Re: [Rats] Composite Evidence

"Eric Voit (evoit)" <evoit@cisco.com> Thu, 23 January 2020 16:19 UTC

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From: "Eric Voit (evoit)" <evoit@cisco.com>
To: "Xialiang (Frank, Network Standard & Patent Dept)" <frank.xialiang@huawei.com>, "Smith, Ned" <ned.smith@intel.com>, "Birkholz, Henk" <henk.birkholz@sit.fraunhofer.de>, Michael Richardson <mcr+ietf@sandelman.ca>, Dave Thaler <dthaler@microsoft.com>
CC: "rats@ietf.org" <rats@ietf.org>
Thread-Topic: Composite Evidence
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Date: Thu, 23 Jan 2020 16:19:33 +0000
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Subject: Re: [Rats] Composite Evidence
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Hi Frank,

 

From: Xialiang, January 23, 2020 2:53 AM



Hi,

I agree with Ned’s categorization of two, and we need to have different terms for them.

 

<eric> I agree that we need different terms for different types of evidence.  And added a third: relayed evidence.

 

I suspect it would be better to separate those from the "composite" concept.  It will be interesting to get others' thoughts on this.

 

I am also wondering the reason why you want to receive a time series snapshots of the same component? 

 

<eric> A Verifier can make an "ok-validated" evaluation on a particular TPM Quote, and send it back to the Attester.  Subsequently, a Relying Party can get a Quote from the same TPM to establish what are current, fresh measurements.   If both Quotes have the same PCR values, then the current state is in an "ok-validated" state.  This minimizes Verifier transactions, and gives quick/fresh response to the Relying Party.

 

If more documentation on this is interesting, I could put together something for the WG.

 

If you want to identify the change of a component’s trustworthiness during a period, maybe a better way is through the on-change evidence notification.

 

<eric> I initially thought on-change would be sufficient.  However there is quite a bit of volatile evidence which needs to be transmitted only under a certain condition.    For example, *only* when a specific PCR changes, send the full signed TPM quote as well as the log entries for that PCR.   It would be unaffordable/unsupportable to send all PCR quotes on-change as things like nonces/clock counters are continuously updating.

 

In a perfect world, this conditional push requirement would be a good match to your draft-bryskin-netconf-automation-yang.  However since this draft isn't close to being picked up by the NETCONF WG yet, we can't depend on that functionality.

 

If we are building from today's available RFCs, I think defining a new "attestation" event stream based on RFC-8639 Subscription to YANG Notifications will be a more stable starting point than starting with RFC-8641 for on-change YANG datastore subscription. 

 

Eric

 

B.R.

Frank

 

 

 

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发件人: RATS [ <mailto:rats-bounces@ietf.org> mailto:rats-bounces@ietf.org] 代表 Smith, Ned
发送时间: 2020年1月23日 9:54
收件人: Eric Voit (evoit) < <mailto:evoit@cisco.com> evoit@cisco.com>; Birkholz, Henk < <mailto:henk.birkholz@sit.fraunhofer.de> henk.birkholz@sit.fraunhofer.de>; Michael Richardson < <mailto:mcr+ietf@sandelman.ca> mcr+ietf@sandelman.ca>; Dave Thaler < <mailto:dthaler@microsoft.com> dthaler@microsoft.com>
抄送:  <mailto:rats@ietf.org> rats@ietf.org
主题: Re: [Rats] 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)" < <mailto:evoit@cisco.com> evoit@cisco.com>
Date: Wednesday, January 22, 2020 at 4:04 PM
To: Henk Berkholz < <mailto:henk.birkholz@sit.fraunhofer.de> henk.birkholz@sit.fraunhofer.de>, Michael Richardson < <mailto:mcr+ietf@sandelman.ca> mcr+ietf@sandelman.ca>, Dave Thaler < <mailto:dthaler@microsoft.com> dthaler@microsoft.com>, "Smith, Ned" < <mailto:ned.smith@intel.com> ned.smith@intel.com>
Cc: " <mailto:rats@ietf.org> rats@ietf.org" < <mailto:rats@ietf.org> rats@ietf.org>
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