Re: [EAT] Scope, Goals & Background for RATS

Denis <denis.ietf@free.fr> Tue, 18 September 2018 10:41 UTC

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From: Denis <denis.ietf@free.fr>
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Subject: Re: [EAT] Scope, Goals & Background for RATS
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Hi Henk,

I browsed through the document and I am a bit puzzled. I was expecting 
some words about the attestation certificates that FIDO is using, but I 
found none.

In FIDO, the concept of an attestation certificate is that a batch of 
devices from the same model have the same attestation private key and 
thus the same certificate.

In this WG, such a restriction should not necessarily be made and when 
the certificate is different for each device model the question is how 
we should name it.

Let me attempt a proposal:

    "shared attestation certificates" to designate attestation
    certificates that are specific to a batch of devices from the same
    model (like in FIDO),
    "individual attestation certificates" to designate attestation
    certificates that are specific to each device.

The introduction section states:

    Authentication presumes some protection: e.g., that there is not an
    illicit copy of the private key available to anyone other than the
    intended subject.

This is insufficient, it also assumes that a legitimate client does not 
perform all the cryptographic computations that another client needs:
preventing the export of private keys does not prevent a client to use a 
private key for the benefits of another client.

The background section states:

    Generally speaking, attestation is applied before data is exchanged
    to avoid disclosure to insufficiently trusted environments.

I don't believe so. There is the need to demonstrate that a data 
exchange really originates from a secure device, acting as a client,
and that that device has some well known properties.

In the terminology section, I would expect to find a definition for an 
attestation certificate.

I read the definition of an "Attestation Evidence" in the terminology 
section several times and I have difficulties to understand it.
I wonder if such a concept really exists. In particular, within this 
definition, the text states:

    Attestation Evidence veracity may be augmented by Claimant
    signatures over portions of the Attestation Evidence.

I don't believe so.  If I am assuming the use of Attestation 
Certificates, there is an attestation private key associated to the 
public that is in it.
The authentication exchange performed by a client should be 
counter-signed by the attestation private key corresponding to the 
public key
placed within the Attestation Certificate.

In the background section, the concept of a remote attestation is the 
following:

    /Remote Attestation/ is where the Attester conveys Attestation
    Evidence to a Verifier that compares it with Reference Values
    (in order for it to be considered, for example, healthy). A Relying
    Party (or the Verifier acting as a Relying Party) appraises the
    verification result
    to assess device trustworthiness and to manage security risk.

Since I could not grasp the concept of Attestation Evidence, I couldn't 
grasp the concept of a Remote Attestation which is based on it.
The goal is not simply to "assess device trustworthiness" but to make 
sure that some data exchange is really coming from a device
that has specific properties. Such properties are going beyond the 
protection, the non-export and the non-import of private keys.

I could not understand either the value of local attestations. IMO, the 
WG should only focus on remote attestations.

Finally, the following invitation was raised:

    Naturally, we are also very interested in feedback about the
    illustrated difference between explicit attestation and implicit
    attestation.

The current definition for Explicit Attestation is:

    /Explicit Attestation/ is where the Attestation Claims/Evidence is
    conveyed authentically (e.g. signed) by the Attester to the Verifier
    where a root-of-trust enumerates the Attestation Claims/Evidence
    associated with an execution environment.

I would rephrase it in the following way:

    /Direct Attestation/ is when a verifier can be confident that some
    data originating from a client originates in practice from a secure
    device
    that has specific *functional and security* properties.

The current definition for Explicit Attestation is:

    /Implicit Attestation/ is where the Attester conveys evidence of the
    availability of an authenticating credential that allows the Verifier
    to infer the Attestation Claims/Evidence associated with an
    execution environment. The Verifier knows that the availability of
    the authenticating credential is bound to certain Attestation
    Claims/Evidence. Inferred knowledge may further be conveyed as
    Attestation Evidence
    by a Trusted Third Party via a credential certificate or other
    out-of-band method.

It is quite hard to understand the concept since three sentences are 
being used.

Let me attempt to define a different but, maybe, close concept:

    /Inferred Attestation/is when a Trusted Third Party (TTP) testifies
    to a verifier that some data originating from a client originated in
    practice from a secure device
    that has specific functional and security properties.

Denis

> Hi all,
>
> we pushed an initial document to the RATS github in order to focus the 
> discussion about remote attestation procedures a bit.
>
>> https://github.com/ietf-rats/charter/blob/master/ietf-rats-charter.md
>
> We included a background section to better highlight the meaning of 
> the term "attestation" in general. Hence, there is a trade-off between 
> clarity and conciseness, which is one of the things we would like to 
> get feedback about.
>
> Naturally, we are also very interested in feedback about the 
> illustrated difference between explicit attestation and implicit 
> attestation.
>
> Viele Grüße,
>
> Henk
>
>
>
>
>
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