Re: [GNAP] Support of FIDO

nadalin@prodigy.net Tue, 18 August 2020 17:35 UTC

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From: <nadalin@prodigy.net>
To: "'Denis'" <denis.ietf@free.fr>, <txauth@ietf.org>
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Date: Tue, 18 Aug 2020 10:35:36 -0700
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Subject: Re: [GNAP] Support of FIDO
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U2F is not CTAP2 or FIDO2, U2F device usually do not store the cryptographic material on the device as the device has limited capabilities 

 

From: Denis <denis.ietf@free.fr> 
Sent: Tuesday, August 18, 2020 9:37 AM
To: nadalin@prodigy.net; txauth@ietf.org
Subject: Re: [GNAP] Support of FIDO

 

Hi Tony,

Not quite Dennis, the U2F device does not store the private key, it only creates the private key.

Please read the Client to Authenticator Protocol (CTAP) specification from the FIDO Alliance:

https://fidoalliance.org/specs/fido-v2.0-ps-20190130/fido-client-to-authenticator-protocol-v2.0-ps-20190130.pdf

Extract:

 (...) the ASPSP  (Account Servicing Payment Service Providers) server sends a challenge message to the authenticator
which is then cryptographically signed by a private key stored in the authenticator.

Denis

 

From: TXAuth  <mailto:txauth-bounces@ietf.org> <txauth-bounces@ietf.org> On Behalf Of Denis
Sent: Friday, August 14, 2020 3:04 AM
To: nadalin@prodigy.net <mailto:nadalin@prodigy.net> ; txauth@ietf.org <mailto:txauth@ietf.org> 
Subject: Re: [GNAP] Support of FIDO

 

Hi Tony, 

Dennis, not all the way correct 

1.	It is also possible to get rid of IDs and passwords using FIDO. FIDO discloses no private information at all about the user 
and no trust relationships need to be defined since there is no AS

Depends on if you only consider “private information” PII, there can be all sorts of information passed in ClientData field of the FIDO response, not desirable but perfectly OK 

2.	None of these two semantics fit with the FIDO use case where the subject value is scoped to be locally unique in the context of one RS. 
Hence, the linkage of users between two RSs (or between one RS and any other server) becomes impossible 

There is nothing that prohibits the RS from sharing registration information between RS 

I am speaking of FIDO U2F where there are two main phases: registration and authentication.

The U2F device gives the public key and a Key Handle to the origin online service or website during the user registration step. 
Later, when the user performs an authentication, the origin online service or website sends the Key Handle back to the U2F device 
via the browser. The U2F device uses the Key Handle to identify the user's private key, and creates a signature which is sent back 
to the origin to verify the presence of the U2F device. Thus, the Key Handle is simply an identifier of a particular key on the U2F device.

There is no other registration information needed. Sharing such an information between RSs does not allow to link user accounts.

Denis

 

From: TXAuth  <mailto:txauth-bounces@ietf.org> <txauth-bounces@ietf.org> On Behalf Of Denis
Sent: Thursday, August 13, 2020 10:31 AM
To: txauth@ietf.org <mailto:txauth@ietf.org> 
Subject: [GNAP] Support of FIDO

 

This topic has already been tackled on the list, but I open a new thread for it.

In OAuth 2.0, one of the goals was to get rid of IDs and passwords. Since the solution in OAuth 2.0 was to use access tokens, 
there have been used everywhere, even when they were not strictly needed. 





It is also possible to get rid of IDs and passwords using FIDO. FIDO discloses no private information at all about the user 
and no trust relationships need to be defined since there is no AS. 





FIDO should be one allowed possibility for the user authentication. In the case of FIDO, the user is authenticated under a pseudonym 
specific to the RS. It may observed that there is no equivalent in OAuth because of the two different semantics of the subject claim.





RFC 7519 states:

The "sub" (subject) claim identifies the principal that is the subject of the JWT.  The claims in a JWT are normally statements about the subject.  
The subject value MUST either be scoped to be locally unique in the context of the issuer or be globally unique.

In one case, it is possible to link the subject claim of two users between two RSs (i.e. using a locally unique identifier in the context of the issuer) 
while in the other case (i.e. using a globally unique identifier) it is possible, in addition, to link the subject claim between one RS and any other server 
(i.e. not supporting OAuth) that is using the same globally unique identifier.





None of these two semantics fit with the FIDO use case where the subject value is scoped to be locally unique in the context of one RS. 

Hence, the linkage of users between two RSs (or between one RS and any other server) becomes impossible. 





There are cases where a user would like to enjoy the unlinkeability properties of FIDO which cannot be met using the claims currently defined in OAuth.





Denis