James said:
>> I would also appreciate your thoughts on whether someone can extend JWS by 
>> specifying: when the “zip” field is present in a JWS the content is 
>> compressed.
That looks legitimate with the current “MUST understand or reject” rule. I 
think it is likely, however, that such an extension will cause security 
problems. Some implementations will confirm that “zip” is on the list of known 
standard fields, but will ignore it in the JWS processing code.

Mike said:
> One would define “zip” in a JWS in the normal way that extensions are made to 
> IETF specs – you’d write a spec defining the meaning for it and register the 
> “zip” value in the IANA JSON Web Signature and Encryption Header Parameters 
> registry for JWS usage.


Lets ignore the process for now (IETF, IANA…), the issue is whether this style 
of extension is sensible: an extension that indicates a change of semantics by 
the presence of a new field. A style that the “MUST understand everything” rule 
encourages.
My argument is that this style is too dangerous a way to indicate new 
semantics. When changing semantics (as “zip” does) it would be much better to 
change a field we are confident implementations will be looking at, as opposed 
to relying on them to notice a new field.

--
James Manger
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