=head1 DESCRIPTION
-SSL_CTX_set_tlsext_ticket_key_cb() sets a callback fuction I<cb> for handling
+SSL_CTX_set_tlsext_ticket_key_cb() sets a callback function I<cb> for handling
session tickets for the ssl context I<sslctx>. Session tickets, defined in
RFC5077 provide an enhanced session resumption capability where the server
implementation is not required to maintain per session state. It only applies
to TLS and there is no SSLv3 implementation.
-The callback is available when the OpenSSL library was built without
-I<OPENSSL_NO_TLSEXT> being defined.
-
The callback function I<cb> will be called for every client instigated TLS
session when session ticket extension is presented in the TLS hello
message. It is the responsibility of this function to create or retrieve the
ticket information or it starts a full TLS handshake to create a new session
ticket.
-Before the callback function is started I<ctx> and I<hctx> have been
+Before the callback function is started I<ctx> and I<hctx> have been
initialised with EVP_CIPHER_CTX_init and HMAC_CTX_init respectively.
For new sessions tickets, when the client doesn't present a session ticket, or
-an attempted retreival of the ticket failed, or a renew option was indicated,
+an attempted retrieval of the ticket failed, or a renew option was indicated,
the callback function will be called with I<enc> equal to 1. The OpenSSL
-library expects that the function will set an arbitary I<name>, initialize
+library expects that the function will set an arbitrary I<name>, initialize
I<iv>, and set the cipher context I<ctx> and the hash context I<hctx>.
-The I<name> is only 16 characters long. The I<iv> is of length
-L<EVP_MAX_IV_LENGTH> defined in B<evp.h>.
+The I<name> is 16 characters long and is used as a key identifier.
+
+The I<iv> length is the length of the IV of the corresponding cipher. The
+maximum IV length is L<EVP_MAX_IV_LENGTH> bytes defined in B<evp.h>.
-The initialization vector I<iv> should be a random value. The cipher context
-I<ctx> should use the initialisation vector I<iv>. The cipher context can be
+The initialization vector I<iv> should be a random value. The cipher context
+I<ctx> should use the initialisation vector I<iv>. The cipher context can be
set using L<EVP_EncryptInit_ex>. The hmac context can be set using L<HMAC_Init_ex>.
When the client presents a session ticket, the callback function with be called
-with I<enc> set to 0 indicating that the I<cb> function should retreive a set
+with I<enc> set to 0 indicating that the I<cb> function should retrieve a set
of parameters. In this case I<name> and I<iv> have already been parsed out of
the session ticket. The OpenSSL library expects that the I<name> will be used
to retrieve a cryptographic parameters and that the cryptographic context
-I<ctx> will be set with the retreived parameters and the initialization vector
+I<ctx> will be set with the retrieved parameters and the initialization vector
I<iv>. using a function like L<EVP_DecryptInit_ex>. The I<hctx> needs to be set
using L<HMAC_Init_ex>.
If the I<name> is still valid but a renewal of the ticket is required the
callback function should return 2. The library will call the callback again
-with an arguement of enc equal to 1 to set the new ticket.
+with an argument of enc equal to 1 to set the new ticket.
The return value of the I<cb> function is used by OpenSSL to determine what
further processing will occur. The following return values have meaning:
=over 4
-=item 2
+=item Z<>2
This indicates that the I<ctx> and I<hctx> have been set and the session can
continue on those parameters. Additionally it indicates that the session
call I<cb> again with an enc argument of 1 to set the new ticket (see RFC5077
3.3 paragraph 2).
-=item 1
+=item Z<>1
This indicates that the I<ctx> and I<hctx> have been set and the session can
continue on those parameters.
-=item 0
+=item Z<>0
This indicates that it was not possible to set/retrieve a session ticket and
-the SSL/TLS session will continue by by negiotationing a set of cryptographic
+the SSL/TLS session will continue by by negotiating a set of cryptographic
parameters or using the alternate SSL/TLS resumption mechanism, session ids.
If called with enc equal to 0 the library will call the I<cb> again to get
=head1 NOTES
Session resumption shortcuts the TLS so that the client certificate
-negiotation don't occur. It makes up for this by storing client certificate
+negotiation don't occur. It makes up for this by storing client certificate
an all other negotiated state information encrypted within the ticket. In a
resumed session the applications will have all this state information available
-exactly as if a full negiotation had occured.
+exactly as if a full negotiation had occurred.
+
+If an attacker can obtain the key used to encrypt a session ticket, they can
+obtain the master secret for any ticket using that key and decrypt any traffic
+using that session: even if the ciphersuite supports forward secrecy. As
+a result applications may wish to use multiple keys and avoid using long term
+keys stored in files.
+
+Applications can use longer keys to maintain a consistent level of security.
+For example if a ciphersuite uses 256 bit ciphers but only a 128 bit ticket key
+the overall security is only 128 bits because breaking the ticket key will
+enable an attacker to obtain the session keys.
=head1 EXAMPLES
-Reference Implemention:
+Reference Implementation:
SSL_CTX_set_tlsext_ticket_key_cb(SSL,ssl_tlsext_ticket_key_cb);
....
=head1 SEE ALSO
-L<ssl(3)|ssl(3)>, L<SSL_set_session(3)|SSL_set_session(3)>,
-L<SSL_session_reused(3)|SSL_session_reused(3)>,
-L<SSL_CTX_add_session(3)|SSL_CTX_add_session(3)>,
-L<SSL_CTX_sess_number(3)|SSL_CTX_sess_number(3)>,
-L<SSL_CTX_sess_set_get_cb(3)|SSL_CTX_sess_set_get_cb(3)>,
-L<SSL_CTX_set_session_id_context(3)|SSL_CTX_set_session_id_context(3)>,
-
-=head1 HISTORY
-
-This function was introduced in OpenSSL 0.9.8h
+L<ssl(3)>, L<SSL_set_session(3)>,
+L<SSL_session_reused(3)>,
+L<SSL_CTX_add_session(3)>,
+L<SSL_CTX_sess_number(3)>,
+L<SSL_CTX_sess_set_get_cb(3)>,
+L<SSL_CTX_set_session_id_context(3)>,
=cut
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