[bitcoin-dev] Fast Merkle Trees

[Russell O'Connor]

In that case, you may as well remove all references to leaves and double
SHA-256 from your BIP since your design has no method for distinguishing
between internal nodes and leaves.

I think that if this design stands, it will play a role in some future
CVEs.  The BIP itself is too abstract about its data contents to
specifically say that it has a vulnerability; however, I believe it is
inviting vulnerabilities.
For example, I might agree with a counterparty to a design of some sort of
smart contract in the form of a MAST.  My counterparty has shown me all the
"leaves" of our MAST and I can verify its Merkle root computation.
After being deployed, I found out that one of the leaves wasn't really a
leaf but is instead a specially crafted "script" with a fake pubkey chosen
by my couterparty so that this leaf can also be interpreted as a fake
internal node (i.e. an internal node with a right branch of 0x8000...100).
Because the Fast Merkle Tree design doesn't distinguish between leaves and
internal nodes my counter party gets away with building an Inclusion Proof
through this "leaf" to reveal the evil code that they had designed into the
MAST at a deeper level.

Turns out my counterparty was grinding their evil code to produce an
internal node that can also be parsed as an innocent script.  They used
their "pubkey" to absorb excess random data from their grinding that they
cannot eliminate.
(The counterparty doesn't actually know the discrete log of this "pubkey",
they just claimed it was their pubkey and I believed them).


Having ambiguity about whether a node is a leaf or an internal node is a
security risk. Furthermore, changing the design so that internal node and
leaves are distinguishable still allows chained invocations.
Arbitrary data can be stored in Fast Merkle Tree leaves, including the
Merkle root of another Fast Merkle Tree.
Applications that are limited to proof with paths no longer than 32
branches can still circumvent this limit by staging these Fast Merkle Trees
in explicit layers (as opposed to the implicit layers with the current
design).

By storing a inner Fast Merkle Tree root inside the (explicit) leaf of an
outer Fast Merkle Tree, the application can verify a Inclusion Proof of the
inner Fast Merkle Tree Root in the outer Fast Merkle Tree Root, and then
verify a second Inclusion Proof of the desired data in the inner Faster
Merkle Tree Root.  The application will need to tag their data to
distinguish between inner Fast Merkle Tree Roots and other application
data, but that is just part of the general expectation that applications
not store ambiguous data inside the leaves of Fast Merkle Trees.


On Wed, Sep 6, 2017 at 10:20 PM, Mark Friedenbach <mark at friedenbach.org>
wrote:

> This design purposefully does not distinguish leaf nodes from internal
> nodes. That way it chained invocations can be used to validate paths longer
> than 32 branches. Do you see a vulnerability due to this lack of
> distinction?
>
> On Sep 6, 2017, at 6:59 PM, Russell O'Connor <roconnor at blockstream.io>
> wrote:
>
> The fast hash for internal nodes needs to use an IV that is not the
> standard SHA-256 IV. Instead needs to use some other fixed value, which
> should itself be the SHA-256 hash of some fixed string (e.g. the string
> "BIP ???" or "Fash SHA-256").
>
> As it stands, I believe someone can claim a leaf node as an internal node
> by creating a proof that provides a phony right-hand branch claiming to
> have hash 0x80000..0000100 (which is really the padding value for the
> second half of a double SHA-256 hash).
>
> (I was schooled by Peter Todd by a similar issue in the past.)
>
> On Wed, Sep 6, 2017 at 8:38 PM, Mark Friedenbach via bitcoin-dev <
> bitcoin-dev at lists.linuxfoundation.org> wrote:
>
>> Fast Merkle Trees
>> BIP: https://gist.github.com/maaku/41b0054de0731321d23e9da90ba4ee0a
>> Code: https://github.com/maaku/bitcoin/tree/fast-merkle-tree
>>
>
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