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/*
* Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
*
* Licensed under the Apache License, Version 2.0 (the "License").
* You may not use this file except in compliance with the License.
* A copy of the License is located at
*
* http://aws.amazon.com/apache2.0
*
* or in the "license" file accompanying this file. This file is distributed
* on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
* express or implied. See the License for the specific language governing
* permissions and limitations under the License.
*/
#include "tls/extensions/s2n_psk_key_exchange_modes.h"
#include <stdint.h>
#include <sys/param.h>
#include "tls/extensions/s2n_client_psk.h"
#include "tls/s2n_tls_parameters.h"
#include "utils/s2n_safety.h"
static bool s2n_psk_key_exchange_modes_should_send(struct s2n_connection *conn);
static int s2n_psk_key_exchange_modes_send(struct s2n_connection *conn, struct s2n_stuffer *out);
static int s2n_psk_key_exchange_modes_recv(struct s2n_connection *conn, struct s2n_stuffer *extension);
const s2n_extension_type s2n_psk_key_exchange_modes_extension = {
.iana_value = TLS_EXTENSION_PSK_KEY_EXCHANGE_MODES,
.minimum_version = S2N_TLS13,
.is_response = false,
.send = s2n_psk_key_exchange_modes_send,
.recv = s2n_psk_key_exchange_modes_recv,
.should_send = s2n_psk_key_exchange_modes_should_send,
.if_missing = s2n_extension_noop_if_missing,
};
static bool s2n_psk_key_exchange_modes_should_send(struct s2n_connection *conn)
{
/**
*= https://tools.ietf.org/rfc/rfc8446#section-4.2.9
*# Servers MUST NOT select a key exchange mode that is not listed by the
*# client. This extension also restricts the modes for use with PSK
*# resumption.
*
* The RFC is ambiguous about whether the psk_kx_modes extension should be
* sent in the first flight of messages, but we choose to do so for
* interoperability with other TLS implementations.
* https://github.com/aws/s2n-tls/issues/4124
* The final check for psk_list.len > 0 is necessary because external
* PSKs are used without setting `use_tickets` to true.
*/
return conn->config->use_tickets || conn->psk_params.psk_list.len > 0;
}
static int s2n_psk_key_exchange_modes_send(struct s2n_connection *conn, struct s2n_stuffer *out)
{
POSIX_ENSURE_REF(conn);
POSIX_GUARD(s2n_stuffer_write_uint8(out, PSK_KEY_EXCHANGE_MODE_SIZE));
/* s2n currently only supports pre-shared keys with (EC)DHE key establishment */
POSIX_GUARD(s2n_stuffer_write_uint8(out, TLS_PSK_DHE_KE_MODE));
return S2N_SUCCESS;
}
static int s2n_psk_key_exchange_modes_recv(struct s2n_connection *conn, struct s2n_stuffer *extension)
{
POSIX_ENSURE_REF(conn);
uint8_t psk_ke_mode_list_len;
POSIX_GUARD(s2n_stuffer_read_uint8(extension, &psk_ke_mode_list_len));
if (psk_ke_mode_list_len > s2n_stuffer_data_available(extension)) {
/* Malformed length, ignore the extension */
return S2N_SUCCESS;
}
for (size_t i = 0; i < psk_ke_mode_list_len; i++) {
uint8_t wire_psk_ke_mode;
POSIX_GUARD(s2n_stuffer_read_uint8(extension, &wire_psk_ke_mode));
/* s2n currently only supports pre-shared keys with (EC)DHE key establishment */
if (wire_psk_ke_mode == TLS_PSK_DHE_KE_MODE) {
conn->psk_params.psk_ke_mode = S2N_PSK_DHE_KE;
return S2N_SUCCESS;
}
}
return S2N_SUCCESS;
}
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