSK_IDENTITY:
         DEBUG("sock_dtls: psk id request\n");
         c = credential.params.psk.id.s;
@@ -279,6 +287,7 @@ int sock_dtls_create(sock_dtls_t *sock, sock_udp_t *udp_sock,
 
     sock->udp_sock = udp_sock;
     sock->buffer.data = NULL;
+    sock->psk_hint[0] = '\0';
 #ifdef SOCK_HAS_ASYNC
     sock->async_cb = NULL;
     sock->buf_ctx = NULL;
@@ -296,6 +305,17 @@ int sock_dtls_create(sock_dtls_t *sock, sock_udp_t *udp_sock,
     return 0;
 }
 
+int sock_dtls_set_server_psk_id_hint(sock_dtls_t *sock, const char *hint)
+{
+    assert(sock);
+    if (strlen(hint) > CONFIG_DTLS_PSK_ID_HINT_MAX_SIZE) {
+        DEBUG("sock_dtls: could not set hint due to buffer size\n");
+        return -1;
+    }
+    strcpy(sock->psk_hint, hint);
+    return 0;
+}
+
 sock_udp_t *sock_dtls_get_udp_sock(sock_dtls_t *sock)
 {
     assert(sock);

pkg/tinydtls/include/sock_dtls_types.h

@@ -22,6 +22,7 @@
 #include "dtls.h"
 #include "net/sock/udp.h"
 #include "net/credman.h"
+#include "net/sock/dtls/creds.h"
 #ifdef SOCK_HAS_ASYNC
 #include "net/sock/async/types.h"
 #endif
@@ -76,6 +77,7 @@ struct sock_dtls {
     } buffer;
     credman_tag_t tag;                      /**< Credential tag of a registered
                                                 (D)TLS credential */
+    char psk_hint[CONFIG_DTLS_PSK_ID_HINT_MAX_SIZE]; /**< PSK Identity hint */
     dtls_peer_type role;                    /**< DTLS role of the socket */
 };
 

sys/include/net/sock/dtls/creds.h

@@ -0,0 +1,60 @@
+/*
+ * Copyright (C) 2021 HAW Hamburg
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+/**
+ * @defgroup    net_sock_dtls_creds    DTLS sock credentials API
+ * @ingroup     net_sock_dtls
+ * @brief       Credential handling for DTLS sock
+ * @{
+ *
+ * @file
+ * @brief   DTLS sock definitions
+ *
+ * @author  Leandro Lanzieri <leandro.lanzieri@haw-hamburg.de>
+ */
+
+#ifndef NET_SOCK_DTLS_CREDS_H
+#define NET_SOCK_DTLS_CREDS_H
+
+#include "net/sock/udp.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @ingroup  net_sock_dtls_conf
+ * @{
+ */
+/**
+ * @brief   Default maximum size for the PSK Identity hint string
+ */
+#ifndef CONFIG_DTLS_PSK_ID_HINT_MAX_SIZE
+#define CONFIG_DTLS_PSK_ID_HINT_MAX_SIZE    32
+#endif
+/** @} */
+
+/**
+ * @brief Sets the PSK Identity hint to be sent to clients during handshake.
+ *
+ * This hint is optional. It helps clients to decide which PSK Identity to use.
+ *
+ * @param[in, out] sock The DTLS sock object to set the hint to.
+ * @param[in] hint      PSK Identity hint as NULL-terminated string.
+ *
+ * @retval 0 on success
+ * @retval -1 on error
+ */
+int sock_dtls_set_server_psk_id_hint(sock_dtls_t *sock, const char *hint);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* NET_SOCK_DTLS_CREDS_H */
+/** @} */

sys/net/sock/Kconfig

@@ -44,4 +44,8 @@ config DTLS_HANDSHAKE_BUFSIZE_EXP
         represents the exponent of 2^n, which will be used as the size of the
         buffer. The buffer is used to hold credentials during DTLS handshakes.
 
+config DTLS_PSK_ID_HINT_MAX_SIZE
+    int "Maximum size for a PSK Identity hint string"
+    default 32
+
 endif # KCONFIG_USEMODULE_SOCK_DTLS