[bitcoin-dev] Trusted merkle tree depth for safe tx inclusion proofs without a soft fork

Peter Todd pete at petertodd.org
Thu Jun 7 22:20:28 UTC 2018

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 at 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
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