[COSE] The one-byte saving from use of a sequence

[Laurence Lundblade <[e-mail]>]

If I’m thinking right, the one-byte saving from using a sequence rather than array for CBORCertificate only happens when using CBORCertificate in a non-CBOR protocol. When you put CBORCertificate in a CBOR protocol, it has to be an array (or a bstr-wrapped sequence) so it can be distinguished from the surrounding CBOR. In non-CBOR protocols, the surrounding protocol (e.g., the DER, or the TLS records) provides the framing for what is the cert, so the array is not needed.

I’d like to see CBORCertificate an array in the main definition. Then when CBORCertificate is put in a non-CBOR protocol, it can be “unwrapped" to save the byte. The CDDL notation for that would be ~CBORCertificate. (I’ve been reading up on CDDL unwrapping lately, so hopefully I got that right).

This also aligns CBORCertificate with the other CBOR protocols I’ve seen so far. They use an array or map to hold the top-level messages together.

Also, I’m pretty sure the CDDL for COSE_C5 is wrong in the -08 draft. It says:

    COSE_C5 = [ + CBORCertificate ]

With CBORCertificate defined as a CDDL group, no framing structure to distinguish the individual CBORCertificates is generated. The array defined by this is not of individual certs, but of an aggregation of all the certificate data items. It would not be possible to decode correctly. Making CBORCertificate an array solves this problem.

We should expect CBORCertificate to be incorporated into lots and lots of other CBOR-based protocols. If we are making a mistake like this because of CBORCertificate being a sequence and not an array, it seems others may as well. So another reason for the main definition of CBORCertificate to be an array.

LL