path: root/contrib/restricted/aws/s2n/tls/s2n_server_hello.c

/*
 * 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 <sys/param.h>

#include <s2n.h>
#include <time.h>

#include "crypto/s2n_fips.h"

#include "error/s2n_errno.h"

#include "tls/s2n_cipher_preferences.h"
#include "tls/s2n_cipher_suites.h"
#include "tls/s2n_connection.h"
#include "tls/s2n_alerts.h"
#include "tls/s2n_server_extensions.h"
#include "tls/s2n_tls.h"
#include "tls/s2n_tls13.h"
#include "tls/s2n_security_policies.h"
#include "tls/s2n_tls13_handshake.h"

#include "stuffer/s2n_stuffer.h"

#include "utils/s2n_safety.h"
#include "utils/s2n_random.h"

/* From RFC5246 7.4.1.2. */
#define S2N_TLS_COMPRESSION_METHOD_NULL 0

/* From RFC8446 4.1.3. */
#define S2N_DOWNGRADE_PROTECTION_SIZE   8
const uint8_t tls12_downgrade_protection_bytes[] = {
    0x44, 0x4F, 0x57, 0x4E, 0x47, 0x52, 0x44, 0x01
};

const uint8_t tls11_downgrade_protection_bytes[] = {
    0x44, 0x4F, 0x57, 0x4E, 0x47, 0x52, 0x44, 0x00
};

static int s2n_hello_retry_validate(struct s2n_connection *conn) {
    notnull_check(conn);

    ENSURE_POSIX(memcmp(hello_retry_req_random, conn->secure.server_random, S2N_TLS_RANDOM_DATA_LEN) == 0,
                 S2N_ERR_INVALID_HELLO_RETRY);

    return S2N_SUCCESS;
}

static int s2n_client_detect_downgrade_mechanism(struct s2n_connection *conn) {
    notnull_check(conn);
    uint8_t *downgrade_bytes = &conn->secure.server_random[S2N_TLS_RANDOM_DATA_LEN - S2N_DOWNGRADE_PROTECTION_SIZE];

    /* Detect downgrade attacks according to RFC 8446 section 4.1.3 */
    if (conn->client_protocol_version == S2N_TLS13 && conn->server_protocol_version == S2N_TLS12) {
        if (s2n_constant_time_equals(downgrade_bytes, tls12_downgrade_protection_bytes, S2N_DOWNGRADE_PROTECTION_SIZE)) {
            S2N_ERROR(S2N_ERR_PROTOCOL_DOWNGRADE_DETECTED);
        }
    } else if (conn->client_protocol_version == S2N_TLS13 && conn->server_protocol_version <= S2N_TLS11) {
        if (s2n_constant_time_equals(downgrade_bytes, tls11_downgrade_protection_bytes, S2N_DOWNGRADE_PROTECTION_SIZE)) {
            S2N_ERROR(S2N_ERR_PROTOCOL_DOWNGRADE_DETECTED);
        }
    }

    return 0;
}

static int s2n_server_add_downgrade_mechanism(struct s2n_connection *conn) {
    notnull_check(conn);
    uint8_t *downgrade_bytes = &conn->secure.server_random[S2N_TLS_RANDOM_DATA_LEN - S2N_DOWNGRADE_PROTECTION_SIZE];

    /* Protect against downgrade attacks according to RFC 8446 section 4.1.3 */
    if (conn->server_protocol_version >= S2N_TLS13 && conn->actual_protocol_version == S2N_TLS12) {
        /* TLS1.3 servers MUST use a special random value when negotiating TLS1.2 */
        memcpy_check(downgrade_bytes, tls12_downgrade_protection_bytes, S2N_DOWNGRADE_PROTECTION_SIZE);
    } else if (conn->server_protocol_version >= S2N_TLS13 && conn->actual_protocol_version <= S2N_TLS11) {
        /* TLS1.3 servers MUST, use a special random value when negotiating TLS1.1 or below */
        memcpy_check(downgrade_bytes, tls11_downgrade_protection_bytes, S2N_DOWNGRADE_PROTECTION_SIZE);
    }

    return 0;
}

static int s2n_server_hello_parse(struct s2n_connection *conn)
{
    notnull_check(conn);

    struct s2n_stuffer *in = &conn->handshake.io;
    uint8_t compression_method;
    uint8_t session_id_len;
    uint8_t protocol_version[S2N_TLS_PROTOCOL_VERSION_LEN];
    uint8_t session_id[S2N_TLS_SESSION_ID_MAX_LEN];

    GUARD(s2n_stuffer_read_bytes(in, protocol_version, S2N_TLS_PROTOCOL_VERSION_LEN));
    GUARD(s2n_stuffer_read_bytes(in, conn->secure.server_random, S2N_TLS_RANDOM_DATA_LEN));

    /* If the client receives a second HelloRetryRequest in the same connection, it MUST send an error. */
    if (s2n_hello_retry_validate(conn) == S2N_SUCCESS) {
        ENSURE_POSIX(!s2n_is_hello_retry_handshake(conn), S2N_ERR_INVALID_HELLO_RETRY);
        GUARD(s2n_set_hello_retry_required(conn));
    }

    GUARD(s2n_stuffer_read_uint8(in, &session_id_len));
    S2N_ERROR_IF(session_id_len > S2N_TLS_SESSION_ID_MAX_LEN, S2N_ERR_BAD_MESSAGE);
    GUARD(s2n_stuffer_read_bytes(in, session_id, session_id_len));

    uint8_t *cipher_suite_wire = s2n_stuffer_raw_read(in, S2N_TLS_CIPHER_SUITE_LEN);
    notnull_check(cipher_suite_wire);

    GUARD(s2n_stuffer_read_uint8(in, &compression_method));
    S2N_ERROR_IF(compression_method != S2N_TLS_COMPRESSION_METHOD_NULL, S2N_ERR_BAD_MESSAGE);

    GUARD(s2n_server_extensions_recv(conn, in));

    if (conn->server_protocol_version >= S2N_TLS13) {
        S2N_ERROR_IF(session_id_len != conn->session_id_len || memcmp(session_id, conn->session_id, session_id_len), S2N_ERR_BAD_MESSAGE);
        conn->actual_protocol_version = conn->server_protocol_version;
        GUARD(s2n_set_cipher_as_client(conn, cipher_suite_wire));
    } else {
        conn->server_protocol_version = (uint8_t)(protocol_version[0] * 10) + protocol_version[1];

        S2N_ERROR_IF(s2n_client_detect_downgrade_mechanism(conn), S2N_ERR_PROTOCOL_DOWNGRADE_DETECTED);
        ENSURE_POSIX(!conn->config->quic_enabled, S2N_ERR_PROTOCOL_VERSION_UNSUPPORTED);

        const struct s2n_security_policy *security_policy;
        GUARD(s2n_connection_get_security_policy(conn, &security_policy));

        if (conn->server_protocol_version < security_policy->minimum_protocol_version
                || conn->server_protocol_version > conn->client_protocol_version) {
            GUARD(s2n_queue_reader_unsupported_protocol_version_alert(conn));
            S2N_ERROR(S2N_ERR_PROTOCOL_VERSION_UNSUPPORTED);
        }

        uint8_t actual_protocol_version = MIN(conn->server_protocol_version, conn->client_protocol_version);
        /* Use the session state if server sent same session id as client sent in client hello */
        if (session_id_len != 0  && session_id_len == conn->session_id_len
                && !memcmp(session_id, conn->session_id, session_id_len)) {
            /* check if the resumed session state is valid */
            S2N_ERROR_IF(conn->actual_protocol_version != actual_protocol_version, S2N_ERR_BAD_MESSAGE);
            S2N_ERROR_IF(memcmp(conn->secure.cipher_suite->iana_value, cipher_suite_wire, S2N_TLS_CIPHER_SUITE_LEN) != 0, S2N_ERR_BAD_MESSAGE);

            /* Session is resumed */
            conn->client_session_resumed = 1;
        } else {
            conn->session_id_len = session_id_len;
            memcpy_check(conn->session_id, session_id, session_id_len);
            conn->actual_protocol_version = actual_protocol_version;
            GUARD(s2n_set_cipher_as_client(conn, cipher_suite_wire));
            /* Erase master secret which might have been set for session resumption */
            memset_check((uint8_t *)conn->secure.master_secret, 0, S2N_TLS_SECRET_LEN);

            /* Erase client session ticket which might have been set for session resumption */
            GUARD(s2n_free(&conn->client_ticket));
        }
    }

    return 0;
}

int s2n_server_hello_recv(struct s2n_connection *conn)
{
    notnull_check(conn);

    /* Read the message off the wire */
    GUARD(s2n_server_hello_parse(conn));

    conn->actual_protocol_version_established = 1;

    GUARD(s2n_conn_set_handshake_type(conn));

    /* If this is a HelloRetryRequest, we don't process the ServerHello.
     * Instead we proceed with retry logic. */
    if (s2n_is_hello_retry_message(conn)) {
        GUARD(s2n_server_hello_retry_recv(conn));
        return 0;
    }

    if (IS_RESUMPTION_HANDSHAKE(conn->handshake.handshake_type)) {
        GUARD(s2n_prf_key_expansion(conn));
    }

    /* Choose a default signature scheme */
    GUARD(s2n_choose_default_sig_scheme(conn, &conn->secure.conn_sig_scheme));

    /* Update the required hashes for this connection */
    GUARD(s2n_conn_update_required_handshake_hashes(conn));

    return 0;
}

int s2n_server_hello_write_message(struct s2n_connection *conn)
{
    notnull_check(conn);

    /* The actual_protocol_version is set while processing the CLIENT_HELLO message, so
     * it could be S2N_TLS13. SERVER_HELLO should always respond with the legacy version.
     * https://tools.ietf.org/html/rfc8446#section-4.1.3 */
    const uint16_t legacy_protocol_version = MIN(conn->actual_protocol_version, S2N_TLS12);
    uint8_t protocol_version[S2N_TLS_PROTOCOL_VERSION_LEN];
    protocol_version[0] = (uint8_t)(legacy_protocol_version / 10);
    protocol_version[1] = (uint8_t)(legacy_protocol_version % 10);

    GUARD(s2n_stuffer_write_bytes(&conn->handshake.io, protocol_version, S2N_TLS_PROTOCOL_VERSION_LEN));
    GUARD(s2n_stuffer_write_bytes(&conn->handshake.io, conn->secure.server_random, S2N_TLS_RANDOM_DATA_LEN));
    GUARD(s2n_stuffer_write_uint8(&conn->handshake.io, conn->session_id_len));
    GUARD(s2n_stuffer_write_bytes(&conn->handshake.io, conn->session_id, conn->session_id_len));
    GUARD(s2n_stuffer_write_bytes(&conn->handshake.io, conn->secure.cipher_suite->iana_value, S2N_TLS_CIPHER_SUITE_LEN));
    GUARD(s2n_stuffer_write_uint8(&conn->handshake.io, S2N_TLS_COMPRESSION_METHOD_NULL));

    return 0;
}

int s2n_server_hello_send(struct s2n_connection *conn)
{
    notnull_check(conn);

    struct s2n_stuffer server_random = {0};
    struct s2n_blob b = {0};
    GUARD(s2n_blob_init(&b, conn->secure.server_random, S2N_TLS_RANDOM_DATA_LEN));

    /* Create the server random data */
    GUARD(s2n_stuffer_init(&server_random, &b));

    struct s2n_blob rand_data = {0};
    GUARD(s2n_blob_init(&rand_data, s2n_stuffer_raw_write(&server_random, S2N_TLS_RANDOM_DATA_LEN), S2N_TLS_RANDOM_DATA_LEN));
    notnull_check(rand_data.data);
    GUARD_AS_POSIX(s2n_get_public_random_data(&rand_data));

    /* Add a downgrade detection mechanism if required */
    GUARD(s2n_server_add_downgrade_mechanism(conn));

    GUARD(s2n_server_hello_write_message(conn));

    GUARD(s2n_server_extensions_send(conn, &conn->handshake.io));

    conn->actual_protocol_version_established = 1;

    return 0;
}