SSL_CTX_SET_SESSION_ID_CONTEXT(3)

NAME
SSL_CTX_set_session_id_context, SSL_set_session_id_context - set context within which session can be reused (server side only)

SYNOPSIS
 #include <openssl/ssl.h>

 int SSL_CTX_set_session_id_context(SSL_CTX *ctx, const unsigned char *sid_ctx,
                                    unsigned int sid_ctx_len);
 int SSL_set_session_id_context(SSL *ssl, const unsigned char *sid_ctx,
                                unsigned int sid_ctx_len);

DESCRIPTION
SSL_CTX_set_session_id_context() sets the context sid_ctx of length sid_ctx_len within which a session can be reused for the ctx object.

SSL_set_session_id_context() sets the context sid_ctx of length sid_ctx_len within which a session can be reused for the ssl object.

NOTES
Sessions are generated within a certain context. When exporting/importing sessions with i2d_SSL_SESSION/d2i_SSL_SESSION it would be possible, to re-import a session generated from another context (e.g. another application), which might lead to malfunctions. Therefore, each application must set its own session id context sid_ctx which is used to distinguish the contexts and is stored in exported sessions. The sid_ctx can be any kind of binary data with a given length, it is therefore possible to use e.g. the name of the application and/or the hostname and/or service name ...

The session id context becomes part of the session. The session id context is set by the SSL/TLS server. The SSL_CTX_set_session_id_context() and SSL_set_session_id_context() functions are therefore only useful on the server side.

The maximum length of the sid_ctx is limited to SSL_MAX_SID_CTX_LENGTH.

WARNINGS
If the session id context is not set on an SSL/TLS server and client certificates are used, stored sessions will not be reused but a fatal error will be flagged and the handshake will fail.

If a client attempts to resume a session and the server detects that the session id context associated with the session is different to the current session id context then the resumption will fail. The handshake will continue normally but no resumption will occur.

It is vital that the session id context is set before any session resumption occurs. Sessions get created early in the handshake. If the session id context is not set by the time the session gets created then the session will be associated with an empty session id context. The already created session will not get updated if the session id context is later set. In particular the callback set via the SSL_CTX_set_tlsext_servername_callback(3) function will be invoked after the session gets created, so if the session id context is set in the callback then this will be too late for the current handshake and the session id context setting will be ignored with respect to resumption. Typically the session id context should be set before the TLS handshake starts, but it may occur as late as in the callback set via the SSL_CTX_set_client_hello_cb(3) function.

RETURN VALUES
SSL_CTX_set_session_id_context() and SSL_set_session_id_context() return the following values:

0   The length sid_ctx_len of the session id context sid_ctx exceeded the maximum allowed length of SSL_MAX_SID_CTX_LENGTH. The error is logged to the error stack.

1   The operation succeeded.

SEE ALSO
ssl(7)

COPYRIGHT
Copyright 2001-2026 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.