Two Identifiers - no caching advantage

[Dick Hardt]

On 19-Oct-06, at 11:12 AM, Josh Hoyt wrote:

> On 10/19/06, Dick Hardt <dick at sxip.com> wrote:
>> > Your attack fails.
>>
>> <sigh> reread the attack. The portable identifier and the IdP do  
>> match.
>
> No the identifiers do not.

They do. The attacker goes to the RP and enters my blog URL. The  
attacker changes the request from the RP so that the IdP-specific  
identifier is what the attacker used in the past. The IdP has an  
incorrect mapping between the attackers IdP-specific identifier and  
the public identifier. The response then has the portable identifier  
(my blog URL) and the attackers IdP-specific identifier. The RP  
verifies (per the spec) that the portable identifier is bound to the  
IdP, and the attacker has successfully logged in as me.

There are two issues here:

	1) the IdP did not verify the binding between the portable  
identifier and the IdP-specific identifier
	2) the RP does not verify the binding between the portable  
identifier and the IdP-specific identifier in the response.
  to the one the attacker controls and the IdP has mapped

The logic presented previously was that the RP is doing discovery and  
that it can save the IdP work. Unfortunately, the discovery the RP  
does of the binding between the portable identifier and the IdP  
specific identifier is useless to the IdP since the message can be  
modified.

-- Dick