[openssl-project] OpenSSL 1.1.1 library(OpenSSL 1.1.0 compile) Postfix to Postfix test

Viktor Dukhovni openssl-users at dukhovni.org
Mon Apr 23 23:16:44 UTC 2018

> On Apr 23, 2018, at 3:35 AM, Matt Caswell <matt at openssl.org> wrote:
>>  * With TLS 1.3 a new session is generated even sessions are
>>    resumed, because the server responds with a new ticket
>>    in the event of session resumption.  With TLS 1.2 sessions
>>    that had sufficient remaining lifetime did not trigger new
>>    ticket generation on the server, and no new session was
>>    stored on the client.  This causes needless wear-and-tear
>>    on the external session cache in Postfix, since each
>>    connection writes out a new session, replacing the one
>>    it just used.  Some might consider this a security feature,
>>    but it is not especially desirable with SMTP.  Any thoughts
>>    about whether this could be tunable?  It would have to be
>>    server-side tuning I think, since the client does not know
>>    why the server issued a new session, perhaps the old one
>>    was not (or will soon not) be valid for re-use.
> Note that some servers may actually issue more than one ticket per
> connection. Notably boring issues 2 by default. I'm not sure if they
> enable configuration of that.

To be clear, I'm looking for server-side controls, so that, for example,
in the Postfix SMTP server when the presented ticket has sufficient
lifetime left (as is the case with TLS 1.2), no new session tickets
are generated.  The Postfix SMTP server sets up a ticket callback:


and so can signal the SSL engine to accept or re-issue the ticket.
Presently tickets are always accepted and periodically a new
handshake takes place when the ticket is no longer valid.

> In servers that accept early data we enforce single use tickets. In
> those scenarios it may make sense to have more than one ticket issued
> per connection.

There's no early data in SMTP STARTTLS.

> I do have a WIP PR for enabling configuration of the number of tickets
> to be sent on the server side:
> https://github.com/openssl/openssl/pull/5227
> I have not been prioritising that at the moment because I have been
> focussing more on fixing defects.

I'll take a look...


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