RE: Open Issue in Part1: path length constraints

["David P. Kemp" <dpkemp@xxxxxxxxxxxxxx>]

Steve,

I've never seen any benefit of the cA flag in the basicConstraints
extension; it's just a vestigial appendix left over from when there
was no keyUsage extension.

Consider the following certificates:

#  cA   CertSign  cRLSign    Effect
- ----- --------  -------   ------------
0  F     0         0        End Entity
1  F     0         1        End Entity (can sign CRLs)
2  F     1         0        End Entity
3  F     1         1        End Entity (can sign CRLs)
4  T     0         0    (Illegal*) Is a CA but can't sign certs or CRLs
5  T     0         1    (Illegal*) Is a CA, can sign CRLs only
6  T     1         0        CA, can sign certs
7  T     1         1        CA, can sign certs and CRLs

* As Dave S. points out, 4.2.1.10 prohibits cert types 4 and 5.

Is there any benefit to having eight different certificate types
instead of just four?   In my example of the online CRL signer, what is
accomplished by setting CA=true (cert #5) instead of CA=false (cert #1)?
You say you would set it to indicate that the CRL issuer is a CA, but
what would the application do with that knowledge? (i.e. what would it
do differently than if the CA flag were false?)

More generally, is there any reason why Section 4.2.1.10 should
not also prohibit cert types 2 and 3? (in other words, require the
keyCertSign bit to always equal the cA flag?).  

Dave



> From: Stephen Kent <kent@xxxxxxx>
> 
> I agree that there is ambiguity in what we mean when we refer to a CA 
> in such circumstances, but algorithmically I have always assumed that 
> only CAs issued CRLs and we can tell whether the entity is a CA by 
> the basic constraints extension. I see there is considerable 
> sentiment to allow for non-CA flagged entities to sign CRLs, but I'm 
> not yet sure I understand why folks consider it important to not turn 
> on the CA flag in certs for such entities. After all, since we have 
> separate key usage bits for cert and CRL signing, we can construct a 
> cert for an entity that signs CRLs and not grant that entity the 
> ability to sign certs, if we so desire.