Nate
On Aug 2, 2012 10:24 PM, "Adam Young" <ayoung@xxxxxxxxxx
<mailto:ayoung@xxxxxxxxxx>> wrote:
On 08/01/2012 11:05 PM, Maru Newby wrote:
Hi Adam,
I apologize if my questions were answered before. I wasn't aware
that what I perceive as a very serious security concern was
openly discussed. The arguments against revocation support, as
you've described them, seem to be:
- it's complicated/messy/expensive to implement and/or execute
- Kerberos doesn't need it, so why would we?
I'm not sure why either of these arguments would justify the
potential security hole that a lack of revocation represents, but
I suppose a 'short enough' token lifespan could minimize that
hole. But how short a span are you suggesting as being acceptable?
The delay between when a user's access permissions change
(whether roles, password or even account deactivation) and when
the ticket reflects that change is my concern. The default in
Keystone has been 24h, which is clearly too long. Something on
the order of 5 minutes would be ideal, but then ticket issuance
could become the bottleneck. Validity that's much longer could
be a real problem, though. Maybe not at the cloud administration
level, but for a given project I can imagine someone being fired
and their access being revoked. How long is an acceptable period
for that ticket to still be valid? How much damage could be done
by someone who should no longer have access to an account if
their access cannot be revoked, by anyone, at all?
I realize that I had been thinking about the revocation list as
something that needs to be broadcast. This is certainly not the case.
A much better approach would be for the Keystone server to have a
list of revoked tokens exposed in an URL. Then, as service like
Glance or Nova can query the Revocation list on a simple
schedule. The time out would be configurable, of course.
There is a question about what to do if the keystone server cannot
be reached during that interval. Since the current behavior is
for authentication to fail, I suppose we would continue doing
that, but also wait a random amount of time and then requery the
Keystone server.
In the future, I would like to make the set of Keystone servers a
configurable list, and the policy for revocation checking should
be able to vary per server: some Keystone servers in a federated
approach might not be accessible. In those cases, it might be
necessary for one Keystone server to proxy the revocation list for
another server.
Let me know if this scheme makes sense to you. If so, we can
write it up as an additional blueprint. It should not be that
hard to implement.
I'm hearing that you, as the implementer of this feature, don't
consider the lack of revocation to be an issue. What am I
missing? Is support for revocation so repugnant that the
potential security hole is preferable? I can see that from a
developer's perspective, but I don't understand why someone
deploying Keystone wouldn't avoid PKI tokens until revocation
support became available.
Thanks,
Maru
On 2012-08-01, at 9:47 PM, Adam Young wrote:
On 08/01/2012 09:19 PM, Maru Newby wrote:
I see that support for PKI Signed Tokens has been added to
Keystone without support for token revocation. I tried to
raise this issue on the bug report:
https://bugs.launchpad.net/keystone/+bug/1003962/comments/4
And the review:
https://review.openstack.org/#/c/7754/
I'm curious as to whether anybody shares my concern and if
there is a specific reason why nobody responded to my question
as to why revocation is not required for this new token scheme.
Anybody?
It was discussed back when I wrote the Blueprint. While it is
possible to do revocations with PKI, it is expensive and
requires a lot of extra checking. Revocation is a policy
decision, and the assumption is that people that are going to
use PKI tokens are comfortable with out revocation. Kerberos
service tickets have the same limitation, and Kerberos has been
in deployment that way for close to 25 years.
Assuming that PKI ticket lifespan is short enough, revocation
should not be required. What will be tricky is to balance the
needs of long lived tokens (delayed operations, long running
operations) against the needs for reasonable token timeout.
PKI Token revocation would look like CRLs in the Certificate
world. While they are used, they are clunky. Each time a token
gets revoked, a blast message would have to go out to all
registered parties informing them of the revocation. Keystone
does not yet have a message queue interface, so doing that is
prohibitive in the first implementation.
Note that users can get disabled, and token chaining will no
longer work: you won't be able to use a token to get a new
token from Keystone.
Thanks,
Maru
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