[tor-bugs] #2505 [Metrics]: Implement bridge descriptor secret manager in metrics-db

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#2505: Implement bridge descriptor secret manager in metrics-db
-------------------------+--------------------------------------------------
 Reporter:  karsten      |       Owner:  karsten
     Type:  enhancement  |      Status:  new    
 Priority:  normal       |   Milestone:         
Component:  Metrics      |     Version:         
 Keywords:               |      Points:         
   Parent:  #2435        |  
-------------------------+--------------------------------------------------
 In #2435 we discussed replacing bridge IP addresses in bridge descriptors
 with

 {{{
 H(IP address + bridge identity + secret)[:3]
 }}}

 This is already implemented for a static `secret`.

 We also discussed changing the `secret` regularly, say, once a month.
 This requires us to extend metrics-db to:

  - generate a new secret when we receive the first descriptor of a new
 month,
  - store secrets to disk and read them on startup,
  - solve the problem that a descriptor can be referenced from statuses of
 two months, and
  - delete secrets when we're sure we don't need them anymore.

-- 
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/2505>
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