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From: Toshi Morita <toshi@peernova.com>
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Cc: Bitcoin Dev <bitcoin-development@lists.sourceforge.net>
Subject: Re: [Bitcoin-development] Possible attack: Keeping unconfirmed
	transactions
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From what I know, Alice does not know to which node Bob will broadcast the
transaction. Therefore, Alice cannot intercept the transaction and prevent
the rest of the network from seeing it.

Toshi



On Fri, Jun 6, 2014 at 3:02 PM, Ra=C3=BAl Mart=C3=ADnez <rme@i-rme.es> wrot=
e:

> I dont know if this attack is even possible, it came to my mind and I wil=
l
> try to explain it as good as possible.
>
> Some transacions keep unconfirmed forever and finally they are purged by
> Bitcoin nodes, mostly due to the lack of fees.
>
>
> Example:
> ---------
>
> Alice is selling a pizza to Bob, Bob is now making the payment with
> Bitcoin.
> The main goal of this attack is to store a unconfirmed transaction send b=
y
> Bob for a few days (it will not be included in the blockchain because it
> has no fee or due to other reason), Bob might resend the payment or might
> just cancel the deal with Alice.
>
> Bob forgets about that failed trade but a couple of days later, Alice, wh=
o
> has stored the signed transacion, relays the transaction to the network (=
or
> mines it directly with his own hashpower).
> Bob does not know what is happening, he believed that that transaction wa=
s
> "canceled forever", he even does not remember the failed pizza deal.
>
> Alice has now the bitcoins and Bob does not know what happened with his
> money.
>
> ---------
>
> This might also work with the Payment Protocol because when using it Bob
> does not relay the transaction to the network, its Alices job to do it,
> Alice stores it and tells Bob to resend the payment, Bob creates another
> transaction (If has the same inputs as the first TX this does not work)
> (this one is relayed by Alice to the network).
>
> Alice comes back a couple of days later and mines with his hashrate the
> first transaction (the one she didnt relayed to the network).
>
> Alice now has two payments, Bob does not know what happened.
>
>
> -----------
>
> I hope that I explained well this possible attack, I dont know if there i=
s
> already a fix for this problem or if it is simply impossible to execute
> this kind of attack.
>
> Thanks for your time.
>
>
>
>
>
>
> -------------------------------------------------------------------------=
-----
> Learn Graph Databases - Download FREE O'Reilly Book
> "Graph Databases" is the definitive new guide to graph databases and thei=
r
> applications. Written by three acclaimed leaders in the field,
> this first edition is now available. Download your free book today!
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>
>

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<div dir=3D"ltr">From what I know, Alice does not know to which node Bob wi=
ll broadcast the transaction. Therefore, Alice cannot intercept the transac=
tion and prevent the rest of the network from seeing it.<div><br></div><div=
>
Toshi</div><div><br></div></div><div class=3D"gmail_extra"><br><br><div cla=
ss=3D"gmail_quote">On Fri, Jun 6, 2014 at 3:02 PM, Ra=C3=BAl Mart=C3=ADnez =
<span dir=3D"ltr">&lt;<a href=3D"mailto:rme@i-rme.es" target=3D"_blank">rme=
@i-rme.es</a>&gt;</span> wrote:<br>
<blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1p=
x #ccc solid;padding-left:1ex"><div dir=3D"ltr">I dont know if this attack =
is even possible, it came to my mind and I will try to explain it as good a=
s possible.<div>
<br></div><div>Some transacions keep unconfirmed forever and finally they a=
re purged by Bitcoin nodes, mostly due to the lack of fees.</div>

<div><br></div><div><br></div><div>Example:</div><div>---------</div><div><=
br></div><div>Alice is selling a pizza to Bob, Bob is now making the paymen=
t with Bitcoin.</div><div>The main goal of this attack is to store a unconf=
irmed transaction send by Bob for a few days (it will not be included in th=
e blockchain because it has no fee or due to other reason), Bob might resen=
d the payment or might just cancel the deal with Alice.</div>


<div><br></div><div>Bob forgets about that failed trade but a couple of day=
s later, Alice, who has stored the signed transacion, relays the transactio=
n to the network (or mines it directly with his own hashpower).</div><div>


Bob does not know what is happening, he believed that that transaction was =
&quot;canceled forever&quot;, he even does not remember the failed pizza de=
al.</div><div><br></div><div>Alice has now the bitcoins and Bob does not kn=
ow what happened with his money.</div>


<div><br></div><div>---------</div><div><br></div><div>This might also work=
 with the Payment Protocol because when using it Bob does not relay the tra=
nsaction to the network, its Alices job to do it, Alice stores it and tells=
 Bob to resend the payment, Bob creates another transaction (If has the sam=
e inputs as the first TX this does not work) (this one is relayed by Alice =
to the network).</div>


<div><br></div><div>Alice comes back a couple of days later and mines with =
his hashrate the first transaction (the one she didnt relayed to the networ=
k).</div><div><br></div><div>Alice now has two payments, Bob does not know =
what happened.</div>


<div><br></div><div><br></div><div>-----------</div><div><br></div><div>I h=
ope that I explained well this possible attack, I dont know if there is alr=
eady a fix for this problem or if it is simply impossible to execute this k=
ind of attack.</div>


<div><br></div><div>Thanks for your time.</div><div><br></div><div><br></di=
v><div><br></div><div><br></div></div>
<br>-----------------------------------------------------------------------=
-------<br>
Learn Graph Databases - Download FREE O&#39;Reilly Book<br>
&quot;Graph Databases&quot; is the definitive new guide to graph databases =
and their<br>
applications. Written by three acclaimed leaders in the field,<br>
this first edition is now available. Download your free book today!<br>
<a href=3D"http://p.sf.net/sfu/NeoTech" target=3D"_blank">http://p.sf.net/s=
fu/NeoTech</a><br>_______________________________________________<br>
Bitcoin-development mailing list<br>
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pment@lists.sourceforge.net</a><br>
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velopment</a><br>
<br></blockquote></div><br></div>

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