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SSL_CTX_SET_SESSION_ID_CONTEXT(3) Library Functions Manual SSL_CTX_SET_SESSION_ID_CONTEXT(3)

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

#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);

TABLE OF CONTENTS

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.

Sessions are generated within a certain context. When exporting/importing sessions with i2d_SSL_SESSION(3) and d2i_SSL_SESSION(3), 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, for instance, the name of the application, the hostname, the 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.
OpenSSL clients will check the session id context returned by the server when reusing a session.
The maximum length of the sid_ctx is limited to SSL_MAX_SSL_SESSION_ID_LENGTH.

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 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, as an OpenSSL server checks the session id context itself before reusing a session as described above.

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_SSL_SESSION_ID_LENGTH. The error is logged to the error stack.
 
 
1
The operation succeeded.

ssl(3), SSL_SESSION_set1_id_context(3)

SSL_set_session_id_context() first appeared in OpenSSL 0.9.2b. SSL_CTX_set_session_id_context() first appeared in OpenSSL 0.9.3. Both functions have been available since OpenBSD 2.6.
March 21, 2018 OpenBSD-current