Re: [saag] ASN.1 vs. DER Encoding

[Nico Williams <[e-mail]>]

On Sat, Mar 30, 2019 at 10:31:02AM -0500, Benjamin Kaduk wrote:
> On Wed, Mar 27, 2019 at 10:15:46AM -0500, Nico Williams wrote:
> > On Wed, Mar 27, 2019 at 09:45:16AM +0000, Peter Gutmann wrote:
> > > >Thus there is almost zero benefit to self-describing encodings.
> > > 
> > > ... apart from the fact that they can be statically analysed to check whether
> > > they're well-formed or not, unlike the encodings in PGP, TLS, IPsec, SSH, ...
> > 
> > The protocols you list don't use a formal syntax, which instantly makes
> > validity checking harder (can't generate the code!).  But if they had
> > used XDR, or ASN.1 with PER/OER/..., you could in fact automatically
> > check the validity of the encoding of a message.
> 
> N.b. that the protocol descriptions in RFC 8446 were run through an
> automated syntax checker (IIRC, by Kazuho, though I'm not confident of that
> and only bought 20MB of data for this  plane flight).  So I'm not entirely
> convinced that this claim applies to TLS 1.3.

ISTR hearing about that, but the RFC does say:

  3.  Presentation Language

     This document deals with the formatting of data in an external
     representation.  The following very basic and somewhat casually
     defined presentation syntax will be used.

which is basically saying it's not a formal syntax.

Now, perhaps it can be be formalized.  If so, was that done for the
checking you mentioned, and was it lost?  That would be unfortunate.

Are there implementations that generate codecs for TLS?

Nico
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