[metrics-bugs] #29624 [Metrics/Exit Scanner]: New version of exit list format

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#29624: New version of exit list format
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 Reporter:  irl                      |          Owner:  irl
     Type:  task                     |         Status:  accepted
 Priority:  Medium                   |      Milestone:
Component:  Metrics/Exit Scanner     |        Version:
 Severity:  Normal                   |     Resolution:
 Keywords:  metrics-roadmap-2019-q2  |  Actual Points:
Parent ID:  #29650                   |         Points:
 Reviewer:  irl                      |        Sponsor:
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Comment (by irl):

 I'm currently working on this, it is taking a little longer than I would
 have hoped because I'm first getting a handle on cert-spec. I'd like us to
 be able to define this in a way that we don't need a version 3 to add
 signatures.

 My thoughts so far are:

 * exit scanners will have Ed25519 keys
 * there may be one long-lived identity key and one shorter-term signing
 key (to allow offline master key)
 * there won't be any RSA keys, it will be "Ed25519-first"
 * we re-use the certificate formats from cert-spec
 * signing is optional, if there is no identity line then no signature
 should be expected

 Two changes are going to be related to unifying the keywords between this
 spec and dir-spec. created->published, software->platform.

 I think the address4|6 lines should be optional, so that we can prevent
 the scanners becoming targets for attack.

 I need to pause on this to look at other tasks, but hopefully will return
 soon and we can get this bit finished off.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/29624#comment:12>
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