Revisit V1.5 protocol design

[mhchia]

We are implementing V1.5 according to the design described in spec. There might be some flaws that compromise correctness or privacy. We should revisit and audit the protocol further.

• Any flaw in the existing protocol?
  • Search protocol
    • Except for setting maximum depth, other ways to prevent the loops?
    • Does that it sense to memorize all seen salt(pubkey) of hubs? (mentioned in Search Protocol: Hub.handleSearch #22 .
  • Proof statements
• Better way to salt public keys