RE: "binding" assertions to business payloads

["Mishra, Prateek" <pmishra@netegrity.com>]

>>2) Alice gets the PO countersigned by a party that has (largely)
>>unrestricted signature powers. [Probably a cryptographic 
>>signing unit in a
>>secure facility].
>>
>>At this point what is effectively being created is an 
>>authorized purchase
>>order.
 
	[PM] I am uncomfortable with the term "authorized purchase
	order". No action has been authorized; in my example, Ravi obtained
	an assertion that described his employment status and classification
	from an asserting party. It is up to the message recipient to
authorize
	any action based on the business payload.  



>>The Authorization statement then wants to either 1) restate the
>>original 'Alice' purchase order or 2) cryptographically bind 
>>to it, e.g. via
>>a MAC.
>>
>>Case (1) is out of scope since it is simply another purchase order.

	[PM] I generally agree that case (1) takes us into somewhat
different
     territory. The bindings group charter is focussed on packaging 
     SAML assertions carried with standard formats such as SOAP, MIME,...

>>
>>Case (2) is potentially in scope and certainly merits a proof of
>>extendability.
>>
>>I think the case implies that the <Subject> of the 
>>authorization would be
>>the purchase order. 

   [PM] I disagree with this statement. The subject of the assertion
obtained
   by Ravi thru an authentication step is some form of "name" for Ravi.
There is a
   need for a link between the assertion to the "action" requested by Ravi
but 
   it would appear to be a distinct field within the assertion.


>>The consistent means of specification 
>>would be as a link
>>with a cryptographic digest - perhaps reusing the XML 
>>signature manifest
>>package.

   [PM] I am broadly in agreement here, except that I would need to
   get a better feeling for the details of this design.

   One concern here, is that the assertion may be unnecessarily over
identified 
   with the business payload. In my example below, the assertion obtained by
   Ravi did not have any dependency on a specific payload. Instead, it
   had a (proposed) dependency on Ravi's public key. It could therefore
   be re-used by Ravi and packaged together with many different business
   messages.


>>
>>Alternatively since the entire assertion is signed the 
>>Authorization could
>>restate the purchase order as an advice element.
>>

   [PM] This approach would present difficulties in the general case 
   when the payload is something other than an XML fragment, for example, 
   a MIME package. 


- prateek 


>>
>>
>>> -----Original Message-----
>>> From: Mishra, Prateek [mailto:pmishra@netegrity.com]
>>> Sent: Sunday, May 06, 2001 9:38 PM
>>> To: 'Philip Hallam-Baker '; ''oasis sstc' '
>>> Cc: 'security-bindings@lists.oasis-open.org'
>>> Subject: "binding" assertions to business payloads
>>> 
>>> 
>>> One issue discussed in the bindings group con-call
>>> this past week is that of providing an authentic binding between
>>> assertions and business payloads. The following scenario
>>> is driven off use-case 3 from the requirements document:
>>> 
>>> End-user Ravi utilizes an authentication service
>>> and obtains assertions ("Ravi is an employee of 
>>> Company X at the supervisor level")
>>> which he packages with a business
>>> payload ("Buy 100 paper clips") to create a composite package. 
>>> One way to ensure authenticity is to have Ravi sign the composite
>>> package with his private key. When the recipient receives the
>>> package, the recipient can check to see if the package has
>>> been tampered with. If not, the recipient can scrutinize the
>>> assertions and appropriately process the business payload.
>>>  
>>> The concern here is with a MITM attack: attacker John intercepts
>>> the package and packages the assertion with a completely
>>> different payload ("Buy 1000 computers"), signs the composite
>>> package and forwards it to the recipient.  
>>> 
>>> In order to avoid an MITM attack, it seems necessary that the
>>> assertion should include Ravi's public key (or more generally
>>> <dsig:keyinfo>) within the assertion. When the composite
>>> package arrives at the recipient, she can validate 
>>> the signature using the public key found within the
>>> assertion. This ensures that only the
>>> original ``holder'' of the assertion could have bound it to
>>> the payload. 
>>> 
>>> It is not clear to me how this is handled
>>> within the current assertion structure. Obviously,
>>> this issue is fairly key to the bindings group; some
>>> clarification is required in this space.
>>> 
>>> 
>>> - prateek
>>> 
>>> 
>>> 
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>>
>>