[OpenAFS-devel] Re: openafs - proposed cache security improvement

[Jeffrey Altman]

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Sean O'Malley wrote:
> I was also thinking that you could hack kaserver to store client keys,

The Kerberos database already stores client keys.  Those keys are what
are used to authenticate the user.  A new session key known only to the
AFS servers and the user is then obtained from Kerberos and that is used
to authenticated and protect the non-authenticated connections between
the cache manager and the AFS servers.

That is how things are done today.  The problem today is that there is
no key available for anonymous connections.

> and transport encryption keys. It could store the client public user key
> to match it with the host key and an encryption key. (and of course put a
> TTL on those keys so they can be cleaned up periodically, and for
> security.) Which does require another server, but kaserver would
> just need to be modified. (well okay, it probably needs to be completely
> overhauled, but not for a prototype.)

There is no need to add public key cryptography in place of Kerberos.
Kerberos already does what we need for authenticated connections.

> Thus I have offered more confusion. :)

You are doing a good job of it.  :)

----

There are two different sets of communications that you need to be
concerned with.  First there is the communication path between the
application and the AFS cache manager.  This is not authenticated by
Kerberos but instead is a local system authentication.  This
communication path is then used to store a token into the cache manager.
 The token is used to perform authentication with the AFS servers for
requests on behalf of the user.  When the token is stored it is bound to
the local user identity within the cache manager.

The cache manager keeps a shared copy of all file/directory status and
content in the cache for all users.  Part of the information that is
cached are the ACLs.

If you want to protect the content of the cache, then it must be
encrypted with a key that is known not to any user but to the cache
manager.  Hence, my recommendation that you turn on encryption of the
cache directory.  Doing so will ensure that only the account under which
the AFS Cache Manager runs will be able to read or write data from the
cache.

The AFS Cache Manager is disconnected mode (when that is available) will
locally restrict access to files based upon the AFS ID and the directory
ACLs.  Only users that are permitted to access the data based upon its
last known state would be permitted to do so.  You don't need additional
keys for this.  If your local system authentication is strong, then only
the locally authenticated user will be able to communicate with the
cache manager using the identifiers for the user that are seen by the
cache manager.

Without a valid token, the cache manager will not be able to write data
back to the server on behalf of the user.  That is a limitation that
disconnected mode will have to address.

Jeffrey Altman
Secure Endpoints Inc.


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