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From: James MacWhyte <keatonatron@gmail.com>
Date: Wed, 30 Jan 2019 12:58:03 -0800
Message-ID: <CAH+Axy77+OTZ12=FFhrdg-1xznKjVQUT58mP6yjVbt7sWtgA7g@mail.gmail.com>
To: rhavar@protonmail.com
Content-Type: multipart/alternative; boundary="0000000000001038d60580b32c9e"
Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] bustapay BIP :: a practical sender/receiver
 coinjoin protocol
Precedence: list

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On Tue, Jan 29, 2019 at 6:46 PM <rhavar@protonmail.com> wrote:

>
> If the sender refuses to sign the final transaction, the receiver just
> propagates the template transaction which pays the receiver! So it's a
> pretty weak attack.
>
> The only real attack is that the sender could double-spend the
> template-transaction before it's propagated, but the cost of doing this
> isn't free, as at the very least you need to pay the transaction fees of
> creating a double spend. It's not an amazingly good defence, but it's good
> enough that it's unlikely to get abused (and an attacker would only learn a
> single utxo of the receiver) .
>

Okay, I see what you mean. I better understand the weaknesses you've
identified, and I can't really think of a better solution than what you've
proposed. I also realized that implementors who aren't capable of
integrating signing and UTXO validation wouldn't be the ones trying to
implement this feature, so my concerns there are also moot. Carry on ;)

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<div dir=3D"ltr"><div dir=3D"ltr"><br></div><br><div class=3D"gmail_quote">=
<div dir=3D"ltr" class=3D"gmail_attr">On Tue, Jan 29, 2019 at 6:46 PM &lt;<=
a href=3D"mailto:rhavar@protonmail.com">rhavar@protonmail.com</a>&gt; wrote=
:</div><blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;=
border-left:1px solid rgb(204,204,204);padding-left:1ex"><div><br></div><di=
v>If the sender refuses to sign the final transaction, the receiver just pr=
opagates the template transaction which pays the receiver! So it&#39;s a pr=
etty weak attack.<br></div><div><br></div><div>The only real attack is that=
 the sender could double-spend the template-transaction before it&#39;s pro=
pagated, but the cost of doing this isn&#39;t free, as at the very least yo=
u need to pay the transaction fees of creating a double spend. It&#39;s not=
 an amazingly good defence, but it&#39;s good enough that it&#39;s unlikely=
 to get abused (and an attacker would only learn a single utxo of the recei=
ver) .</div></blockquote><div><br></div><div>Okay, I see what you mean. I b=
etter understand the weaknesses you&#39;ve identified, and I can&#39;t real=
ly think of a better solution than what you&#39;ve proposed. I also realize=
d that implementors who aren&#39;t capable of integrating signing and UTXO =
validation wouldn&#39;t be the ones trying to implement this feature, so my=
 concerns there are also moot. Carry on ;)</div></div></div>

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