[openssl-dev] Fwd: [openssl-users] How to get list of TLS protocols supported by OpenSSL?

Fri Nov 13 19:37:46 UTC 2015

Hi guys,

I recently confirmed, from the openssl-users mailing list, that there's no
suitable API call to determine what TLS versions a given compiled copy of
the OpenSSL library is capable of. This would be a functionality that would
be useful to a lot of real-world users.

I was thinking of implementing this and sending a patch to you guys at
rt at openssl.org (or whatever the recommended procedure is), since I think
it'll benefit a lot of users. What do you guys think?
Moreover, since the list of TLS protocols supported is a property of a
particular version of the library, I was thinking of doing it the way
SSLeay_version(SSLEAY_VERSION) works. i.e.
#define OPENSSL_VERSION_TEXT    "OpenSSL 1.0.2a 19 Mar 2015"

I'm thinking of adding a #define with a comma-separated list of TLS
versions (eg for OpenSSL 1.0.2a):
#define OPENSSL_SUPPORTED_TLS_VERSIONS
"TLSv1.2,TLSv1.1,TLSv1,SSLv3".              This would obvously have to be
updated carefully in every release (if needed) just like
OPENSSL_VERSION_TEXT must be.

I'm interested in contributing code to the openssl project since I use it a
lot, and this looks likes a small enough change that'll get me started.
Please let me know what you guys think.

Thanks,
Pratyush Parimal.
Software Security Engineer | HPE Vertica

---------- Forwarded message ----------
From: Jakob Bohm <jb-openssl at wisemo.com>
Date: Fri, Nov 13, 2015 at 7:38 AM
Subject: Re: [openssl-users] How to get list of TLS protocols supported by
OpenSSL?
To: openssl-users at openssl.org

On 13/11/2015 10:34, Matt Caswell wrote:

On 13/11/15 02:56, pratyush parimal wrote:

Hi,

I'm writing a client-server program that uses TLS for communication.
I'm wondering if there's any way to programmatically find out which TLS
protocol versions are supported by the OpenSSL library installed on my
system.

I'm currently aware of three ways which "sort of" provide this information:
(1) After setting up the TLS communication, call: SSL_get_version(ssl);
which returns "TLSV1.2", etc.
(2) Try to connect to a server using TLS by specifying all possible TLS
versions in the client program, and see which connections pass/fail.
(3) Call: SSL_get_ciphers(), print their names, and try to correlate
them with the protocol they're associated with.

Unfortunately, none of the above answer my question completely.

So is it possible to ascertain which TLS protocol versions are actually
supported by my server-program, without trying the above methods? My
purpose is not to simply make a list for my own reference, but rather
finding it out on-the-fly in the server-side program, since I may run it
on different versions of OpenSSL.

You can use the define TLS_MAX_VERSION to determine the highest protocol
version supported by the library.

If you also want to know the lowest version then that's a bit more
tricky. All current released versions will define OPENSSL_NO_SSL3 if
SSLv3 support has been compiled out, and OPENSSL_NO_SSL2 if SSLv2
support has been compiled out (its not currently possible to compile out
other protocol versions). In the forthcoming 1.1.0 release SSLv2 support
has been completely removed so you don't get OPENSSL_NO_SSL2 even though
there is no SSLv2 support available (hmmmm...I wonder if we should add
that?). There are other SSLv2 defines in ssl2.h that are removed in
1.1.0 which you could use to detect whether you are on a version with
ssl2.h completely removed such as SSL2_MT_ERROR, i.e. something like
(completely untested):

#ifdef OPENSSL_NO_SSL3
#define TLS_MIN_VERSION TLS1_VERSION
#elif defined(OPENSSL_NO_SSL2) || !defined(SSL2_MT_ERROR)
#define TLS_MIN_VERSION SSL3_VERSION
#else
#define TLS_MIN_VERSION SSL2_VERSION
#endif

How future proof that is if we ever remove SSLv3 support I'm not sure.

Matt

Unfortunately that presumes that the client is compiled
against configure headers from the library build.

This is absolutely useless if you try to share an OpenSSL
shared library compiled by a 3rd party (such as an OS
distribution or an end user).

What lots of people need is an ability to interrogate a
compiled shared library about what is enabled in that copy,
similar to how the SSL_get_ciphers() or similar can be used
to determine if the current copy has been compiled without
IDEA, ECC or other optional cipher suites.

This is what happens in the real world when end users run your
compiled program on various Linux distributions, such as Red
Hat vs. OpenSUSE vs. Ubuntu...

Enjoy

Jakob
-- 
Jakob Bohm, CIO, Partner, WiseMo A/S.  https://www.wisemo.com
Transformervej 29, 2860 Søborg, Denmark.  Direct +45 31 13 16 10
This public discussion message is non-binding and may contain errors.
WiseMo - Remote Service Management for PCs, Phones and Embedded

_______________________________________________
openssl-users mailing list
To unsubscribe: https://mta.openssl.org/mailman/listinfo/openssl-users
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mta.openssl.org/pipermail/openssl-dev/attachments/20151113/bc585b7e/attachment.html>