Date: Thu, 7 Jun 2018 18:20:28 -0400
From: Peter Todd <pete@petertodd.org>
To: Bram Cohen <bram@chia.net>
Message-ID: <20180607222028.zbva4vrv64dzrmxy@petertodd.org>
References: <20180607171311.6qdjohfuuy3ufriv@petertodd.org>
	<CAHUJnBB7UL3mH6SixP_M4yooMVP3DgZa+5hiQOmF=AiqfdpfOg@mail.gmail.com>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Trusted merkle tree depth for safe tx inclusion
 proofs without a soft fork
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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? Th=
at
> sounds like a good idea.
>=20
> On Thu, Jun 7, 2018 at 10:13 AM, Peter Todd via bitcoin-dev <
> bitcoin-dev@lists.linuxfoundation.org> wrote:
>=20
> > It's well known that the Bitcoin merkle tree algorithm fails to disting=
uish
> > between inner nodes and 64 byte transactions, as both txs and inner nod=
es
> > 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 s=
afe
> > 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 impleme=
nt
this. In fact, I think you can argue that this is an accidental feature, no=
t a
bug, as it further encourages the use of safe full verifiaction rather than
unsafe lite clients.

--=20
https://petertodd.org 'peter'[:-1]@petertodd.org

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