[Rats] Composite Evidence

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

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