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From: Sergio Demian Lerner <sergio.d.lerner@gmail.com>
Date: Sat, 9 Jun 2018 14:21:17 +0200
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Subject: Re: [bitcoin-dev] Trusted merkle tree depth for safe tx inclusion
proofs without a soft fork
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Also it must be noted that an attacker having only 1.3M USD that can
brute-force 72 bits (4 days of hashing on capable ASICs) can perform the
same attack, so the attack is entirely feasible and no person should accept
more than 1M USD using a SPV wallet.
Also the attack can be repeated: once you create the "extension point"
block, you can attack more and more parties without any additional
computation.
On Sat, Jun 9, 2018 at 1:03 PM Sergio Demian Lerner <
sergio.d.lerner@gmail.com> wrote:
> Hi Peter,
> We reported this as CVE-2017-12842, although it may have been known by
> developers before us.
> There are hundreds of SPV wallets out there, without even considering
> other more sensitive systems relying on SPV proofs.
> As I said we, at RSK, discovered this problem in 2017. For RSK it's very
> important this is fixed because our SPV bridge uses SPV proofs.
> I urge all people participating in this mailing list and the rest of the
> Bitcoin community to work on this issue for the security and clean-design
> of Bitcoin.
>
> My suggestion is to use in the version bits 4 bits indicating the tree
> depth (-1), as a soft-fork, so
> 00=1 depth,
> 0F = 16 depth (maximum 64K transactions). Very clean.
>
> The other option is to ban transaction with size lower or equal to 64.
>
> Best regards,
> Sergio.
>
> On Sat, Jun 9, 2018 at 5:31 AM Bram Cohen via bitcoin-dev <
> bitcoin-dev@lists.linuxfoundation.org> wrote:
>
>> So are you saying that if fully validating nodes wish to prune they can
>> maintain the ability to validate old transactions by cacheing the number of
>> transactions in each previous block?
>>
>> On Thu, Jun 7, 2018 at 3:20 PM, Peter Todd <pete@petertodd.org> wrote:
>>
>>> On Thu, Jun 07, 2018 at 02:15:35PM -0700, Bram Cohen wrote:
>>> > Are you proposing a soft fork to include the number of transactions in
>>> a
>>> > block in the block headers to compensate for the broken Merkle format?
>>> That
>>> > sounds like a good idea.
>>> >
>>> > On Thu, Jun 7, 2018 at 10:13 AM, Peter Todd via bitcoin-dev <
>>> > bitcoin-dev@lists.linuxfoundation.org> wrote:
>>> >
>>> > > It's well known that the Bitcoin merkle tree algorithm fails to
>>> distinguish
>>> > > between inner nodes and 64 byte transactions, as both txs and inner
>>> nodes
>>> > > are
>>> > > hashed the same way. This potentially poses a problem for tx
>>> inclusion
>>> > > proofs,
>>> > > as a miner could (with ~60 bits of brute forcing) create a
>>> transaction that
>>> > > committed to a transaction that was not in fact in the blockchain.
>>> > >
>>> > > Since odd-numbered inner/leaf nodes are concatenated with themselves
>>> and
>>> > > hashed
>>> > > twice, the depth of all leaves (txs) in the tree is fixed.
>>> > >
>>> > > It occured to me that if the depth of the merkle tree is known, this
>>> > > vulnerability can be trivially avoided by simply comparing the
>>> length of
>>> > > the
>>> > > merkle path to that known depth. For pruned nodes, if the depth is
>>> saved
>>> > > prior
>>> > > to pruning the block contents itself, this would allow for
>>> completely safe
>>> > > verification of tx inclusion proofs, without a soft-fork; storing
>>> this
>>> ^^^^^^^^^^^^^^^^^^^
>>>
>>> Re-read my post: I specifically said you do not need a soft-fork to
>>> implement
>>> this. In fact, I think you can argue that this is an accidental feature,
>>> not a
>>> bug, as it further encourages the use of safe full verifiaction rather
>>> than
>>> unsafe lite clients.
>>>
>>> --
>>> https://petertodd.org 'peter'[:-1]@petertodd.org
>>>
>>
>> _______________________________________________
>> bitcoin-dev mailing list
>> bitcoin-dev@lists.linuxfoundation.org
>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>
>
--000000000000df6c04056e348f08
Content-Type: text/html; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
<div dir=3D"ltr">Also it must be noted that an attacker having only 1.3M US=
D that can brute-force 72 bits (4 days of hashing on capable ASICs) can per=
form the same attack, so the attack is entirely feasible and no person shou=
ld accept more than 1M USD using a SPV wallet.<div><br></div><div>Also the =
attack can be repeated: once you create the "extension point" blo=
ck, you can attack more and more parties without any additional computation=
.</div><div><br></div></div><br><div class=3D"gmail_quote"><div dir=3D"ltr"=
>On Sat, Jun 9, 2018 at 1:03 PM Sergio Demian Lerner <<a href=3D"mailto:=
sergio.d.lerner@gmail.com">sergio.d.lerner@gmail.com</a>> wrote:<br></di=
v><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:=
1px #ccc solid;padding-left:1ex"><div dir=3D"ltr">Hi Peter,<div>We reported=
this as CVE-2017-12842, although it may have been known by developers befo=
re us.=C2=A0</div><div>There are=C2=A0hundreds of SPV wallets out there, wi=
thout even considering other more sensitive systems relying on SPV proofs.=
=C2=A0</div><div>As I said we, at RSK, discovered this problem in 2017. For=
RSK it's very important this is fixed because our SPV bridge uses SPV =
proofs.</div><div>I urge all people participating in this mailing list and =
the rest of the Bitcoin community to work on this issue for the security an=
d clean-design of Bitcoin.</div><div><br></div><div>My suggestion is to use=
in the version bits 4 bits indicating the tree depth (-1), as a soft-fork,=
so=C2=A0</div><div>00=3D1 depth,=C2=A0</div><div>0F =3D 16 depth (maximum =
64K transactions).=20
<span style=3D"background-color:rgb(255,255,255);text-decoration-style:init=
ial;text-decoration-color:initial;float:none;display:inline">Very clean.</s=
pan></div><div>=C2=A0</div><div>The other option is to ban transaction with=
size lower or equal to 64.=C2=A0</div><div><br></div><div>Best regards,</d=
iv><div>=C2=A0Sergio.</div></div><br><div class=3D"gmail_quote"><div dir=3D=
"ltr">On Sat, Jun 9, 2018 at 5:31 AM Bram Cohen via bitcoin-dev <<a href=
=3D"mailto:bitcoin-dev@lists.linuxfoundation.org" target=3D"_blank">bitcoin=
-dev@lists.linuxfoundation.org</a>> wrote:<br></div><blockquote class=3D=
"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding=
-left:1ex"><div dir=3D"ltr">So are you saying that if fully validating node=
s wish to prune they can maintain the ability to validate old transactions =
by cacheing the number of transactions in each previous block?</div><div cl=
ass=3D"gmail_extra"><br><div class=3D"gmail_quote">On Thu, Jun 7, 2018 at 3=
:20 PM, Peter Todd <span dir=3D"ltr"><<a href=3D"mailto:pete@petertodd.o=
rg" target=3D"_blank">pete@petertodd.org</a>></span> wrote:<br><blockquo=
te class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc so=
lid;padding-left:1ex"><span>On Thu, Jun 07, 2018 at 02:15:35PM -0700, Bram =
Cohen wrote:<br>
> Are you proposing a soft fork to include the number of transactions in=
a<br>
> block in the block headers to compensate for the broken Merkle format?=
That<br>
> sounds like a good idea.<br>
> <br>
> On Thu, Jun 7, 2018 at 10:13 AM, Peter Todd via bitcoin-dev <<br>
> <a href=3D"mailto:bitcoin-dev@lists.linuxfoundation.org" target=3D"_bl=
ank">bitcoin-dev@lists.linuxfoundation.org</a>> wrote:<br>
> <br>
> > It's well known that the Bitcoin merkle tree algorithm fails =
to distinguish<br>
> > between inner nodes and 64 byte transactions, as both txs and inn=
er nodes<br>
> > are<br>
> > hashed the same way. This potentially poses a problem for tx incl=
usion<br>
> > proofs,<br>
> > as a miner could (with ~60 bits of brute forcing) create a transa=
ction that<br>
> > committed to a transaction that was not in fact in the blockchain=
.<br>
> ><br>
> > Since odd-numbered inner/leaf nodes are concatenated with themsel=
ves and<br>
> > hashed<br>
> > twice, the depth of all leaves (txs) in the tree is fixed.<br>
> ><br>
> > It occured to me that if the depth of the merkle tree is known, t=
his<br>
> > vulnerability can be trivially avoided by simply comparing the le=
ngth of<br>
> > the<br>
> > merkle path to that known depth. For pruned nodes, if the depth i=
s saved<br>
> > prior<br>
> > to pruning the block contents itself, this would allow for comple=
tely safe<br>
> > verification of tx inclusion proofs, without a soft-fork; storing=
this<br>
</span>=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=
=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =
=C2=A0^^^^^^^^^^^^^^^^^^^<br>
<br>
Re-read my post: I specifically said you do not need a soft-fork to impleme=
nt<br>
this. In fact, I think you can argue that this is an accidental feature, no=
t a<br>
bug, as it further encourages the use of safe full verifiaction rather than=
<br>
unsafe lite clients.<br>
<div class=3D"m_-7137015386548983706m_-1263791027191071155HOEnZb"><div clas=
s=3D"m_-7137015386548983706m_-1263791027191071155h5"><br>
-- <br>
<a href=3D"https://petertodd.org" rel=3D"noreferrer" target=3D"_blank">http=
s://petertodd.org</a> 'peter'[:-1]@<a href=3D"http://petertodd.org"=
rel=3D"noreferrer" target=3D"_blank">petertodd.org</a><br>
</div></div></blockquote></div><br></div>
_______________________________________________<br>
bitcoin-dev mailing list<br>
<a href=3D"mailto:bitcoin-dev@lists.linuxfoundation.org" target=3D"_blank">=
bitcoin-dev@lists.linuxfoundation.org</a><br>
<a href=3D"https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev" =
rel=3D"noreferrer" target=3D"_blank">https://lists.linuxfoundation.org/mail=
man/listinfo/bitcoin-dev</a><br>
</blockquote></div>
</blockquote></div>
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