On 06/19/2013 02:36 PM, Adam Back wrote:
This maybe simpler and trivially compatible with
existing type2 public keys
(ones that are multiples of a parent public key): send an ECDSA
signature of
the multiplier, and as we know you can compute ("recover") the
parent public
key from an the ECDSA signature made using it.
Adam
On Wed, Jun 19, 2013 at 05:28:15PM +0200, Adam Back wrote:
[q-th root with unknown no discrete log
artefact]
If it was a concern I guess you could require a proof of
knowledge of
discrete log. ie as well as public key parent, multiplier the
address must
include ECDSA sig or Schnorr proof of knowledge (which both
demonstrate
knowledge of the discrete log of Q to base G.)
It's a cool trick but requiring a signature on each multiplier
defeats one of the purposes of a deterministic wallet. I don't want
to have to explicitly export a whole bunch of signatures from my
offline system just to exercise this address option. The "observer
wallet" should be able to do anything it needs to on its own,
without help from the offline wallet.
Unless you mean that there is a one-time signature from the offline
computer that works for all addresses, that can be exported with the
observer wallet...? If all you want to do is prove that someone
owns that private key, you could send {Sign(MagicString),
Multiplier}. So it becomes one signature operation per wallet,
but creating new wallets would require going back to the offline
computer for that one-time signature. That's better than the
alternative, but it's still extra bloat for the wallet apps.
Either way, I'm not convinced that these are a problem for the
specified use cases I outlined. In cases where you have a
persistent business relationship, they need to verify the parent
public key exchange anyway. After that, the software doesn't
technically require the transmission of the PubKey, it only needs
the Name/ID of the party and the multiplier and it will fetch the
PubKey from its data store. Or it is transmitted and the payer
verifies it's correct. Computing an alternate {PubKey', Mult'} that
produces the same address and then using it in a MitM attack doesn't
work here if the two parties pre-verified the public keys.
In the case that a business is checking whether the cashout address
of a customer is the same as the last time: if the first payout was
not replaced by an attacker, then the business already has the
correct public key in their DB and a replacement of further payout
requests will fail validation. If the first payout was replaced...
well that could've been done anyway (with or without this alternate
form), and the customer wouldn't have received their money and the
whole process would be flagged and terminated before further
transactions.
-Alan