Re: [Ace] Group Communication Security Disagreements

[Michael StJohns <[e-mail]>]

On 9/16/2016 6:03 AM, Eliot Lear wrote:
>
> Hi Mike,
>
>
> On 9/15/16 5:36 PM, Michael StJohns wrote:
>> Hi Elliot et al -
>>
>> Sorry, I think you're still missing the point:
>>
>>   * Source Authentication (A) cannot be accomplished securely by
>>     Symmetric Key Multicast  (^B):   (A -> ^B)
>>   * Cyber Physical control functions (C)  require source
>>     authentication  (A):  (C -> A)
>>   * Turning on and off lights (D) is a Cyber Physical Control
>>     Function (C):  (D -> C)
>>   * Therefore Turning on and off lights (D) requires source
>>     authentication (B):  (D -> C -> A) (D -> A)
>>   * Therefore Turning on and off lights (D) cannot be accomplished
>>     securely by Symmetric Key Multicast (^B):  (D -> C) ( C -> A) (D
>>     -> C -> A) ( D->A) (A -> ^B) (D -> ^B).
>>
>> Apologies if I got the formal logic wrong - its been a while.
>>
> All of this seems about right to me, but with two big caveats:
>
>  1. We are, I think, talking about group-based communications and not
>     necessarily group-based authorization for device control.  There
>     is a difference, albeit subtle.  One could reasonably envision
>     borrowing from lower layers to satisfy device authorization
>     requirements.
>
Sorry - we've really been talking about using symmetric key multicast 
for control functions.  (cf all the discussions on lighting control and 
the ever reducing maximum latency requirement).   Authorization is 
really about whether or not a given entity accepts and acts on a given 
authenticated message.  Trying to parse this as group comms (which I 
took you to mean "secure group comms") vs group authorization is really 
a red herring here as you can't have meaningful authorization without 
meaningful authentication and so far, we haven't gotten that far.



>  1. The question here is whether this is the right level to address
>     the problem.  And I'll ask my clarifying question again: is there
>     a more logical place to anchor identity, like above or below this
>     layer?
>

This is mostly an irrelevant question at this point.  Once we agree that 
symmetric key multiparty (N>2 or 3 in the case of a kerberos system) 
authentication is off the table, then talking about which level to put 
this reduces down to either per application  or per device and the 
associated protocols for each of those.  It's entirely possible a 
message might be authenticated by credentials at multiple protocol 
layers depending on who owns what in the mesh.

Later, Mike

> Eliot