Session Ticket Rotation

[Richard Fussenegger, BSc]

I'd like to implement built-in session ticket rotation. I know that it 
this was discussed before but it was never implemented. Right now a 
custom external ticket key system is supported. Admins with single 
installations and not enough knowledge about the topic are left with 
keys that are valid for the complete lifetime nginx is running.

I thought about a rotation scheme that introduces a single new 
configuration variable (e.g. ssl_session_ticket_key_rotation or 
ssl_session_ticket_key_interval) that defines the interval in which the 
ticket key should be rotated. I think a default setting of 24 hours 
would be enough for most installations. One key is always used for 
decryption and encryption and the most recently expired key is only used 
for decryption. This means that we have a de- and encrypt key for 24 
hours and a decrypt key for 48 hours with the default setting. The best 
place for this would be in ngx_event_openssl.c#2640 after the if (paths 
== NULL) by checking if the aforementioned variable is set (default) and 
if the currently used key has expired. If it hasn't return NGX_OK (as it 
is now) otherwise copy current key to old key and generate new key.

Unsolved questions:

Implementation: Currently OpenSSL is generating and keeping track of the 
key (this would still be the case if the newly introduced setting is set 
to "0"). We'd have to introduce two variables - one for the current and 
one for the old key plus timestamps - and let OpenSSL know about the key 
before attempting to decrypt sent session data.

Per server keys: CVE-2014-3616 from Antoine Delignat-Lavaud was dealing 
with this. I have to review his patches but I guess that the variables 
have to be arrays and we'd have to keep different current and old keys 
per server. But what if one wants to share the sessions among servers? I 
guess this has to depend on the location the configuration was set. As I 
said, I have to review his patches to gather more knowledge for this 
problem.

Restarts: Apache is storing the key to file, that would allow the 
process to read the last used keys. But of course this isn't very secure 
and we may need an additional setting to define the path where these 
intermediate keys should be stored. Of course we could also simply 
ignore restarts, as it is now. But we'd loose the ability to decrypt old 
sessions tickets.

Any thoughts from your side are highly appreciated.

Richard