Re: [apps-discuss] Concise Binary Object Representation (CBOR)

[Nico Williams <[e-mail]>]

On Fri, May 24, 2013 at 12:14 PM, Phillip Hallam-Baker <[e-mail]> wrote:
> On Fri, May 24, 2013 at 1:02 PM, Joe Hildebrand (jhildebr)
> <[e-mail]> wrote:
>> >My experience of coding ASN.1 DER (in C and javascript) leads me to
>> >reject any counted scheme as a non starter.
>>
>> I've read the whole thread, and didn't see adequate justification for that
>> worldview.  Could you reiterate it, please?
>
>
> If you have a counter scheme it is necessary to have the whole data
> structure in memory to serialize it. Many network applications require the
> use of an intermediary with a fixed buffer length.
>
> Now ASN.1 BER definite length encoding makes matters even worse by
> specifying lengths in octets rather than the number of entries in an object
> but the same objection applies. And DER encoding is just plain insane as the
> lengths of the lengths also varies.

Yes, but note that CBOR does much better than DER: it counts elements
(of arrays, of objects), not bytes (except for strings, which are
byte-counted).

It might be nice to have an array/object of indefinite length encoding
where the end is indicated by a special type whose only purpose is to
mark the end of the array/object (CER has this).  But don't try to use
this for strings: you end up having to escape the end-of-string marker
-- ugh!

> A very frequent use case for an encoding format is to take a stream of data
> of unknown size and package it in real time. For example I would like to be
> able to take a video stream and encrypt it using JSON encryption and then
> put a digital signature at the end.

Note BTW that JSON *is* amenable to online encoding (and decoding).
Binary encodings should keep this feature.

> BSON does not do that quite like I would like but I can fake the same effect
> by using an array of Binary rather than one Binary chunk and the same for
> strings.

I think that's as well as you can expect to do without having to
resort to escaping binary/string characters.

Anyways, +1.

Nico
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