Re: [Cbor] Using cddl-control to dissect byte strings

[Christian Amsüss <[e-mail]>]

> ## Using cddl-control to make CBOR even less schema-less

An action from this very fitting observation could be to decorate any
use of such a construction with text like:

| This concatenation touches the fringes of CBOR applicability: We
| sacrifice discernible CBOR structure to save off single bytes, which
| runs counter to CBOR's priorization of being schemaless over
| serialization compactness {{Section E of RFC8949}}. This approach is
| not recommended for general use without clear size boundaries that
| need to be met.

> I’d probably use .abnfb for many of these; at least if (like in this
> example) you don't need to go back to CDDL names (which is a
> shortcoming I have a plan for, but that will happen later this year).

I'm curious to see that.

(For this particular case, ABNF might not easily cut it, for the string
length will only be known at runtime, and the steps to get the generic
length into the ABNF processor will make this exercise look tame in
comparison).

> You mentioned the cddl tool generator — does the validator choke on this?

    $ cat minhoc.cddl
    message_alg42 = [message<4>]
    message<L> = (number: uint, key<L> .cat data)
    key<L> = bstr .size L
    data = bstr
    $ cddl minhoc.cddl generate | tee minhoc.diag
    [1146, h'B3BFE8437065727369737473']
    $ diag2cbor.rb minhoc.diag > minhoc.cbor
    $ cddl minhoc.cddl validate minhoc.cbor
    CDDL validation failure
    [ full output at the end of the mail ]

> (I don’t want to think about .det here.)

For purposes of parser building (which is even stricter than validation,
for validation probably doesn't need to find a *unique* assignment),
there are reversible and irreversible controls; det is just
irreversible, as is `bstr .cat bstr` without further constraints.

> (BTW^2: Why the [()]?  It does look very important, but maybe not
> necessary?)

Just an effect of me editing around; needless, and fixed in current
example.

BR
c

---

CDDL validation failure (nil for [1146, "\xB3\xBF\xE8Cpersists"]):
["\xB3\xBF\xE8Cpersists",
 [:anno, :cat, [:anno, :size, [:prim, 2], [:int, 4]], [:prim, 2]],
 "\n" +
 "occur 0 < 1, not reached at 1 in array [1146, \"\\xB3\\xBF\\xE8Cpersists\"] for [:array, [:member, 1, 1, [:text, \"number\"], [:prim, 0]], [:member, 1, 1, nil, [:anno, :cat, [:anno, :size, [:prim, 2], [:int, 4]], [:prim, 2]]]][\"\\xB3\\xBF\\xE8Cpersists\", [:anno, :cat, [:anno, :size, [:prim, 2], [:int, 4]], [:prim, 2]], \"\\noccur 0 < 1, not reached at 1 in array [1146, \\\"\\\\xB3\\\\xBF\\\\xE8Cpersists\\\"] for [:array, [:member, 1, 1, [:text, \\\"number\\\"], [:prim, 0]], [:member, 1, 1, nil, [:anno, :cat, [:anno, :size, [:prim, 2], [:int, 4]], [:prim, 2]]]]\"] -- cannot complete (false, 2) array [1146, \"\\xB3\\xBF\\xE8Cpersists\"] for [:array, [:member, 1, 1, [:text, \"number\"], [:prim, 0]], [:member, 1, 1, nil, [:anno, :cat, [:anno, :size, [:prim, 2], [:int, 4]], [:prim, 2]]]]"]
[h'B3BFE8437065727369737473', [:anno, :cat, [:anno, :size, [:prim, 2], [:int, 4]], [:prim, 2]], "\noccur 0 < 1, not reached at 1 in array [1146, \"\\xB3\\xBF\\xE8Cpersists\"] for [:array, [:member, 1, 1, [:text, \"number\"], [:prim, 0]], [:member, 1, 1, nil, [:anno, :cat, [:anno, :size, [:prim, 2], [:int, 4]], [:prim, 2]]]][\"\\xB3\\xBF\\xE8Cpersists\", [:anno, :cat, [:anno, :size, [:prim, 2], [:int, 4]], [:prim, 2]], \"\\noccur 0 < 1, not reached at 1 in array [1146, \\\"\\\\xB3\\\\xBF\\\\xE8Cpersists\\\"] for [:array, [:member, 1, 1, [:text, \\\"number\\\"], [:prim, 0]], [:member, 1, 1, nil, [:anno, :cat, [:anno, :size, [:prim, 2], [:int, 4]], [:prim, 2]]]]\"] -- cannot complete (false, 2) array [1146, \"\\xB3\\xBF\\xE8Cpersists\"] for [:array, [:member, 1, 1, [:text, \"number\"], [:prim, 0]], [:member, 1, 1, nil, [:anno, :cat, [:anno, :size, [:prim, 2], [:int, 4]], [:prim, 2]]]]"]


-- 
To use raw power is to make yourself infinitely vulnerable to greater powers.
  -- Bene Gesserit axiom