On 4/16/2013 6:51 PM, Burns, Robert
wrote:
Mike,
Firstly, the word 'opaque' means that the token has no requirement
to decode/dissect/encode/interpret the data stored in that object.
Your descriptions require both encoding and decoding of this data
-- that is the opposite of 'opaque'. So no, as you've described
it,
this field is not opaque.
I said that CKA_VALUE on an X509 cert object was opaque. I stand by
that statement. There is no requirement in PKCS11 to enforce that
this object actually parses as X509 (or WTLS or X509 attribute cert,
etc, or maps to a private key).
I was NOT referring to CKA_PUBLIC_KEY_INFO. But as both Peter and I
have said, in most of the RSA cases, and in all of the ECDSA cases,
both the parsing of such DER encoding and the encoding of such data
are fairly trivial.
Secondly, this attribute is on ALL public and private key objects,
not just RSA -- so a use case for RSA rings hollow.
Still don't quite understand this. For EC private keys, you can
*always* generate the public key. I don't know if this is the case
for DH keys, for DSA keys or for GOST keys - I just know that there
is at least one form of RSA private key where you can't regenerate
the public key. And if this attribute is going to be implemented
for C_CreateObject for private keys, you need to insure coherence
between the private key and the public key. Why is this such a
difficult concept?
Thirdly, if the APPLICATION has the subject public key info data,
it can itself decode it and provide the necessary public
attributes
during C_CreateObject() -- why is this the token's responsibility?
This doesn't make sense. Furthermore, if the application does not
have the relevant fields, then too bad so sad -- they can't create
the object then.
Because the entity that stores the key, may not be the entity that
retrieves the key. If the token doesn't understand the
CKA_PUBLIC_KEY_INFO attribute for C_CreateObject for private keys
then it throws an error. If it does understand the attribute, then
it has to do minimal validation that the data in this attribute
matches the private key.
Seems like we're hacking P11 to support poorly constructed P11
applications, and that just seems like the wrong approach to
me.
Nope. P11 does NOT require/enforce that you always have both the
public and private key objects on the token. We've already talked
about the whole CKA_ID and CKA_LABEL mess where it may actually not
be possible to relate the the public and private keys that DO exist
to each other and to a certificate.
The whole idea here is to place key pair information on the private
key. It probably should have been there all along, but I can't fix
2.20 and before.
Later, Mike
Bob
-----Original Message-----
From: Michael StJohns [msj@nthpermutation.com]
Sent: Tuesday, April 16, 2013 03:31 PM Eastern Standard
Time
To: Burns, Robert
Cc: 'Peter Gutmann'; 'pkcs11@lists.oasis-open.org'
Subject: Re: [pkcs11] CKA_PUBLIC_KEY_INFO
On 4/16/2013 11:24 AM, Burns, Robert
wrote:
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?
You're missing the case where the private key is generated off
token, and the form of the private bits does not include
sufficient
information to recover the public bits. (The one that springs
to mind is the { n, d } form of an RSA private key).
Here's what the decision tree for a C_CreateObject for an RSA
private key looks like:
if (CKAPUBLICKEYINFO_IS_SUPPORTED) {
switch (keyformat) {
case RSA_N_D_FORM:
if (attributes include CKA_PUBLIC_KEY_INFO)
{
-- verify
the data represents a valid RSA public key
-- verify
this public key matches the private key represented by
CKA_MODULUS,
--
CKA_PRIVATE_EXPONENT on this call
-- save the
opaque CKA_PUBLIC_KEY_INFO data for later use
} else { // no public key data provided
-- set the
value of CKA_PUBLIC_KEY_INFO for this private key to zero
length
}
break;
case RSA_N_D_P_Q_FORM:
case RSA_CRT_FORM:
if (attributes include CKA_PUBLIC_KEY_INFO) {
-- verify
this is a valid RSA public key
-- verify
this public key matches the private key represented by
CKA_MODULUS,
--
CKA_PRIVATE_EXPONENT, P, Q and possibly the CRT data
-- save the
opaque CKA_PUBLIC_KEY_INFO data for later use
} else { // no public key data provided
-- derive
the public key from the private key data (e.g. p and q)
provided
-- encode it
as a DER SubjectPublicKeyInfo
-- set
CKA_PUBLIC_KEY_INFO for this private key to that value
}
break;
}
}
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?
You do a C_GetAttributeValue for CKA_PUBLIC_KEY_INFO to get that
value.
Then you do a C_CreateObject for a public key using
CKA_PUBLIC_KEY_INFO in the template
and THEN you do a signature verification using that brand new
public 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.
Nope. The public key info on the private key is just data on
the private key. It is not a key. It has the
permissions appropriate to data associated with a private key (and
I'm ok if that data is Private - I think it should be public
though
ala CKA_LABEL and CKA_ID).
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?
There's also the case where someone accidentally deletes the
public
key by accident (e.g. I didn't need the public key because I have
a
certificate - what do you mean I don't have a certificate???)
... it would really be nice if your private key were still
useful.
Later, Mike
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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