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X-Mailman-Approved-At: Fri, 22 Nov 2019 15:13:32 +0000
Subject: Re: [bitcoin-dev] Draft BIP for SNICKER
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Hi, AFAIK snicker is limited to 2 party mixes for the foreseeable future.
What makes this a useful anonymity system for cryptocurrency/Bitcoin?
Thanks
On 11/22/19 3:02 PM, AdamISZ via bitcoin-dev wrote:
> Hi Riccardo,
> Apologies for not answering before, this slipped my mind.
> Clearly what you propose is possible, and adding the proposer's own
> signed transaction is a nice touch to make it more privacy-viable.
> For now my inclination is not to add this complexity, especially because
> of the cost implication.
> I'd note though that your idea about adding in second-stage transactions
> aligns with the CoinJoinXT idea (or perhaps, just the segwit idea!).
> Proposers could send sequences of transactions with various patterns,
> including backouts and promises, but it would clearly be way more
> complicated than what we're considering right now.
> Regards,
> Adam/waxwing
>
>
> Sent with ProtonMail <https://protonmail.com> Secure Email.
>
> ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
> On Wednesday, November 6, 2019 4:52 PM, Riccardo Casatta via bitcoin-dev
> <bitcoin-dev@lists.linuxfoundation.org> wrote:
>
>> Hello Adam,
>>
>> are you sure you can't tackle the watch-only issue?
>>
>> What if the proposer create the coinjoin-tx, plus another tx
>> (encrypted with the shared secret) which is a 1 input-1 output (1to1)
>> tx which spend his output to another of his key.
>> At this point when the receiver accept the proposal tx he could create
>> other tx 1to1 which are spending his tweaked output to pure bip32
>> derived key, he than broadcast together the coinjoin tx and for every
>> output of the coinjoin tx one other tx which is a 1to1 tx.
>>
>> Notes:
>> * We are obviously spending more fee because there are more txs
>> involved but the receiver ends up having only bip32 derived outputs.
>>
>> * The receiver must create the 1to1 tx or the receiver lose privacy by
>> being the only one to create 1to1 tx
>> * a good strategy could be to let the coinjoin tx have a very low fee,
>> while the 1to1 tx an higher one so there is less risk that only the
>> coinjoin gets mined
>> * Whit this spending strategy, the wallet initial scan does not need
>> to be modified
>>
>>
>> Il giorno mar 22 ott 2019 alle ore 15:29 AdamISZ via bitcoin-dev
>> <bitcoin-dev@lists.linuxfoundation.org
>> <mailto:bitcoin-dev@lists.linuxfoundation.org>> ha scritto:
>>
>> Just to chime in on these points:
>>
>> My discussions with ghost43 and ThomasV led me to the same
>> conclusion, at least in general, for the whole watch-only issue:
>>
>> It's necessary that the key tweak (`c` as per draft BIP) be known
>> by Proposer (because has to add it to transaction before signing)
>> and Receiver (to check ownership), but must not be known by anyone
>> else (else Coinjoin function fails), hence it can't be publically
>> derivable in any way but must require information secret to the
>> two parties. This can be a pure random sent along with the
>> encrypted proposal (the original concept), or based on such, or
>> implicit via ECDH (arubi's suggestion, now in the draft, requiring
>> each party to access their own secret key). So I reached the same
>> conclusion: the classic watch-only use case of monitoring a wallet
>> in real time with no privkey access is incompatible with this.
>>
>> It's worth mentioning a nuance, however: distinguish two
>> requirements: (1) to recover from zero information and (2) to
>> monitor in real time as new SNICKER transactions arrive.
>>
>> For (2) it's interesting to observe that the tweak `c` is not a
>> money-controlling secret; it's only a privacy-controlling secret.
>> If you imagined two wallets, one hot and one cold, with the second
>> tracking the first but having a lower security requirement because
>> cold, then the `c` values could be sent along from the hot to the
>> cold, as they are created, without changing the cold's security
>> model as they are not money-controlling private keys. They should
>> still be encrypted of course, but that's largely a technical
>> detail, if they were exposed it would only break the effect of the
>> coinjoin outputs being indistinguishable.
>>
>> For (1) the above does not apply; for there, we don't have anyone
>> telling us what `c` values to look for, we have to somehow
>> rederive, and to do that we need key access, so it reverts to the
>> discussion above about whether it might be possible to interact
>> with the cold wallet 'manually' so to speak.
>>
>> To be clear, I don't think either of the above paragraphs describe
>> things that are particularly likely to be implemented, but the
>> hot/cold monitoring is at least feasible, if there were enough
>> desire for it.
>>
>> At the higher level, how important is this? I guess it just
>> depends; there are similar problems (not identical, and perhaps
>> more addressable?) in Lightning; importing keys is generally
>> non-trivial; one can always sweep non-standard keys back into the
>> HD tree, but clearly that is not really a solution in general; one
>> can mark out wallets/seeds of this type as distinct; not all
>> wallets need to have watch-only (phone wallets? small wallets?
>> lower security?) one can prioritise spends of these coins. Etc.
>>
>> Some more general comments:
>>
>> Note Elichai's comment on the draft (repeated here for local
>> convenience:
>> https://gist.github.com/AdamISZ/2c13fb5819bd469ca318156e2cf25d79#gistcomment-3014924)
>> about AES-GCM vs AES-CBC, any thoughts?
>>
>> I didn't discuss the security of the construction for a Receiver
>> from a Proposer who should after all be assumed to be an attacker
>> (except, I emphasised that PSBT parsing could be sensitive on this
>> point); I hope it's clear to everyone that the construction Q = P
>> + cG is only controllable by the owner of the discrete log of P
>> (trivial reduction: if an attacker who knows c, can find the
>> private key q of Q, he can derive the private key p of P as q - c,
>> thus he is an ECDLP cracker).
>>
>> Thanks for all the comments so far, it's been very useful.
>>
>> AdamISZ/waxwing/Adam Gibson
>>
>> Sent with ProtonMail Secure Email.
>>
>> ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
>> On Monday, October 21, 2019 4:04 PM, SomberNight via bitcoin-dev
>> <bitcoin-dev@lists.linuxfoundation.org
>> <mailto:bitcoin-dev@lists.linuxfoundation.org>> wrote:
>>
>> > > The SNICKER recovery process is, of course, only required for
>> wallet
>> >
>> > recovery and not normal wallet use, so I don't think a small
>> amount of
>> > round-trip communication between the hot wallet and the cold
>> wallet is
>> > too much to ask---especially since anyone using SNICKER with a
>> > watching-only wallet must be regularly interacting with their cold
>> > wallet anyway to sign the coinjoins.
>> >
>> > What you described only considers the "initial setup" of a
>> watch-only wallet. There are many usecases for watch-only wallets.
>> There doesn't even necessarily need to be any offline-signing
>> involved. For example, consider a user who has a hot wallet on
>> their laptop with xprv; and wants to watch their addresses using
>> an xpub from their mobile. Or consider giving an xpub to an
>> accountant. Or giving an xpub to your Electrum Personal Server
>> (which is how it works).
>> >
>> > Note that all these usecases require "on-going" discovery of
>> addresses, and so they would break.
>> >
>> > ghost43
>> >
>> > (ps: Apologies Dave for the double-email; forgot to cc list
>> originally)
>> >
>> > bitcoin-dev mailing list
>> > bitcoin-dev@lists.linuxfoundation.org
>> <mailto:bitcoin-dev@lists.linuxfoundation.org>
>> > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>
>>
>> _______________________________________________
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>>
>>
>> --
>> Riccardo Casatta - @RCasatta <https://twitter.com/RCasatta>
>
>
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