Re: review of draft-wierenga-ietf-sasl-saml-00

[Klaas Wierenga <[e-mail]>]

On 5/26/10 8:28 PM, Sam Hartman wrote:

Sam,

Just to understand, what does channel binding buy you if you still can 
not prevent the MiTM? The SAML interactions can:

- give you a return url to the Relying Party and a 'transaction id'
- have you authenticate at your IdP
- have the IdP verify that the RP is a 'trusted' one
- have the (trusted) SP verify that it indeed issued that authentication 
request

Klaas

>>>>>> "Klaas" == Klaas Wierenga<[e-mail]>  writes:
>
>      Klaas>  right, so the server would have a list of realm to idp-url
>      Klaas>  mappings?
>
> In order for the proposal I made to work, the following must be true:
>
> 1) The IDP MUST verify that channel binding data is asserted by the
> expected RP and has been modified by no other party
>
> 2) The client MUST verify that the channel binding data corresponds to
> the channel that is in use.
>
> Those two are enough that the user will learn through the web browser if
> the connection is under attack.  However, the that's not enough to
> defend against a server that fakes successful authentication or to learn
> at the SASL protocol level about a channel binding failure.
>
> 3)  The client inserts a cookie into the return URL
>
> 4) The server or agents under the control of the server not see this
> cookie before succesful authentication
>
> 5) The server returns the cookie to the client.
>
> I'm not actually sure 3 is possible: I'm not sure whether the return URL
> is covered by the signature.  Assuming it's possible, 4 means that the
> client must be able to verify that the IDP URL it goes to is one that it
> trusts.  So, ultimately, the server can pick however you like, but it
> MUST pick from a list of *URLs* that the client trusts.  Anything like
> the server mapping an IDP hint to a URL destroys the security property.
>
> Note that it's possible to have multiple modes of operation with
> different security properties.  That creates UI challenges, but that is
> potentially doable.