Nat and Torsten,
My responses ares embedded in your text.
I agree, we should analyse the threat. From my first impression it
feels like injection with some specialties.
Not exactly. In most scenario attacks, we have two good guys (Alice and
Bob) and one bad guy (Carol) acting as the single attacker..
In this scenario, we have two bad guys (Alice and Bob willing to
collaborate) and one good guy (Carol) acting as a relying party.
@Denis:
So far, I'm struggeling to understand how this attack is performed
from a practical perspective.
Every token/assertion issued to the uncle is bound to its identity.
This key question is to which "identity" since I am considering a scheme
where privacy considerations are as important as security considerations.
So the goal is only to reveal to the third party that the user making
the access is more than 18, without revealing anything else than the
relying party
would already know about the user making the access request.
So if the niece wants to "upgrade" her age, she would need to somehow
mix identity data for two identities
(her's and her uncle's identity) into a single token, which needs to
be signed by the respective AS. How is this gone work?
As yourself, I don't believe this is a solution. As I already said:
Whatever kind of cryptographic is being used, when two users
collaborate, a software-only solution will be unable
to prevent the transfer of an attribute of a user that possess it to
another user that does not possess it .
The use of a secure element simply protecting the confidentiality
and the integrity of some secret key or private key
will be ineffective to counter the Alice and Bob collusion attack.
Additional properties will be required for the secure element
(i.e. some physical device with security properties).
kind regards,
Torsten.
Am 11.11.2016 um 16:27 schrieb Nat Sakimura:
Thanks Denis for pointing it out. It may be desirable to add ABC
attack to the list of threats.
Torsten et al. are updating Threat Model and Security Considerations
so it could potentially be included in there.
Some remarks:
* I suppose the assumption is that the Bob does not share his
credentials with Alice: Otherwise, sharing the credential would
achieve something worse.
The assumption is correct.
* In addition, it assumes that Bob does not give his device to
Alice: Otherwise, something similar to ABC attack can be achieved
by Bob
giving Alice his Laptop or Phone, and I guess this happens more
often than shipping Bob's access token to Alice.
The assumption is correct. If Bob is using a smart card that protects
some keys, he will never give the smart card nor the PIN to Alice.
*
* With these assumptions:
o It looks like a variation of token injection attack that we
have been talking about for many years.
Not exactly.
o If we token bind the refresh and access tokens, the ABC
attack as described does not work.
I am not sure I understand what you mean here, since my belief is still :
Whatever kind of cryptographic is being used, when two users
collaborate, a software-only solution will be unable
to prevent the transfer of an attribute of a user that possess it to
another user that does not possess it .
o For something like Age verification, recognizing such
attacks, it probably is a bad practice to rely on
refresh/access token.
The service should do more active check, e.g., through OpenID
Connect.
Same comment as above.
Denis
Best,
Nat
On Tue, Nov 8, 2016 at 2:54 AM Denis <denis.i...@free.fr> wrote:
Section 5 of "draft-ietf-oauth-pop-architecture-08.txt"
identifies requirements.
One of them (which, BTW, should be moved into Section 4 -
Threats) is :
Collusion:
Resource servers that collude ...
This threat addresses the case of "/collusion between servers"/
while the case of "/collusion between clients"/
has not been considered. When access tokens are being used,
/collusion between clients /is of primary importance.
Let us consider the following "Alice and Bob Collusion attack"
(ABC attack).
An uncle (Bob) is willing to collaborate with his young niece
(Alice) who is less than 18 during a short period of time.
The niece is opening her own session and creates an account on a
server. The uncle does not hand over his own session to her niece
at any point of time.
Let us assume that some crypto expert has written two specific
pieces of software. One has been installed on the laptop
of the uncle and another one on the laptop of the niece. The two
laptops are able to communicate using a network (e.g. a WAN or a
LAN).
The niece creates an account on a resource server. Later on, the
resource server asks her (or him ?) to demonstrate that she (or
his ?)
is more than 18. She forwards the information received from the
resource server to her uncle using the network. The uncle receives
that information and connects to an Authorization Server. The
uncle requests an access token containing information demonstrating
that he is older than 18 and passed it back to his niece. The
niece then presents it to the resource server. The access token
is accepted.
Since the niece has been able to demonstrate once that she is
more than 18, the resource server will remember this attribute
and in the future she will not need to demonstrate it again. She
will keep the advantages related to this attribute associated
with her account on that resource server until she does not need
it anymore, i.e. when she will really be over 18.
Whatever kind of cryptographic is being used, when two users
collaborate, a software-only solution will be
unable to prevent the transfer of an attribute of a user that
possess it to another user that does not possess it .
The use of a secure element simply protecting the confidentiality
and the integrity of some secret key or private key will be
ineffective
to counter the Alice and Bob collusion attack. Additional
properties will be required for the secure element.
RFC 6819 (OAuth 2.0 Threat Model and Security Considerations)
issued in January 2013 has omitted to take into consideration
the Alice and Bob Collusion attack.
Section 2.3 of the ABC4Trust project about key-binding in
Deliverable D2.2 available at:
https://abc4trust.eu/download/Deliverable_D2.2.pdf states on page
17 :
To prevent “credential pooling”, i.e., multiple Users sharing
their credentials, credentials can optionally be bound to a
secret key,
i.e. a cryptographically strong random value that is assumed to
be known only to a particular user. The credential specification
specifies whether the credentials issued according to this
specification are to employ key binding or not.
A presentation token derived from such a key-bound credential
always contains an implicit proof of knowledge of the underlying
secret key,
so that the Verifier can be sure that the rightful owner of the
credential was involved in the creation of the presentation token.
As an extra protection layer, the credentials can also be bound
to a trusted physical device, such as a smart card, by keeping
the secret key in a protected area of the device. That is, the
key cannot be extracted from the device, but the device does
participate
in the presentation token generation to include an implicit proof
of knowledge of this key in the token. Thus, for credentials that
are key-bound
to a physical device it is impossible to create a presentation
token without the device.
The rightful owner of the credential was indeed involved in
real-time in the creation of the presentation token but in the
collaboration scenario,
the key binding mechanism is not sufficient to counter that
specific attack. ABC4Trust, Idemix (IBM) and U-Prove
(Microsoft)are currently
not resistant to the "ABC attack".
The IRMA card project (https://www.irmacard.org/) based on the
use of a smart card and of the Idemix scheme claims to provide
security
and privacy simultaneously. However, this project will not be
resistant either to the ABC attack.
*draft-ietf-oauth-pop-architecture-08 should take into
consideration the ABC attack.*
The threat related to the ABC attack should be identified in the
security considerations section
and the core of the document should attempt to identify one or
more ways to counter it.
The scope of draft-ietf-oauth-token-exchange-06 is limited to the
definition of a basic request and response protocol for
an STS-style token exchange utilizing OAuth 2.0. Section 6
(Security Considerations) has omitted to take into consideration
the ABC attack and therefore the currently described "basic
request and response protocol" will allow Bob to obtain an access
token and to pass it successfully to Alice so that she can use it.
*draft-ietf-oauth-token-exchange-06 **should take into
consideration the ABC attack.*
The threat related to the ABC attack should be identified in the
security considerations section
and the core of the document should attempt to identify one or
more ways to counter it.
Denis
PS. I have recently registered to the OAuth mailing list.
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Nat Sakimura
Chairman of the Board, OpenID Foundation
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