Your restatement of my point misses the point entirely:
IF there is a signature that the recipient can use
THEN the recpient should know that there is such a signature
If this condition is not met an attacker can perform upgrade and downgrade
attacks in which the attacker attaches a bogus signature.
-----Original Message-----
From: Bill(_dot_)Oxley(_at_)cox(_dot_)com
[mailto:Bill(_dot_)Oxley(_at_)cox(_dot_)com]
Sent: Tuesday, October 24, 2006 9:39 AM
To: Hallam-Baker, Phillip; stephen(_dot_)farrell(_at_)cs(_dot_)tcd(_dot_)ie
Cc: ietf-dkim(_at_)mipassoc(_dot_)org
Subject: RE: [ietf-dkim] Comments on
draft-ietf-dkim-ssp-requirements-02.txt
Phillip,
"since we will not be specifying compromised algorithms
initially the only code path that requires implementation is
what to do when you have a signature present that you do not
support. We absolutely do need to have a mechanism that
allows the verifier to know that it should expect there to
also be a signature that it can use."
We need to have a policy record that indicates what the
receiptient should expect. Im not sure of the requirement
that there be a signature that the receiptient can use.
thanks,
Bill
-----Original Message-----
From: ietf-dkim-bounces(_at_)mipassoc(_dot_)org on behalf of
Hallam-Baker, Phillip
Sent: Mon 10/23/2006 5:29 PM
To: Stephen Farrell
Cc: ietf-dkim(_at_)mipassoc(_dot_)org
Subject: RE: [ietf-dkim] Comments on
draft-ietf-dkim-ssp-requirements-02.txt
I don't think that we are re-opening anything here.
I am not proposing to re-open base or to place any algorithm
identifiers in the policy record. In fact quite the opposite.
My reading of the requirements as written is that they
suggest that the policy record should enumerate the
acceptable signature algorithms. That is the opposite of what I want.
My position is that every key record, by its very existence
indicates an acceptable signing profile.
Where we disagree is that I believe that it is necessary for
the policy language to specify that multiple signatures are
present meeting different selector matching criteria.
This is not a repeat of any closed issue since the response
each time in the past has been 'this is not currently in
scope because it is policy'. Ergo the issue cannot be closed
because it was never open.
The requirement is that the policy language be able to state:
"There is a signature present of type A" AND "There is a
signature present of type B".
If a verifier finds a signature present that is 1) valid and
2) meets their security requirements the message is
considered to be authentica and no further checking is required.
Where a difference is made is in the behavior when a
signature is found that does not meet the verifier's security
requirements, either because the algorithm is unsuported
(upgrade attack) or because the algorithm is considered
compromised (downgrade attack). In these cases the verifier
MUST pull the SSP record even though a signature is present
because the signature that is present is not one that the
verifier can use.
Since we will not be specifying compromised algorithms
initially the only code path that requires implementation is
what to do when you have a signature present that you do not
support. We absolutely do need to have a mechanism that
allows the verifier to know that it should expect there to
also be a signature that it can use.
The spec is broken without this.
-----Original Message-----
From: Stephen Farrell
[mailto:stephen(_dot_)farrell(_at_)cs(_dot_)tcd(_dot_)ie]
Sent: Monday, October 23, 2006 5:07 PM
To: Hallam-Baker, Phillip
Cc: ietf-dkim(_at_)mipassoc(_dot_)org
Subject: Re: [ietf-dkim] Comments on
draft-ietf-dkim-ssp-requirements-02.txt
Hi Phill,
Before we get into the wording of the latest ssp-reqs, can
we try to
figure out whether this is really different from the set of similar
*closed* issues on base?
I am not at all clear why such flags are useful in SSP,
given that we
have rough consensus that they are not needed for base.
As I see it:
1. The current WG consensus for base is that its up to each
recipient
to decide for itself which algorithms it considers acceptable.
2. The current WG consensus (I think) is that SSP need not be used
when a signature that the recipient considers "good" is
present. (I.e.
I see no consensus for a position that SSP MUST be used
even if what
the verifier thinks is a good signature is present.)
Given 1 & 2 above, I just don't see the point of algorithm
transition
information in SSP since no-one will read it who might act on it.
Please also expliticly explain why this is not simply
revisiting some
*closed* issues on base.
Thanks,
Stephen.
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