SSL_CTX_SET_SESSION_TICKET_CB

Section: OpenSSL (3)
Updated: 2020-07-28
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NAME

SSL_CTX_set_session_ticket_cb, SSL_SESSION_get0_ticket_appdata, SSL_SESSION_set1_ticket_appdata, SSL_CTX_generate_session_ticket_fn, SSL_CTX_decrypt_session_ticket_fn - manage session ticket application data

SYNOPSIS

 #include <openssl/ssl.h>

 typedef int (*SSL_CTX_generate_session_ticket_fn)(SSL *s, void *arg);
 typedef SSL_TICKET_RETURN (*SSL_CTX_decrypt_session_ticket_fn)(SSL *s, SSL_SESSION *ss,
                                                                const unsigned char *keyname,
                                                                size_t keyname_len,
                                                                SSL_TICKET_STATUS status,
                                                                void *arg);
 int SSL_CTX_set_session_ticket_cb(SSL_CTX *ctx,
                                   SSL_CTX_generate_session_ticket_fn gen_cb,
                                   SSL_CTX_decrypt_session_ticket_fn dec_cb,
                                   void *arg);
 int SSL_SESSION_set1_ticket_appdata(SSL_SESSION *ss, const void *data, size_t len);
 int SSL_SESSION_get0_ticket_appdata(SSL_SESSION *ss, void **data, size_t *len);

DESCRIPTION

SSL_CTX_set_set_session_ticket_cb() sets the application callbacks gen_cb and dec_cb that are used by a server to set and get application data stored with a session, and placed into a session ticket. Either callback function may be set to NULL. The value of arg is passed to the callbacks.

gen_cb is the application defined callback invoked when a session ticket is about to be created. The application can call SSL_SESSION_set1_ticket_appdata() at this time to add application data to the session ticket. The value of arg is the same as that given to SSL_CTX_set_session_ticket_cb(). The gen_cb callback is defined as type SSL_CTX_generate_session_ticket_fn.

dec_cb is the application defined callback invoked after session ticket decryption has been attempted and any session ticket application data is available. If ticket decryption was successful then the ss argument contains the session data. The keyname and keyname_len arguments identify the key used to decrypt the session ticket. The status argument is the result of the ticket decryption. See the NOTES section below for further details. The value of arg is the same as that given to SSL_CTX_set_session_ticket_cb(). The dec_cb callback is defined as type SSL_CTX_decrypt_session_ticket_fn.

SSL_SESSION_set1_ticket_appdata() sets the application data specified by data and len into ss which is then placed into any generated session tickets. It can be called at any time before a session ticket is created to update the data placed into the session ticket. However, given that sessions and tickets are created by the handshake, the gen_cb is provided to notify the application that a session ticket is about to be generated.

SSL_SESSION_get0_ticket_appdata() assigns data to the session ticket application data and assigns len to the length of the session ticket application data from ss. The application data can be set via SSL_SESSION_set1_ticket_appdata() or by a session ticket. NULL will be assigned to data and 0 will be assigned to len if there is no session ticket application data. SSL_SESSION_get0_ticket_appdata() can be called any time after a session has been created. The dec_cb is provided to notify the application that a session ticket has just been decrypted.

NOTES

When the dec_cb callback is invoked, the SSL_SESSION ss has not yet been assigned to the SSL s. The status indicates the result of the ticket decryption. The callback must check the status value before performing any action, as it is called even if ticket decryption fails.

The keyname and keyname_len arguments to dec_cb may be used to identify the key that was used to encrypt the session ticket.

The status argument can be any of these values:

SSL_TICKET_EMPTY: Empty ticket present. No ticket data will be used and a new ticket should be sent to the client. This only occurs in TLSv1.2 or below. In TLSv1.3 it is not valid for a client to send an empty ticket.
SSL_TICKET_NO_DECRYPT: The ticket couldn't be decrypted. No ticket data will be used and a new ticket should be sent to the client.
SSL_TICKET_SUCCESS: A ticket was successfully decrypted, any session ticket application data should be available. A new ticket should not be sent to the client.
SSL_TICKET_SUCCESS_RENEW: Same as SSL_TICKET_SUCCESS, but a new ticket should be sent to the client.

The return value can be any of these values:

SSL_TICKET_RETURN_ABORT: The handshake should be aborted, either because of an error or because of some policy. Note that in TLSv1.3 a client may send more than one ticket in a single handshake. Therefore just because one ticket is unacceptable it does not mean that all of them are. For this reason this option should be used with caution.
SSL_TICKET_RETURN_IGNORE: Do not use a ticket (if one was available). Do not send a renewed ticket to the client.
SSL_TICKET_RETURN_IGNORE_RENEW: Do not use a ticket (if one was available). Send a renewed ticket to the client.
If the callback does not wish to change the default ticket behaviour then it should return this value if status is SSL_TICKET_EMPTY or SSL_TICKET_NO_DECRYPT.
SSL_TICKET_RETURN_USE: Use the ticket. Do not send a renewed ticket to the client. It is an error for the callback to return this value if status has a value other than SSL_TICKET_SUCCESS or SSL_TICKET_SUCCESS_RENEW.
If the callback does not wish to change the default ticket behaviour then it should return this value if status is SSL_TICKET_SUCCESS.
SSL_TICKET_RETURN_USE_RENEW: Use the ticket. Send a renewed ticket to the client. It is an error for the callback to return this value if status has a value other than SSL_TICKET_SUCCESS or SSL_TICKET_SUCCESS_RENEW.
If the callback does not wish to change the default ticket behaviour then it should return this value if status is SSL_TICKET_SUCCESS_RENEW.

If status has the value SSL_TICKET_EMPTY or SSL_TICKET_NO_DECRYPT then no session data will be available and the callback must not use the ss argument. If status has the value SSL_TICKET_SUCCESS or SSL_TICKET_SUCCESS_RENEW then the application can call SSL_SESSION_get0_ticket_appdata() using the session provided in the ss argument to retrieve the application data.

When the gen_cb callback is invoked, the SSL_get_session() function can be used to retrieve the SSL_SESSION for SSL_SESSION_set1_ticket_appdata().

By default, in TLSv1.2 and below, a new session ticket is not issued on a successful resumption and therefore gen_cb will not be called. In TLSv1.3 the default behaviour is to always issue a new ticket on resumption. In both cases this behaviour can be changed if a ticket key callback is in use (see SSL_CTX_set_tlsext_ticket_key_cb(3)).

RETURN VALUES

The SSL_CTX_set_session_ticket_cb(), SSL_SESSION_set1_ticket_appdata() and SSL_SESSION_get0_ticket_appdata() functions return 1 on success and 0 on failure.

The gen_cb callback must return 1 to continue the connection. A return of 0 will terminate the connection with an INTERNAL_ERROR alert.

The dec_cb callback must return a value as described in NOTES above.

HISTORY

The SSL_CTX_set_session_ticket_cb(), SSSL_SESSION_set1_ticket_appdata() and SSL_SESSION_get_ticket_appdata() functions were added in OpenSSL 1.1.1.

COPYRIGHT

Licensed under the OpenSSL license (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>.