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Re: Proposal: Chrome privacy CA

From: Denis Pinkas <denis.w3c@free.fr>
Date: Thu, 9 Nov 2017 11:28:44 +0100
To: Adam Langley <agl@google.com>
Cc: W3C Web Authn WG <public-webauthn@w3.org>
Message-ID: <038fd3fb-3f71-c09e-16a3-6d4e6705423d@free.fr>
Hi Adam,

> On Wed, Nov 8, 2017 at 9:02 AM, Denis Pinkas <denis.w3c@free.fr 
> <mailto:denis.w3c@free.fr>> wrote:
>     By default, self-attestations should be used. So nothing at all
>     would be given away. This is what comes out when you apply
>     "Privacy by Design" and "Privacy by Default" rules.
> By "self-attestation" I'm assuming that you have in mind that the 
> private key itself signs the registration data, as opposed to some 
> attestation key in the token. This would show possession of the 
> private key.
> However, no U2F device that I'm aware of does this and the CTAP2 spec 
> doesn't have this either, I think. So, while I'd be very happy for 
> tokens to do that by default, that ship may have sailed.
> But we might agree that privacy by default makes sense. Our requests 
> (2) and (3) say that, by default, RPs should not get attestation data. 
> In that case, no information need be sent to any Privacy CA.

You say that "by default, RPs should not get attestation data". I am not 
sure how to interpret this sentence.
An attestation is always needed, either a certificate attestation (where 
currently a batch of hardware tokens share the same private key)
or a self-attestation (where a self-signed certificate is generated 
using the private key used to open the account).
This means that the RP must always *get and verify* attestation data.

>     Revocation of a specific token is currently impossible in FIDO
>     since all RPs are considered to be equal. If you remove this
>     equality, then a new bunch of possibilities emerges.
>     So saying that is is a "non-goal of FIDO" may not be an
>     appropriate statement. From a functional point of view, it would
>     be nice to be able to revoke a specific token
>     and this is possible as soon as you introduce a distinction between :
>       * RPs that need to know whether an authenticator conforms to
>         some security and functional requirements,
>         and in this case the use of an attestation certificate will be
>         required.
>       * RPs that don't need to know whether an authenticator conforms
>         to some security and functional requirements,
>         and in this case the use of an self-signed certificate will be
>         required.
>     This means that a given hardware token should be able either to
>     generate a self-signed certificate or to use its attestation
>     certificate upon the request from the client.
> I think we're in agreement there, although we do have to deal with the 
> existing population of tokens in the world. Thus we're asking for a 
> flag from RPs to indicate which of those two groups that they're in 
> (point #5) and we'll have Chrome generate a dummy attestation 
> certificate for RPs that don't care about requirements. That's not as 
> good as having the token sign the registration message with the 
> registration private key, but it's the best that we can do with the 
> current population of tokens.

It is not a good way at all to solve the problem. By doing so, you 
introduce an easy way to create a bunch of dummy registrations,
because at the time of the registration you do not even know that the 
user is knowing the private key corresponding to the public key that is 
being registered.
In other words, you do not have any PoP (Proof of Possession of the 
private key) at registration time. Adding a signature of the 
registration message
with the private key would be the smartest and the simplest way to 
correct the problem rather that, as currently, asking to create a 
self-signed certificate
to keep a single profile for the registration protocol.

> If CTAP2 were to define a "no-op" attestation form that tokens would 
> use on request, we would happy with that.

Hardware authenticators do not last for ever. Since current hardware 
authenticators are unable to support revocation,
it may be time to think of an architecture able to support revocation. 
My proposal paves the way for such an architecture.

The topic of the Webinar of the FIDO Alliance from Wednesday, September 
20, 2017 was : "NIST 800-63 Guidance and FIDO Authentication".

The agenda was:

  * Introduction/Updates on FIDO Alliance
  * Overview of NIST 800-63-3
  * Exceeding NIST Guidance with FIDO Authentication

One of the speakers was Paul Grassi, Senior Standards and Technology 
Advisor, NIST.

According" to NISP SP 800-63A, FIDO hardware authenticators are in the 
category of "Multi-Factor Cryptographic Device Authenticators".
However currently FIDO authenticators do not comply with a requirement 
from section of NISP SP 800-63A which states:
 Multi-Factor Cryptographic Device Authenticators

Multi-factor cryptographic device authenticators use tamper-resistant 
to encapsulate a secret key that is unique to the authenticator.

FIDO authenticators do not**encapsulate a private key that is unique to 
the authenticator but to a batch of authenticators.

FIDO authenticators do not comply either to another requirement from 
section 5.2.1 of NISP SP 800-63B which states:

        5.2.1 Physical Authenticators

CSPs SHALL provide subscriber instructions on how to appropriately 
protect the authenticator against theft or loss.
The CSP SHALL provide a mechanism to revoke or suspend the authenticator 
immediately upon notification from subscriber
that loss or theft of the authenticator is suspected.

If revocation is "a non-goal" for FIDO authenticators, is it reasonable 
to claim at the same time that FIDO authenticators are exceeding NIST 
Guidance ?

Both within an enterprise environment and in an Internet environment 
revocation is an important concern. It is time to address and to solve 
this issue.

>     If you want to remain compatible with the current situation, you
>     always request the use of an attestation certificate, ... if some
>     way or another
>     you know that the same certificate is being used for a batch of at
>     least 100K tokens. BTW, today there is no mean being defined to
>     know whether the attestation certificate
>     applies to a single token or a batch of tokens.
>     Making the distinction between two kinds of RPs opens the door to
>     new possibilities, in particular, the revocation of a specific token.
>>     That is a balance between the wider interests of common users
>>     (who do not wish to have a tracking token) and the interests of
>>     closed, enterprise deployments,
>>     who might like to be able to identify specific tokens, track them
>>     in inventory systems etc.
>     The balance is simply not between two parties (common users and
>     RPs) but between three parties :
>       * common users,
>       * RPs that need to know whether an authenticator conforms to
>         some security and functional requirements, and
>       * RPs that don't need to know whether an authenticator conforms
>         to some security and functional requirements.
>>     Item #5 in our requests attempts to reconcile this a little
>>     differently and envisions a signal to tokens that they are being
>>     used in a context where individual attestation is appropriate,
>>     and might thus return a different, unique attestation certificate.
>     By default, the use of an attestation certificate (whether unique
>     or shared) would be inappropriate. By exception (i.e. upon
>     request), it would be.
>>     In Chrome, that signal would be wired up to our enterprise policy
>>     to allow administrators to enable direct attestation for
>>     enumerated RP IDs.
>     FIDO is intended to be usable on the Internet, I mean not only
>     within an enterprise environment.
>     Within an enterprise environment the added value of FIDO will be
>     "ease of use" and "security", it will not be "privacy".
>     Within an enterprise environment privacy is not the major concern,
>     but on the Internet it is.
> I do not object to any of that. While I don't think tokens should use 
> individual attestation certificates by default (because not all 
> browsers will use a Privacy CA in all likelihood), we've no problem 
> with them doing so when signalled that they're in an enterprise 
> environment.

Coming back to the Privacy CA proposal, you postulate that many 
consumers services in reality don’t care about provenance of 
authenticators. That's true.

You also consider that, "in enterprise environments, there are usually 
the need for a much stricter enforcement of authenticator types
and as such it is important in these cases for the device to disclose 
information about itself to the RP". I would not phrase this this way,
but I would say that some RPs (/whether or not in an enterprise 
environment/) need for a much stricter enforcement of authenticator types.

So the requirement is not necessarily associated with the enterprise 

You are trying to find a solution for *hardware authenticators that are 
not compliant with **NISP SP 800-63*.

I am proposing a solution for hardware authenticators that would be 
compliant with NISP SP 800-63.

Since the functionalities of the Google Private CA are going to remain 
private and non verifiable by independent parties,
the information that could be gathered, as well as its use, is going to 
be unknown. This is a serious concern.

If the Privacy CA were being deployed, this would introduce a *Big 
Brother component* into the FIDO architecture.
Such a component should be and can be avoided.

If the project was going anyway to proceed, at the minimum, the 
drawbacks of the approach should be mentioned in the proposal.


> Cheers
Received on Thursday, 9 November 2017 10:29:09 UTC

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