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

Sergio Demian Lerner sergio.d.lerner at gmail.com
Sat Jun 9 11:03:53 UTC 2018


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 at 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 at 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 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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