On Tuesday, 26 March 2019 16:38:11 CET Yoav Nir wrote: > > On 26 Mar 2019, at 14:45, Hubert Kario <hka...@redhat.com> wrote: > > > > On Monday, 25 March 2019 22:09:35 CET Yoav Nir wrote: > >> Hi. Today at the TLS meeting, there was a discussion at the mic about > >> 1-bit extensions that only serve to indicate support for an optional > >> feature. EKR commented that such extensions take 4 bytes each and that > >> maybe we need to replace them with a flags extension. > >> > >> So I threw together a quick -00 draft with an extension that does just > >> that > >> [1]. > >> > >> Comments are welcome. > > > > I don't think that "penny-pinching" the 4 bytes necessary to send a flag > > is > > worth the interoperability problems, and increased complexing of parsing > > Client Hello. Especially if we go the route of actual bit flags. > > Right. Which is why I went with a 1-byte encoding rather than a bitstring. > > > I think the likelihood of bugs in that code over the possible bytes saved > > makes it a net negative. > > I don’t think so. My encoding is not all that complex. > > > yes, TLS is quite chatty protocol, it could encode values much more > > tightly, but I think we all remember the bugs related to ASN.1 parsing > > from inside of PKCS#1 v1.5 signatures > > Complexity is on a spectrum. DER encoding is pretty far on this spectrum. > A list of 1-octet identifiers is on the other end. A bitstring is more > complex than the identifier list, but not anywhere near DER.
1-octet identifiers may not be considered extensible enough (yes, you can add another extension, but the first extension to use it will be paying an additional price of 2 bytes on top of the extension overhead; same if you just need to use only one flag, then you are paying the same price for every connection) 2-octet identifiers asymptotically approach 2-octet saved per flag, which is about 50% saved per flag, I don't see it as much to approach it from another way: while I think we will, sometime in the future, reach a situation when we have few hundred flag extensions *defined* , I do not see a future in which we will need to *use* more than few dozen flag extensions in any real world client. So we are talking about a possible saving of around 100 bytes in ClientHello (36 extensions * 3 bytes saved) in this proposal won't this be completely erased by any post-quantum key share? > I don’t think we should project the failings of DER parsing to the parsing > of much simpler structures. yes, that was an extreme example; I was replying to all sent messages so far -- Regards, Hubert Kario Senior Quality Engineer, QE BaseOS Security team Web: www.cz.redhat.com Red Hat Czech s.r.o., Purkyňova 115, 612 00 Brno, Czech Republic
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