Sharing tickets between multiple instances of stud #30

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My previous pull requests was about sharing sessions between multiple instances of stud using memcached. In this case, use of TLS tickets was disabled because they cannot be shared between multiple instances (keys are randomly generated when SSL context is initialized).

With this patch, keys used to protect and encrypt TLS tickets are generated by signing some seed provided by the user with the private key. It does not really depends on the previous commits but it is a bit useless standalone (because there is a lot of clients not supporting tickets). Moreover, it alters the memcached patch by allowing tickets if this feature is enabled.

Since the memcached patch is a bit rusty, there is no hurry to merge this pull request. I just make it available to gather comments on this kind of feature. I did not find such a feature in any web server. It is an exclusive for stud ! ;-)

jamwt commented Oct 12, 2011

Interesting stuff... I'll have to read up on all this.


There is more information about tickets here:


OK, I have rebased this branch on top of Emeric's branch (pull request 50). You should only consider the last 3 commits (mine). The first commit use the shared secret (20 bytes) to build the keys used to protect tickets (48 bytes but only 32 should be considered secrets since the 16 first one are key name). I consider this is a bit weak to build the secret this way.

The second commit replaces the use of SHA1 by SHA384. We get a 48 bytes secret that we can use to protect tickets. However, this may hinder the performances of session sharing between multiple hosts (but I think this does not really matter, SHA384 is a lot faster than public key operations).

The third commit is more invasive but compute a shared secret with SHA384 and only uses the first 20 bytes with SHA1 to protect session sharing between nodes. This should remove the performance hit introduced by the previous commit.

EmericBr commented Nov 7, 2011

Good for me!

Emeric and others added some commits Nov 8, 2011
Emeric Fix memory leak using shared cache:
Openssl online documentation is not up to date:
"The new_session_cb() is called," ... "If the callback returns 0, the session will be immediately removed again."

Checking source code and archive doc:
"Return 1 if a 'copy' is required, otherwise, return 0.  This return value just causes the reference count to be incremented (on return of a 1), this means the application does not need to worry about incrementing the refernece count (and the locking that implies in a multi-threaded application)."
emeric Shared cache: add callback and function to receive/send cache updates
from/to a remote host
Note: Send call back provide encoded session into an aligned buffer
      large enougth to add a footer and send it directly to a peer
      using a sendto
emeric Send and receive cache updates to/from remotes hosts on udp. a07b7b6
emeric Add multicast support for cache updates exchange a85f71d
Emeric Disable tls tickets using multiple host sharing session. 7191c81
@vincentbernat vincentbernat Share TLS tickets between multiple instances of stud
When multiple hosts share a common session cache, TLS tickets need to
be shared as well. By default, keys protecting them are randomly
generated when SSL context is initialized.

With this patch, keys used to protect and encrypt TLS tickets are
generated from RSA private key.
@vincentbernat vincentbernat Replace SHA1 by SHA384 to compute shared secret.
This allows a stronger ticket keys by may hinder a bit the
performances of session transmission process.
@vincentbernat vincentbernat Use SHA1 to protect inter-node traffic instead of SHA384.
The shared secret used to protect inter-node traffic and to generate
tickets is computed from the RSA private key using SHA384 (48
bytes). However, only the first 20 bytes are used to protect
inter-node traffic, with SHA1. This allows to keep performance of SHA1
to protect inside traffic and have good security with SHA384 for
traffic that can be tampered by an outsider.

Rebased on top of emeric/UpdateSHP (cc @emericBr)


OK, the new method is worse than the old one. The problem with sharing tickets this way is that we break forward secrecy. If the user enables a DHE or a ECDHE cipher suite thinking that it will get forward secrecy, we just break this by encrypting tickets using something directly derived from the private key. When I was using a user provided seed, this was better: the private key does not allow alone to decrypt tickets, but the seed may be as unsecure (appears on the command line, may be stolen with the key, may be too short). Therefore, this is not the good method either.

Since Emeric introduced a protocol to exchange sessions, I think that we can extend it a bit to share keys. Each node will generate at regular intervals a new key it will broadcast to other nodes. A key contains a keyname to ensure that we know which key was used to protect a ticket. Moreover, a node should wait 1-5 seconds before using a newly generated key (to ensure other nodes know it). Each node will use its own key to protect a ticket but will know keys from other nodes. A key will be expired after some timer. Therefore, the protocol can be very simple: we just need to broadcast keys.


But when a process receive a new request how choose the right key? In addition, sub process openssl ctx is not in shared memory, so if your reveive update keys or recreate key you need to store the keys in a shared memory and all sub-process will need to lock shared memory to check if a new key is available on each request (there is a cost to do that). I don't see any callback in openssl to call user space when a new connection appear using a ticket.


The first 16 bytes are the name of the key. The next 16 are an IV and
the remaining is the ticket while the last 32 bytes are a MAC
construct. This is not well documented but we can define a custom
callback to handle tickets:

        /* Callback to support customisation of ticket key setting */
        int (*tlsext_ticket_key_cb)(SSL *ssl,
                                        unsigned char *name, unsigned char *iv,
                                        EVP_CIPHER_CTX *ectx,
                                        HMAC_CTX *hctx, int enc);

enc is 1 when we need to create the ticket (name, iv, ectx and hctx are
output variables) and 0 when we need to check it (name, iv, ectx and
hctx are input variables).

About shared memory, I haven't thought of it. Write will be pretty
rare. We could replicate tickets in all processes. Or use some RCU


That is what we need. I think the best way is to generate anew key each N callsof callback creation.

Do you know if ticket have a limited validity time?


Per RFC 5077, there is a lifetime for each ticket (enforced on server
side). It is encoded in the first 4 octets. In OpenSSL, this is encoded
in tlsext_tick_lifetime_hint in the session. I haven't found any code
that check its value.

I think it would be easier for the user if we allow to configure how
long a key is valid. For example, a user could ask to generate a new key
every hour. We keep 5 keys. This means that with n peers, we need to
keep 5*n keys in memory and a ticket is valid 5 hours. Memory bound and
time bound.

If we go for a new key each N calls of callback creation, we need to
ensure that a ticket is valid 5 (for example) hours and therefore, we
could have to store a lot of keys. There is no universal value. A busy
server could handle 3 millions of handshake per hour and a not busy one
a thousand. Seems more difficult to configure.

jamwt commented Dec 12, 2011

Vincent and Emeric, I'm not pulling this in until it seems like you guys have consensus on the right approach. (but I am monitoring this thread)


We have a consensus. The problem is that we need to implement it. ;-)


What's the status of this PR?

@lkarsten lkarsten referenced this pull request in varnish/hitch May 19, 2015

Add support for RFC5077 session tickets #16


The SSL_CTX_set_tlsext_ticket_key_cb is pretty well documented now after they took my document out of their patch graveyard system

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