David Wierbowski writes:
> I agree with Tero that Yoav's proposed text adds clarity and effectively it
> does not add a new MUST; however, to address Paul's concern can we just
> change the words "MUST be" to the word "are" or lower case "should be?"
> For example:
> 
>    o  X.509 Certificate - Signature (4) contains a DER encoded X.509
>       certificate whose public key is used to validate the sender's AUTH
>       payload. Note that with this encoding if a chain of certificates
>       needs to be sent, multiple CERT payloads are used, only the
>       first of which holds the public key used to validate the sender's
>       AUTH payload.

This text looks good for me. And I think it is important to add this
to X.509 Certificate - Signature (4) case, even when we have some
generic text elsewhere saying same thing, as this is the most common
case people are using now. 

> As Tero points out, this is the only way to send a chain this using X.509
> Certificate - Signature.encoding.  MUST terminology really is not needed in
> my opinion.

Agreed.

I think in addition to that text we might want to add some generic
text to the final paragraph saying:

   Some of the certificate formats can only contain one certificate
   (for example formats 4, 7, 11 and 12), and some can contain
   multiple certificates (for example 1, 13). In case those formats
   which contain one certificate are used and multiple certificates
   are to be sent then each certificate are sent as separate
   Certificate Payload.

Or add similar text than what is in the 3.7 Certificate Request
Payload which have following sentence at the end of first paragraph
"If multiple CAs are trusted and the certificate encoding does not
allow a list, then multiple Certificate Request payloads would need to
be transmitted."

One more thing, do we want to explictly mention that it is very common
to mix multiple types of certificate payloads, i.e. send certificate
multiple payloads some having X.509 Certificate - Signature (4) format
and some having Certificate Revocation List (7) format.

Another combination could be Raw RSA Key (11) and X.509 Certificate -
Signature (4). In that case the Raw RSA Key (11) format is meant for
minimal implementations who have the raw RSA key of the other end
statically configured to their policy, and the X.509 Certificate -
Signature (4) format is meant for full implementations which can
process and validate certificates.

Note also that not all certificates are there to form a chain, they
can also provide other things like CRLs or even multiple different
chains towards the different CAs (in case the private/public key use
has certificates from multiple CAs, and other end didn't indicate any
known CA), so the extra certificate payloads (after the first one used
to sign the AUTH payload) are there just to help validation of the
first certificate, not necessarely to form a chain. 
-- 
kivi...@iki.fi
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