Dan Harkins <> scribbled on Saturday, February 02, 2008 8:46 AM:
Hello again,
Pardon my repetition but I have come up with a very valid reason
why naming keys using HMAC-SHA-256 is a bad idea.
If one wants to administratively remove all keys in a particular
key hierarchy (which seems like an entirely reasonable request)
It doesn't seem particularly reasonable to me. The one reason I can
think of for this is to disable access for a particular user in some
domain; to do that it doesn't seem necessary to explicitly remove all
the keys in the hierarchy, just the root key (after disconnecting the
user). Disconnecting the user should delete the PMK, TSKs, etc., right?
one
must do a linear search of all keys in all key hierarchies that a
particular server maintains!
all identifying key information for all key hierarchies to the same
name space. It precludes using something like a hash table for fast
lookup of all related keys.
If keys were named by concatenating the EAP Session-ID with a
string that identified the particular key in the key hierarchy rooted
at the MK derived in that EAP Session-ID then the EAP Session-ID
could be used as an index into a hash table and all keys for that
particular key hierarchy could be looked up very efficiently.
I won't argue that indexing the keystore by Session-ID is a bad idea
(though Username might be better), but I can't see how that depends upon
the actual key name.
regards,
Dan.
On Fri, February 1, 2008 5:16 pm, Dan Harkins wrote:
Hello,
I believe this is a well organized and complete document. On
numerous occasions while reviewing it I made a mental question
regarding something only to have the question answered in a
subsequent paragraph.
I do have several comments though:
1. this protocol can be used in the presence of AAA proxies. Due
to the nature of AAA proxies a peer or authenticator may not
even know whether they are part of the communication chain.
Therefore, from the view of a security threat their presence
must be assumed by the peer and authenticator.
The Domain referred to in section 2 (part of the EMSK key
hierarchy draft) specifically allows for proxies as part of
the distributed system of computers that define the Domain.
This brings up many significant issues that are not addressed
in this draft.
- It cannot be claimed that a key is being bound to its
context when the context cannot even be scoped. (Section 6)
- The domino effect is not prevented because compromise of a
proxy will compromise keying material on (other)
authenticators.
- A pairwise key is being given by one of the entities that share
it, e.g. the server, to a 3rd entity, e.g. a proxy, without the
consent of the other peer that shares the key, e.g. the peer.
This brings up security considerations that are not discussed.
- During discussions at a HOKEY meeting and on the list the
rationale that justified proxies was that the peer is more
concerned about receiving network access (which is confirmed
in the ERP document when it says, "The primary purpose [of
ERP] is network access control.") than about specifically
authenticating "the network". Provided that service is
obtained with no surprises in the bill at the end of the month
the rationale was that the peer didn't care if the key was
distributed to proxies if it was necessary to continue to
provide network access. Which is a reasonable rationale. But
it needs to be mentioned in this document. It has a unique
threat model that is not discussed anywhere.
- The aforementioned rationale begs the question of why have
"Domain Specific Keys". If the peer doesn't care whether
proxies have a key, potentially for a different domain, then
it doesn't care about key separation between domains. This is
significant added complexity for no benefit.
- RFC4962 REQUIRES things-- bind key to its context, prevent the
domino effect-- that ERP cannot support. ERP is a AAA key
management protocol though and falls under the scope of
RFC4962. There needs to be justification for why ERP is not
meeting the mandatory requirements of RFC4962.
I think all of these issues need addressing before advancement of
this Internet-Draft.
2. Inter-Domain ERP
It is this reviewers recollection that consensus was reached in
HOKEY to require a peer to reauthenticate back to the home AAA
server every time it attached to a POP in different domain.
Therefore, I wonder why a "Domain-Specific" key, the DSRK, and all
it's progeny-- DS-rIK, DS-rRK, DSUSRK, etc.-- continue to be used
by this protocol. A "HOKEY-KEY", a USRK, should be derived from
the EMSK and that is the key given, through proxies if need be, to
the ER server in the visited domain. If the peer goes to a
different domain then it does a full reauthentication resulting in
a _new_ USRK, that has no relation to the previous USRK, being
given to the ER server in the new domain. Again, it was my
understanding that the group already reached consensus on this
matter.
3. HMAC-SHA256 as a key naming technique
SHA-256 is a computationally intensive operation; HMAC-SHA256
doubly so. There should, therefore, be some justification to use
such a strong cryptographic mixing function if all one wants to do
is "uniquely name a key". EAP methods export a Session-ID. An rIK
can be named by the concatenation of Session-ID and "rIK".
Similarly for the rMSK, rRK and the other keys being generated in
ERP.
This has the added benefit of allowing for key management to
quickly identify keys based on common queries-- all the keys for a
specific Session-ID, or all rIKs held by a particular entity. By
using a strong cryptographic mixing function all specificity of
the key names has been lost across every single key hierarchy that
a HOKEY server may end up managing.
This is a really bad idea and it should be changed before this
Internet-Draft is advanced.
4. Section 5.3.2 EAP-Initiate/Re-Auth Packet
This packet has the following field:
+-+-+-+-+-+-+-+-+
|R|B|L| Reserved|
+-+-+-+-+-+-+-+-+
And "R" is, itself, reserved. This makes no sense. Please << 1
this field.
regards,
Dan.
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