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tor-spec: Extends accept all-zero ed25519 keys

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The spec gives conficting advice about all-zero ed25519 keys in extends.
Resolve this conflict by documenting tor's current behaviour.

Also move a sentence about circuit IDs, so it's closer to the associated
paragraph.
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teor 2020-04-28 17:30:30 +10:00
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21
tor-spec.txt
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@ -1333,15 +1333,20 @@ see tor-design.pdf.
When an onion router receives an EXTEND2 relay cell, it sends a CREATE2
cell to the next onion router, with the enclosed HLEN, HTYPE, and HDATA
as its payload.
as its payload. The initiating onion router chooses some circID not yet
used on the connection between the two onion routers. (But see section
5.1.1 above, concerning choosing circIDs.)
As special cases, if the EXTEND/EXTEND2 cell includes a legacy
identity, identity fingerprint, or Ed25519 identity of all zeroes, or
asks to extend back to the relay that sent the extend cell, the
circuit will fail and be torn down. The initiating onion router
chooses some circID not yet used on the connection between the two
onion routers. (But see section 5.1.1 above, concerning choosing
circIDs.)
As special cases, if the EXTEND/EXTEND2 cell includes a legacy identity, or
identity fingerprint of all zeroes, or asks to extend back to the relay
that sent the extend cell, the circuit will fail and be torn down.
Ed25519 identity keys are not required in EXTEND2 cells, so all zero
keys SHOULD be accepted. If the extending relay knows the ed25519 key from
the consensus, it SHOULD also check that key. (See section 5.1.2.)
If an EXTEND2 cell contains the ed25519 key of the relay that sent the
extend cell, the circuit will fail and be torn down.
When an onion router receives a CREATE/CREATE2 cell, if it already has a
circuit on the given connection with the given circID, it drops the