[openssl-dev] Circumstances cause CBC often to be preferred over GCM modes

[Kurt Roeckx]

On Tue, Dec 16, 2014 at 07:57:08PM +0000, Viktor Dukhovni wrote:
> On Tue, Dec 16, 2014 at 08:46:35PM +0100, Kurt Roeckx wrote:
> 
> > On Tue, Dec 16, 2014 at 06:56:14PM +0000, Viktor Dukhovni wrote:
> > > And the browsers should implement SHA-384, and why the hell are we
> > > using SHA-384 with AES256-GCM instead of SHA-256 anyway?  Surely
> > > the SHA256 HMAC construction has adequate strength in this context?
> > 
> > With GCM the collision resistance is important and SHA-256
> > only provides an 128 bit strength for that.
> 
> I've not looked into this, can you elaborate (citation)? Which
> attacker controls the SHA2-256 inputs to the TLS PRF?  Why are
> collisions rather than 2nd preimages the relevant issue?

I think the best reference I can find at this time is:
http://www.ietf.org/mail-archive/web/tls/current/msg13313.html

But I'm sure I can find others if needed.


Kurt