Re: [Rats] Composite Evidence

["Eric Voit (evoit)" <[e-mail]>]

Hi Ned,

Hi Dave,

 

I agree with Ned's framing of (i) and (ii).  These two are embedded in my passport example below.    We also have (iii): evidence which is relayed from outside the Attester.  I think (iii) needs to be broken out as an evidence type because signed information from the Verifier should be trusted differently from other evidence the Attester.

 

That makes:

 (i) component evidence

(ii) time-series evidence

(iii) relayed evidence

 

Beyond (i), (ii), & (iii), do we need term combinations of (i), (ii), & (iii) with the word "composite"?  Hopefully we wouldn't need to define six terms for each combination of these three variants.  Otherwise my example might be called "component, time-series, relayed composite evidence".  And that is a mouthful.

 

Eric

 

 

From: Dave Thaler, January 22, 2020 9:05 PM



Agree with Ned that we should keep composite devices separate from time series snapshots of the same component in terms of concepts.

 

From: Smith, Ned < <mailto:[e-mail]> [e-mail]> 
Sent: Wednesday, January 22, 2020 5:54 PM
To: Eric Voit (evoit) < <mailto:[e-mail]> [e-mail]>; Birkholz, Henk < <mailto:[e-mail]> [e-mail]>; Michael Richardson < <mailto:[e-mail]> [e-mail]>; Dave Thaler < <mailto:[e-mail]> [e-mail]>
Cc:  <mailto:[e-mail]> [e-mail]
Subject: Re: Composite Evidence

 

Eric,

I think there are two types of “composite” things that could be considered (i) decompositions of hardware and software such as a motherboard and the sub-components that could be plugged into it and (ii) time series snapshots of the same component. 

 

You seem to be asserting (ii) where a time series snapshot of the same thing could be considered “composite evidence”. 

 

I think both cases are valid, but maybe it makes sense to use different terms for each as they seem to have distinct properties. (e.g. evidence having multiple entries of a component with the same name  under (i) implies multiple instances of the same type of device – such as two NIC cards. Whereas under (ii) multiple entries implies there is one instance of the NIC sampled over a time interval. 

 

-Ned

 

From: "Eric Voit (evoit)" <[e-mail] <mailto:[e-mail]> >
Date: Wednesday, January 22, 2020 at 4:04 PM
To: Henk Berkholz <[e-mail] <mailto:[e-mail]> >, Michael Richardson <[e-mail] <mailto:[e-mail]> >, Dave Thaler <[e-mail] <mailto:[e-mail]> >, "Smith, Ned" <[e-mail] <mailto:[e-mail]> >
Cc: "[e-mail] <mailto:[e-mail]> " <[e-mail] <mailto:[e-mail]> >
Subject: Composite Evidence

 

Henk,         Dave,

Michael,    Ned,

 

I promised you a definition for Composite Evidence.  You can see my proposed definition directly the text below, but I am not willing yet to place it in a Pull Request. I thought an email thread might be helpful first.

 

Anyway my strawman definition for Composite Evidence is:  Evidence which includes multiple sub-elements of evidence, more than one of which can be computationally verified to have been generated by a specific Attester Subcomponent or Verifier.

 

I built this definition considering the passport model, which looks like it will often needs to use composite evidence.  As an example of why I believe this, see the use case below.

 

    .--------------.             

    |  Verifier A  |

    '--------------'     

        ^     [2]           

        |     Verifier A signed Attestation Results @time(x) (

    evidence(  |  determination, hash(TpmQuote@time(x)))  

    TpmQuote   |                  

    @time(x))  |                        

       [1]     V                        

     .-------------.                           .---------------.

     |  Attester   |<------nonce @time(y)---[3]|  Verifier B   |

     |    .-----.  |                           |       /       |

     |    | Tpm |  |[4]-composite evidence ( ->| Relying Party |

     |    '-----'  |      TpmQuote@time(y),    '---------------'

     '-------------'      TpmQuote@time(x),  

                          Verifier A signed Attestation Results @time(x) )                      

 

 

In the example above, evidence at time x is generated and signed within a TPM.  This would *not* be composite evidence.   This evidence would be evaluated by Verifier A, signed, and returned as Attestation Results to the Attester.   A subsequent request from a Relying Party at time y could pull three independently signed elements of evidence from the Attester.  These three would comprise the composite evidence which when taken together would allow Verifier B / Relying Party to evaluate the current trustworthiness of the Attester.

 

Does this definition meet your needs?

 

Thanks,

Eric