Re: [Cbor] CDDL and b64u for JSON and byte array for CBOR

[Carsten Bormann <[e-mail]>]

Hi Oliver,

> On 2024-01-24, at 21:14, Oliver Terbu <oliver.terbu=[e-mail]> wrote:
> 
> Hi,
> 
> I have a CDDL for a data model with potential serialization to JSON/CBOR and I have map entry that could be either a byte array or a base64url-encoded text representation of the byte array. 
> 
> Some = {
>   "key": #6.21(bstr)
> }
> 
> Does the CDDL above express that correctly?

The CDDL expresses that the value under “key” needs to be a tag 21 on a byte string.  It does not directly allow the base64url-encoded text representation that JSON would need.

The purpose of the tag 21 would be to "indicate that a byte string might require a specific encoding when interoperating with a text-based representation” (Section 3.4.5.2 of RFC 8949 [1]).

[1]: https://www.rfc-editor.org/rfc/rfc8949.html#name-expected-later-encoding-for

It goes on to say:

  "These tags are useful when an encoder knows that the
   byte string data it is writing is likely to be later converted to a
   particular JSON-based usage.”

So the assumption here is that a processor that converts the CBOR to JSON can do this without knowing the CDDL data description, based on the mere presence of the tag 21.

If you can assume that the processor will be aware of the data definition, you can put a description both for a CBOR representation and a JSON representation into a single piece of CDDL, e.g., by using

  JC<J,C> = J .feature "json" / C .feature "cbor"

from RFC 9165, combined with .b64u from draft-ietf-cddl-cbor-more-control:

  binary = JC<text .b64u bstr, bstr>

or, more generally:

  binary-data<D> = JC<text .b64u D, D>

so you can be more specific what those data D is.
By relying on the processor’s awareness of the data definition, you save one byte that is needed for actually encoding tag 21 otherwise.

Examples that the CDDL tool generates for JSON and CBOR, respectively, are shown below.

Grüße, Carsten

$ cat jc.cddl
binary = JC<text .b64u bstr, bstr>
JC<J,C> = J .feature "json" / C .feature “cbor"

$ CDDL_FEATURE_OK=cbor,^json cddl jc.cddl gp 10
/binary/ h'717569766572697368'
/binary/ h'656E646F7374796C6172'
/binary/ h'6D616C6C6F736569736D6963'
/binary/ h'73757065726571756976616C656E74'
/binary/ h'706F726B776F6F64'
/binary/ h'44696374796F736970686F6E'
/binary/ h'4672616E636F6E69616E'
/binary/ h'73636C65726F7469636F63686F72696F696469746973'
/binary/ h'6174696C74'
/binary/ h’657061676F676963'

$ CDDL_FEATURE_OK=json,^cbor cddl jc.cddl gp 10
/binary/ "c25haWw"
/binary/ "VGV0cmFkeW5hbWlh"
/binary/ "aW50ZXJmZXJl"
/binary/ "cHJlY2F2YWw"
/binary/ "Y291bnRlcnN0YXRhbnQ"
/binary/ "ZW1lcmdlbmN5"
/binary/ "c2xlZXBlcmVk"
/binary/ "aG9ybndvcnQ"
/binary/ "cGhhcm1hY29tZXRlcg"
/binary/ “c3VwcmFzcGluYXRl"