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Subject: RE: [pkcs11] CKA_PUBLIC_KEY_INFO
- From: "Burns, Robert" <Robert.Burns@thalesesec.com>
- To: 'Peter Gutmann' <pgut001@cs.auckland.ac.nz>, "'msj@nthpermutation.com'" <msj@nthpermutation.com>
- Date: Tue, 16 Apr 2013 11:24:54 -0400
Title: RE: [pkcs11] CKA_PUBLIC_KEY_INFO
Peter,
Sorry for the late response -- traveling.
I understand now where you're coming from.
So a couple of follow up statements and then perhaps this will be a
discussion that needs to happen 'out-of-band'?
I'm having difficulty imagining other use cases where a user will
only have access to the private key, yet needs access to the public
bits as well. I do understand your experiences with tokens which
fit this mold, but the error (it seems) is in the fact that the
token distributors were not also including the public key object as
well? Seems like we're hacking P11 to make up for the deficiencies
in how someone decided to deploy their tokens. Again, I don't doubt
that this is a real problem that you're facing, just not sure if
this is one which P11 needs to solve?
Secondly, I'm still feeling like adding SPKI to a private key
object is not keeping within the spirit of P11 objects -- the
public key bits should be on a separate object (yes, I know RSA is
the exception in this case, but it is only a single attribute
(public exp) and relatively small). Putting in a marshaled form of
the public key bits is relatively straight forward, but introduces
ambiguities, etc. For example, how much time will pass before
people start asking for the ability to do signature verifications
using the SPKI info on a key?
One of the purposes of having two unique objects in P11 for
asymmetric keys was to be able to support different permission
attributes, as well as different user ACLs. By shoving everything
(even marshaled and 'opaque') onto a private object introduces a
number of *potential* ambiguities which will become more difficult
to manage going forward.
I definitely understand where you're coming from, and I'm trying to
take a pragmatic view, but at some point we do have to honor the
P11 model or else we end up with an unmanageable specification.
So is the problem really that there are definite cryptographic
reasons for needing the public key attributes on a private key, or
is this just a convenient way to solve the problem that token
vendors have introduced by not providing enough objects on their
tokens?
Thanks,
Bob
-----Original Message-----
From: Peter Gutmann [pgut001@cs.auckland.ac.nz]
Sent: Saturday, April 13, 2013 01:44 AM Eastern
Standard Time
To: msj@nthpermutation.com; pgut001@cs.auckland.ac.nz;
Burns, Robert
Cc: pkcs11@lists.oasis-open.org
Subject: RE: [pkcs11] CKA_PUBLIC_KEY_INFO
"Burns, Robert"
<Robert.Burns@thalesesec.com> writes:
>You are proposing this mechanism to fix the issue of non-RSA
keys not having
>the appropriate public bits, rather than the mechanism for
tying the keys
>together with a cert? (e.g. the ECDSA private key
issue...)
Absolutely. The "tying keys together" seems to have gotten
added later, but
it was never the original intent, which was to fix the problem that
if you
have a token with a private key object that's anything but RSA then
you pretty
much can't use it because you can't get the public key for
it. The most
obvious example of this is that you can't get a certificate for the
key
because the CSR requires the public-key components.
>If my interpretation is correct, shouldn't this be solved
similar to how the
>RSA private key handles it? That is, but requiring the
public key attributes
>on the object too?
Yes, but it evolved over time:
Step 1: Slightly abuse the derive functionality to get a public-key
object
from the private-key object.
Objection: It's a bit of a misuse of derive, and in any case we
don't need all
that, just the public components.
Step 2: Add public-key values to private-key objects.
Objection: Since the only real need for them is as
subjectPublicKeyInfo for
certificates, why not just return the SPKI directly?
Step 3: Add SPKI as an attribute.
>Finally, tangent to our DER discussion, using these public key
blobs on the
>private keys would then REQUIRE all tokens be able to DER
decode them to make
>effective use of the public bits, contradicting the assertion
that most
>tokens won't need to DER decode anything.
There's no need to decode them since the token never uses them,
they're there
purely for the convenience of PKCS #11-using applications. In
fact there's no
need to store them at all, you just generate the SPKI on the fly
from whatever
public-key data you have in the token.
Peter.
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