SSL_CTX_SET_...N_ID_CONTEXT(3ossl)

💅 SSL_CTX_SET_...N_ID_CONTEXT 💅

okay so bestie, this is an OpenSSL manpage — the 3ossl one — and it's about setting the session id context, which is the little name-tag that says "this session belongs to ME" 🎀. Heads up: it's server side only, so if you're writing a client, this is not your moment (still cute of you to read it tho 🫶).

✨ NAME ✨

SSL_CTX_set_session_id_context, SSL_set_session_id_context — set the context within which a session can be reused. Server side only, no cap. 👑

🌸 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. it's giving "whole-context energy" ✨
• 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. same vibe, but for one single ssl 💫

🧚 NOTES 🧚

So here's the lore, bestie: sessions are generated within a certain context. and like, when you're exporting/importing sessions with i2d_SSL_SESSION/d2i_SSL_SESSION, it would be possible to re-import a session that was generated from a different context (e.g. another application), which might lead to malfunctions 😭 — not the cute kind of chaos.

So. Every application must set its own session id context sid_ctx, which is used to distinguish the contexts and is stored in exported sessions. literally the whole point. 🫶

The sid_ctx can be any kind of binary data with a given length, so it's totally possible to use e.g. the name of the application and/or the hostname and/or service name ... 🌈 (yes, the "..." is in the original, iconic of it frankly).

The session id context becomes part of the session. it's set by the SSL/TLS server. so SSL_CTX_set_session_id_context() and SSL_set_session_id_context() are therefore only useful on the server side. periodt. 👑

OpenSSL clients will check the session id context returned by the server when reusing a session — trust but verify, iykyk 💅.

And the maximum length of the sid_ctx is limited to SSL_MAX_SID_CTX_LENGTH. don't get greedy with it 🍓.

💀 WARNINGS 💀

okay this is the serious part, listen up 🫶

If the session id context is not set on an SSL/TLS server and client certificates are used, stored sessions will not be reused — instead a fatal error will be flagged and the handshake will fail. like, full send into the wall. 💀

Also: if a server returns a different session id context to an OpenSSL client when reusing a session, an error will be flagged and the handshake will fail. OpenSSL servers will always return the correct session id context, because an OpenSSL server checks the session id context itself before reusing a session as described above. so the servers are behaving, bestie — it's the impostors you gotta watch for 🙈.

⭐ RETURN VALUES ⭐

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

1. 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. (too long, too iconic, too much 💀)
2. 1 — the operation succeeded. we love a win 🎉👑

🔥 SEE ALSO 🔥

• ssl(7)

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🎀 COPYRIGHT 🎀

Copyright 2001-2020 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>.

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k that's the whole page bestie, go set your sid_ctx and slay 💖✨

signing off — OpenSSL 3.5.6, dated 2026-04-07, manual page 3ossl. mwah 🫶💜