[Development] QSsl: finer-grained protocol selection

[Mikkel Krautz]

On Fri, Dec 26, 2014 at 10:12 PM, Thiago Macieira
<thiago.macieira at intel.com> wrote:
> On Friday 26 December 2014 16:55:34 Mikkel Krautz wrote:
>> We definitely wouldn't keep supporting TLS 1.0 in that case.
>>
>> But we would like to still be able to accept connections where the
>> handshake ends up using TLS 1.0. Then we can handle the rejection at
>> the application protocol level, and show an appropriate message to the
>> user and potentially disconnect them immediately. This would be
>> handled through an update to the server software. Clients could be
>> updated with a similar mechanism, too.
>>
>> If we do it this way, it allows us to display an appropriate message,
>> via the server, even for clients that are not TLS 1.1 or TLS
>> 1.2-aware. (Which is pretty much the whole user base.)
>>
>> However, if there really is no interest in allowing a finer-grained
>> protocol selection in Qt itself, I suppose we could live with using
>> QSsl::SecureProtocols. That'll at least allow us to add a generic
>> error if a modern client attempts to connect to a server that only
>> allows TLS 1.0. But older clients would, unfortunately, be stuck with
>> a generic handshake error.
>>
>> But in my opinion, that would not be ideal for our users.
>
> I don't think we need fine-grained detection, but we do need something better
> than what we have right now.
>
> My suggestion is to set a level. For example, if you set to TlsV10, then you
> get TLS v1.0 and anything newer, existing today or not, and disable anything
> older. The client will negotiate the highest version at connection time. The
> only reason to disable newer versions is when the server is buggy, but the
> application should not have to care about that. That's OpenSSL's job.
>
> That means old clients will use newer and more secure protocol versions as
> soon as OpenSSL and Qt are upgraded, with no change to the application.

I really like this approach.

I've taken a stab at implementing this for Mumble's Qt 5 tree - and it
works great. (I'll open a code review soon, for further discussion)

However, while doing that I stumbled upon the qsslsocket_winrt.cpp
file, and by extension I spotted the way WinRT implements this leveled
approach:

http://msdn.microsoft.com/en-US/library/windows/apps/windows.networking.sockets.socketprotectionlevel.aspx

Setting TLS 1.2 gives you TLS 1.2 and below (no SSL 3)
And so on...

However, your suggested approach of having QSsl::Tlsv1_0 mean "TLS 1.0
or greater" is obviously still achievable if we limit ourselves to
only doing it for QSsl::TlsV1_0, QSsl::TlsV1_1 and QSsl::TlsV1_2. (On
WinRT, all of these would simply be set to "Tls12".). But I think
that's fine.

> --
> Thiago Macieira - thiago.macieira (AT) intel.com
>   Software Architect - Intel Open Source Technology Center
>
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