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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_cookie.h"
#include "tls/s2n_tls.h"
#include "utils/s2n_random.h"
/*
*= https://tools.ietf.org/rfc/rfc8446#section-4.2.2
*# Cookies serve two primary purposes:
*#
*# - Allowing the server to force the client to demonstrate
*# reachability at their apparent network address (thus providing a
*# measure of DoS protection). This is primarily useful for
*# non-connection-oriented transports (see [RFC6347] for an example
*# of this).
*#
*# - Allowing the server to offload state to the client, thus allowing
*# it to send a HelloRetryRequest without storing any state. The
*# server can do this by storing the hash of the ClientHello in the
*# HelloRetryRequest cookie (protected with some suitable integrity
*# protection algorithm).
*#
*# When sending a HelloRetryRequest, the server MAY provide a "cookie"
*# extension to the client (this is an exception to the usual rule that
*# the only extensions that may be sent are those that appear in the
*# ClientHello).
*
* So our server does not send cookies in production,
* because it doesn't support DTLS and isn't stateless.
*
* However, we will sometimes send cookies for testing.
*/
static bool s2n_server_cookie_should_send(struct s2n_connection *conn)
{
return conn && conn->cookie.size > 0 && s2n_in_unit_test();
}
/*
*= https://tools.ietf.org/rfc/rfc8446#section-4.2.2
*# When sending the new ClientHello, the client MUST copy
*# the contents of the extension received in the HelloRetryRequest into
*# a "cookie" extension in the new ClientHello.
*
* Store the server's cookie for later use in the new ClientHello.
*/
static int s2n_server_cookie_recv(struct s2n_connection *conn, struct s2n_stuffer *extension)
{
POSIX_ENSURE_REF(conn);
/* This extension should only appear on the hello retry request extension list,
* but verify the retry anyway.
*/
POSIX_ENSURE(s2n_is_hello_retry_handshake(conn), S2N_ERR_UNSUPPORTED_EXTENSION);
uint16_t size = 0;
POSIX_GUARD(s2n_stuffer_read_uint16(extension, &size));
POSIX_ENSURE(s2n_stuffer_data_available(extension) >= size, S2N_ERR_BAD_MESSAGE);
POSIX_GUARD(s2n_realloc(&conn->cookie, size));
POSIX_GUARD(s2n_stuffer_read(extension, &conn->cookie));
return S2N_SUCCESS;
}
const s2n_extension_type s2n_server_cookie_extension = {
.iana_value = TLS_EXTENSION_COOKIE,
.minimum_version = S2N_TLS13,
.is_response = false,
.send = s2n_cookie_send,
.recv = s2n_server_cookie_recv,
.should_send = s2n_server_cookie_should_send,
.if_missing = s2n_extension_noop_if_missing,
};
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