.\"	$OpenBSD: SSL_CTX_set_tlsext_ticket_key_cb.3,v 1.8 2022/01/25 18:01:20 tb Exp $
.\"	OpenSSL b97fdb57 Nov 11 09:33:09 2016 +0100
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.Dd $Mdocdate: January 25 2022 $
.Dt SSL_CTX_SET_TLSEXT_TICKET_KEY_CB 3
.Os
.Sh NAME
.Nm SSL_CTX_set_tlsext_ticket_key_cb
.Nd set a callback for session ticket processing
.Sh SYNOPSIS
.In openssl/tls1.h
.Ft long
.Fo SSL_CTX_set_tlsext_ticket_key_cb
.Fa "SSL_CTX sslctx"
.Fa "int (*cb)(SSL *s, unsigned char key_name[16],\
 unsigned char iv[EVP_MAX_IV_LENGTH],\
 EVP_CIPHER_CTX *ctx, HMAC_CTX *hctx, int enc)"
.Fc
.Sh DESCRIPTION
.Fn SSL_CTX_set_tlsext_ticket_key_cb
sets a callback function
.Fa cb
for handling session tickets for the ssl context
.Fa sslctx .
Session tickets, defined in RFC 5077, provide an enhanced session
resumption capability where the server implementation is not required to
maintain per session state.
.Pp
The callback function
.Fa 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
cryptographic parameters and to maintain their state.
.Pp
The OpenSSL library uses the callback function to help implement a
common TLS ticket construction state according to RFC 5077 Section 4 such
that per session state is unnecessary and a small set of cryptographic
variables needs to be maintained by the callback function
implementation.
.Pp
In order to reuse a session, a TLS client must send a session ticket
extension to the server.
The client can only send exactly one session ticket.
The server, through the callback function, either agrees to reuse the
session ticket information or it starts a full TLS handshake to create a
new session ticket.
.Pp
The callback is called with
.Fa ctx
and
.Fa hctx
which were newly allocated with
.Xr EVP_CIPHER_CTX_new 3
and
.Xr HMAC_CTX_new 3 ,
respectively.
.Pp
For new sessions tickets, when the client doesn't present a session
ticket, or an attempted retrieval of the ticket failed, or a renew
option was indicated, the callback function will be called with
.Fa enc
equal to 1.
The OpenSSL library expects that the function will set an arbitrary
.Fa key_name ,
initialize
.Fa iv ,
and set the cipher context
.Fa ctx
and the hash context
.Fa hctx .
.Pp
The
.Fa key_name
is 16 characters long and is used as a key identifier.
.Pp
The
.Fa iv
length is the length of the IV of the corresponding cipher.
The maximum IV length is
.Dv EVP_MAX_IV_LENGTH
bytes defined in
.In openssl/evp.h .
.Pp
The initialization vector
.Fa iv
should be a random value.
The cipher context
.Fa ctx
should use the initialisation vector
.Fa iv .
The cipher context can be set using
.Xr EVP_EncryptInit_ex 3 .
The hmac context can be set using
.Xr HMAC_Init_ex 3 .
.Pp
When the client presents a session ticket, the callback function
with be called with
.Fa enc
set to 0 indicating that the
.Fa cb
function should retrieve a set of parameters.
In this case
.Fa key_name
and
.Fa iv
have already been parsed out of the session ticket.
The OpenSSL library expects that the
.Em key_name
will be used to retrieve a cryptographic parameters and that the
cryptographic context
.Fa ctx
will be set with the retrieved parameters and the initialization vector
.Fa iv
using a function like
.Xr EVP_DecryptInit_ex 3 .
The
.Fa hctx
needs to be set using
.Xr HMAC_Init_ex 3 .
.Pp
If the
.Fa key_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 argument of
.Fa enc
equal to 1 to set the new ticket.
.Pp
The return value of the
.Fa cb
function is used by OpenSSL to determine what further processing will
occur.
The following return values have meaning:
.Bl -tag -width Ds
.It 2
This indicates that the
.Fa ctx
and
.Fa hctx
have been set and the session can continue on those parameters.
Additionally it indicates that the session ticket is in a renewal period
and should be replaced.
The OpenSSL library will call
.Fa cb
again with an
.Fa enc
argument of 1 to set the new ticket (see RFC 5077 3.3 paragraph 2).
.It 1
This indicates that the
.Fa ctx
and
.Fa hctx
have been set and the session can continue on those parameters.
.It 0
This indicates that it was not possible to set/retrieve a session ticket
and the SSL/TLS session will continue by negotiating a set of
cryptographic parameters or using the alternate SSL/TLS resumption
mechanism, session ids.
.Pp
If called with
.Fa enc
equal to 0, the library will call the
.Fa cb
again to get a new set of parameters.
.It less than 0
This indicates an error.
.El
.Pp
Session resumption shortcuts the TLS so that the client certificate
negotiation don't occur.
It makes up for this by storing client certificate and 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 negotiation had occurred.
.Pp
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.
.Pp
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.
.Sh RETURN VALUES
This function returns 0 to indicate that the callback function was set.
.Sh EXAMPLES
Reference Implementation:
.Bd -literal
SSL_CTX_set_tlsext_ticket_key_cb(SSL, ssl_tlsext_ticket_key_cb);
\&....
static int ssl_tlsext_ticket_key_cb(SSL *s, unsigned char key_name[16],
    unsigned char *iv, EVP_CIPHER_CTX *ctx, HMAC_CTX *hctx, int enc)
{
	if (enc) { /* create new session */
		if (RAND_bytes(iv, EVP_MAX_IV_LENGTH))
			return -1; /* insufficient random */

		key = currentkey(); /* something you need to implement */
		if (!key) {
			/* current key doesn't exist or isn't valid */
			key = createkey();
			    /* something that you need to implement.
			     * createkey needs to initialise a name,
			     * an aes_key, a hmac_key, and optionally
			     * an expire time. */
			if (!key) /* key couldn't be created */
				return 0;
		}
		memcpy(key_name, key->name, 16);

		EVP_EncryptInit_ex(&ctx, EVP_aes_128_cbc(), NULL,
		    key->aes_key, iv);
		HMAC_Init_ex(&hctx, key->hmac_key, 16, EVP_sha256(), NULL);

		return 1;

	} else { /* retrieve session */
		key = findkey(name);

		if  (!key || key->expire < now())
			return 0;

		HMAC_Init_ex(&hctx, key->hmac_key, 16, EVP_sha256(), NULL);
		EVP_DecryptInit_ex(&ctx, EVP_aes_128_cbc(), NULL,
		    key->aes_key, iv );

		if (key->expire < (now() - RENEW_TIME))
		    /* this session will get a new ticket
		     * even though the current is still valid */
		    return 2;

		return 1;
	}
}
.Ed
.Sh SEE ALSO
.Xr ssl 3 ,
.Xr SSL_CTX_add_session 3 ,
.Xr SSL_CTX_callback_ctrl 3 ,
.Xr SSL_CTX_sess_number 3 ,
.Xr SSL_CTX_sess_set_get_cb 3 ,
.Xr SSL_CTX_set_session_id_context 3 ,
.Xr SSL_session_reused 3 ,
.Xr SSL_set_session 3
.Sh HISTORY
.Fn SSL_CTX_set_tlsext_ticket_key_cb
first appeared in OpenSSL 0.9.8h and has been available since
.Ox 4.5 .