[tor-bugs] #28651 [Obfuscation/Snowflake]: Prepare all pieces of the snowflake pipeline for a second snowflake bridge

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#28651: Prepare all pieces of the snowflake pipeline for a second snowflake bridge
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     Reporter:  arma                   |      Owner:  (none)
         Type:  enhancement            |     Status:  new
     Priority:  Medium                 |  Milestone:
    Component:  Obfuscation/Snowflake  |    Version:
     Severity:  Normal                 |   Keywords:
Actual Points:                         |  Parent ID:
       Points:                         |   Reviewer:
      Sponsor:                         |
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 Right now there is one snowflake bridge, and its fingerprint is hard-coded
 in tor browser.

 Eventually we will have enough load, and/or want more resiliency, that we
 want to set up a second snowflake bridge.

 To be able to do that, I think we need changes at the client, changes at
 the snowflake, and changes at the broker.

 (A) At the snowflake side, the snowflake needs to tell the broker which
 bridge(s) it is willing to send traffic to. Additionally, we either want
 to declare that each snowflake sends to only one bridge, or we need to add
 a way for the client to tell the snowflake which bridge it wants to reach.

 (B) At the broker side, we need it to be able to learn from snowflakes
 which bridge(s) they use, and we need it to be able to learn from clients
 which bridge they want to use, and we need it to match clients with
 snowflakes that will reach that bridge.

 (C) At the client side, we need it to tell the broker which bridge it
 wants to use, and (depending on our design choice in A above) we might
 also need the client to be able to tell the snowflake which bridge it
 wants to use.

 (There is an alternative approach, where we assume that every snowflake is
 always running the newest javascript, so it is willing to reach every
 bridge on our master list. Then the broker doesn't need to do anything
 new, and we just need to add a way for the client to tell the snowflake
 which bridge it wants. I don't have a good handle on how realistic this
 assumption is.)

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/28651>
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