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From: Sanket Kanjalkar <sanket1729@gmail.com>
Date: Sat, 14 Jun 2025 17:20:06 -0700
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Subject: Re: [bitcoindev] CTV + CSFS: a letter
To: Greg Maxwell <gmaxwell@gmail.com>
Cc: Jameson Lopp <jameson.lopp@gmail.com>, Antoine Poinsot <darosior@protonmail.com>, 
	Matt Corallo <lf-lists@mattcorallo.com>, Andrew Poelstra <apoelstra@wpsoftware.net>, 
	Bitcoin Development Mailing List <bitcoindev@googlegroups.com>
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On Sat, Jun 14, 2025 at 5:01=E2=80=AFPM Greg Maxwell <gmaxwell@gmail.com> w=
rote:

> On Sat, Jun 14, 2025 at 11:50=E2=80=AFPM Sanket Kanjalkar <sanket1729@gma=
il.com>
> wrote:
>
>> Do you mean arbitrary output address that is unknown at commitment time?
>> Otherwise, I think the current CTV vault does allow abort/allowing from
>> "stage area" to "hot area" or abort to "rescue area". While general purp=
ose
>> recursive vaults will allow funds back into same "cold area", I think it=
 is
>> possible to also move funds back into same back under the same cold keys
>> with a bounded recursion CTV provides.
>>
>
> Moving funds back to the initial key that the attacker already has
> demonstrated the ability to release from doesn't seem useful to me.  --
> though that is a thing the presigned example I gave doesn't do.
>
>
>> Finally, on the usefulness of vaults; based on my own observation of all
>> the hacks (bitcoin and wider crypto), in most cases it is not the key th=
at
>> is stolen but rather the authorization process or UI/UX hacks or somethi=
ng
>> else up the signing stack is compromised. Having reactive security to
>> "undo" feels valuable in this scenario.
>>
>
> Is there an example of a hack that has been defeated by one?  It would be
> interesting to see the exact workflow.
>
I presume in this case any rational attacker will not attempt something
like this until they know they can succeed. A weaker argument here might be
that this setup by itself would discourage attackers to attempt to continue
:). I can try to look up examples for defeated hacks, but they might be
hard to find.

>
> If the scheme is just released into a 'hot area' and the hot area keys
> have the power to send the coins anywhere, presumably the attacker will
> attack the hot area keys and wait for funds to be moved there and
> instantly sweep once they're there.  If the hot area keys are presumed
> secure, then they can be multisig on the release from 'cold'.
>
Any amount of money that you move in hot wallet can be stolen. The point
here would be limit the theft exposure, for example, you never have more
than X BTC in hot wallets. I agree that this might be awkward to do in
practice with CTV vaults because you have to move the entire UTXO into "hot
area" in order to send change back. Having a powerful vault primitive helps
avoid this issue with change amounts. But in case of CTV, this can be
somewhat mitigated with careful UTXO management.

When moving larger amounts across wallets, you would move them in
increments of X BTC one by one limiting exposure.


--=20
Sanket Kanjalkar

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<div dir=3D"ltr"><div dir=3D"ltr"><br></div><br><div class=3D"gmail_quote g=
mail_quote_container"><div dir=3D"ltr" class=3D"gmail_attr">On Sat, Jun 14,=
 2025 at 5:01=E2=80=AFPM Greg Maxwell &lt;<a href=3D"mailto:gmaxwell@gmail.=
com">gmaxwell@gmail.com</a>&gt; wrote:<br></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 dir=3D"ltr"><div dir=3D"ltr">On Sat, Jun 14, 20=
25 at 11:50=E2=80=AFPM Sanket Kanjalkar &lt;<a href=3D"mailto:sanket1729@gm=
ail.com" target=3D"_blank">sanket1729@gmail.com</a>&gt; wrote:<br></div><di=
v class=3D"gmail_quote"><blockquote class=3D"gmail_quote" style=3D"margin:0=
px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><=
div dir=3D"ltr"><div class=3D"gmail_quote"><div>Do you mean arbitrary outpu=
t address that is unknown at commitment time? Otherwise, I think the curren=
t CTV vault does allow abort/allowing from &quot;stage area&quot; to &quot;=
hot area&quot; or abort to &quot;rescue area&quot;. While general purpose r=
ecursive vaults will allow funds back into same &quot;cold area&quot;, I th=
ink it is possible to also move funds back into same back under the same co=
ld keys with a bounded recursion CTV provides.<br></div></div></div></block=
quote><div><br></div><div>Moving funds back to the initial key that the att=
acker already has demonstrated the ability to release from doesn&#39;t seem=
 useful to me.=C2=A0 -- though that is a thing=C2=A0the presigned example I=
 gave doesn&#39;t do.</div><div><br></div><blockquote class=3D"gmail_quote"=
 style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);p=
adding-left:1ex"><div dir=3D"ltr"><div class=3D"gmail_quote"><div><br>Final=
ly, on the usefulness of vaults; based on my own observation of all the hac=
ks (bitcoin and wider crypto), in most cases it is not the key that is stol=
en but rather the authorization process or UI/UX hacks or something else up=
 the signing stack is compromised. Having reactive security to &quot;undo&q=
uot; feels valuable in this scenario.=C2=A0</div></div></div></blockquote><=
div><br></div><div>Is there an example of a hack that has been defeated by =
one?=C2=A0 It would be interesting to see the exact workflow.</div></div></=
div></blockquote><div>I presume in this case any rational attacker will not=
 attempt something like this until they know they can succeed. A weaker arg=
ument here might be that this setup by itself would discourage attackers to=
 attempt to continue :). I can try to look up examples for defeated hacks, =
but they might be hard to find.<br></div><blockquote class=3D"gmail_quote" =
style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);pa=
dding-left:1ex"><div dir=3D"ltr"><div class=3D"gmail_quote"><div><br></div>=
<div>If the scheme is just released into a &#39;hot area&#39; and the hot a=
rea keys have the power to send the coins anywhere, presumably the attacker=
 will attack the hot area keys and wait for funds to be moved there and ins=
tantly=C2=A0sweep once they&#39;re there.=C2=A0 If the hot area keys are pr=
esumed secure, then they can be multisig on the release from &#39;cold&#39;=
.</div></div></div></blockquote><div>Any amount of money that you move in h=
ot wallet can be stolen. The point here would be limit the theft exposure, =
for example, you never have more than X BTC in hot wallets. I agree that th=
is might be awkward to do in practice with CTV vaults because you have to m=
ove the entire UTXO into &quot;hot area&quot; in order to send change back.=
 Having a powerful vault primitive helps avoid this issue with change amoun=
ts. But in case of CTV, this can be somewhat mitigated with careful UTXO ma=
nagement.<br><br>When moving larger amounts across wallets, you would move =
them in increments of X BTC one by one limiting exposure.=C2=A0</div></div>=
<div><br clear=3D"all"></div><div><br></div><span class=3D"gmail_signature_=
prefix">-- </span><br><div dir=3D"ltr" class=3D"gmail_signature"><div dir=
=3D"ltr">Sanket Kanjalkar<br></div></div></div>

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