Questions about signing key pair in IdP

[Tom Scavo]

On Tue, Jul 10, 2012 at 9:55 AM, Cantor, Scott <cantor.2 at osu.edu> wrote:
>> As I understand from the above article, the key pair for signing and the
>> key pair for the SSL/TLS on port 8443 MUST always be the same and this
>> single public key is published through <md:KeyDescriptor use="signing">
>> in metadata, right?
>
> No, not at all. The keys are what you want them to be. The only requirement is to document them in the metadata you provide to SPs.

I think this needs further clarification. Suppose you have a key
descriptor with use="signing" in the IDPSSODescriptor element and
another key descriptor with use="signing" in the
AttributeAuthorityDescriptor element. If the two certificates are the
same, then the same key pair is used for everything:

- message signing on the front channel
- message signing on the back channel (if the IdP supports that)
- SSL/TLS on the back channel

If, OTOH, the two certificates are different, then key pair #2 is used for:

- message signing in conjunction with attribute query (if the IdP supports that)
- SSL/TLS in conjunction with artifact query

while key pair #1 is used for everything else, including message
signing on the front channel, message signing in conjunction with the
artifact binding, and SSL/TLS in conjunction with the artifact
binding.

That said, I've never seen two different use="signing" certificates in
IdP metadata, which is not surprising since such a deployment would be
very complicated. Likewise, I've never seen a use="encryption"
certificate in IdP metadata since I don't believe there's anything an
SP could send an IdP that could be encrypted. Oh, I take that back, an
SP could send an encrypted NameID in the AuthnRequest...not sure if
there's any software that supports this edge case, however.

Tom