Hi,

Good that Laurance brought up the "trust relationship" that the x5u text talks 
about during the interim, this seem quite unspecified in the draft. Also, 
forcing the implementation to support pkcs7 in addition to COSE_X509 seems 
unnecessary.

https://github.com/cose-wg/X509/issues/31

I updated the text for 'c5u'. It does not align with x5u as much anymore. We 
definitely do not want to force implementation to support pkcs7. The current 
plan is to let c5u just be a way to do c5bag/c5chain out of band. Should it be 
bag or chain?

Below is the current updated text in GitHub.

-------------------------------------------------

COSE Header Parameters Registry {#cose}

EDITORS NOTE: Should x5u refer to a bag or a chain? The text should be moved a 
section and not be in the IANA Section.

This document registers the following entries in the "COSE Header Parameters" 
registry under the "CBOR Object Signing and Encryption (COSE)" heading. The 
formatting and processing for c5bag, c5chain, and c5t are the same as the 
corresponding x5bag, x5chain, and x5t defined in {{I-D.ietf-cose-x509}} except 
that the certificates are CBOR encoded instead of DER encoded and that c5t MUST 
refer to an end-entity certificate. c5u provides an alternative way to identify 
an untrusted certificate bag/chain by reference with a URI. The content is a 
COSE_X509 item served with the application/cbor content format.

As the contents of c5bag, c5chain, c5t, and c5u are untrusted input, the header 
parameters can be in either the protected or unprotected header bucket. The 
trust mechanism MUST process any certificates in the c5bag, c5chain, and c5u 
parameters as untrusted input. The presence of a self-signed certificate in the 
parameter MUST NOT cause the update of the set of trust anchors without some 
out-of-band confirmation.

Note that certificates can also be identified with a 'kid' header parameter by 
storing 'kid' and the associated bag or chain in a dictionary.

+-----------+-------+----------------+------------------------------+
| Name      | Label | Value Type     | Description                  |
+===========+=======+================+==============================+
| c5bag     |  TBD1 | COSE_X509      | An unordered bag of CBOR     |
|           |       |                | certificates                 |
+-----------+-------+----------------+------------------------------+
| c5chain   |  TBD2 | COSE_X509      | An ordered chain of CBOR     |
|           |       |                | certificates                 |
+-----------+-------+----------------+------------------------------+
| c5t       |  TBD3 | COSE_CertHash  | Hash of a CBOR certificate   |
+-----------+-------+----------------+------------------------------+
| c5u       |  TBD4 | uri            | URI pointing to a bag/chain  |
|           |       |                | of CBOR certificates         |
+-----------+-------+----------------+------------------------------+

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