[Anima-bootstrap] brsky concern1: separating audit-log retrieval from voucher generation

[Toerless Eckert <[e-mail]>]

As discussed today on the call, restated for the rest of the team with more detail:

-> pledge connects to two networks, A, B.
-> Pledge tries to well-behave, only offers nonce to one network first, A.
-> (registrar of) A gets voucher. A looks at audit-log. Audit log is fine.
   Ultimately, A decides  not to enroll pledge though.
-> Pledge offers nonce to B, B gets voucher and audit-log.
   Audit log shows A, so B is concerned. B rejects device.
   Without the entry in audit log, B would have enrolled device.

My worry is that the current audit log approach can too easily create
false positives that will make enrolment fail if a device has multiple network
connections:

It's impossible for A to get an audit-log without also getting
a voucher, which in return would make another domain suspicious and likely
make it decide not to accept the device.

Separating out request for voucher from request for audit-log could work like this:

  A->MASA: request audit-log for (pledge,nonce1)
  MASA:    audit-log entry: "A requested audit log for pledge, hash(nonce1)" [1]
  MASA->A: reply: audit-log

  ... A makes up its mind if it wants pledge and decides that it does NOT.

  -> A doesn't do anything more. Audit log would show B that A did see device,
     but also that A never got voucher so that it could not have modified
     device == B will happily enroll pledge.


  ... A makes up its mind if it wants pledge and decides that it does want to:

  A->MASA: request voucher for (pledge,nonce1)
  MASA:    Examine audit-log that no voucher was granted since the audit-log
           was given to A [1]. requests for audit-logs since [1] are
	   ignored in this determination.
  MASA->A: If audit-log ok:
           reply OK: voucher(nonce1)
           audit-log entry: "A received voucher for pledge, hash(nonce1)" [2]

	   If audit-log nok:
           reply NOK: audit-log
           audit-log entry: "A failed voucher request for pledge, hash(nonce1), hash(nonce2)"
	   hash(nonce2) would be from an audit-log entry that did happen after
	   [1] and 
          

Cheers
    Toerless