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Possible use of seconds instead of minutes in HS blinding parameter

In crates/tor-hscrypto/src/pk.rs in fn blinding_parameter, the period length is hashed into the blinding parameter.

Currently, period_length is hashed in the Arti code as a BE u64 number of seconds. But elsewhere in the spec, period_length is stated to be a number of minutes.

If C Tor uses seconds here, then:

  • Arti is correct
  • The spec doesn't describe the behaviour (so, near-term there should be a spec MR to state the behaviour explicitly)
  • The spec is illogical, since the HS period_length ought only to be hashed one way

If C Tor uses minutes here, then:

  • Arti must be fixed
  • The test vectors in Arti are wrong (where did they come from?)
  • The spec accurately describes the behaviour

Anyone addressing #767 is likely to either trip over this, or be able to confirm that Arti matches C Tor.

Whatever conclusion is reached ought to be stated in the Icebox spec ticket torspec#188 so that we fix the behaviour if we get a chance to update this part of the protocol.