[Bitcoin-development] About Compact SPV proofs via block header commitments

Sergio Lerner sergiolerner at certimix.com
Sun Apr 27 01:08:15 UTC 2014


I read the post in this threads about Compact SPV proofs via block
header commitments (archived e-mail in
https://www.mail-archive.com/bitcoin-development@lists.sourceforge.net/msg04318.html).
I was working on the same problem almost at the same time, which is
something that's becoming very common nowadays..

The proposal starts with the following claim:

"In simple payment verification (SPV) proofs it is currently necessary
that every intervening block header be provided between two blocks in
order to establish both connectivity and proof of work."

I think this is false. Let's first assume that at the time of an attack
we're connected only to the attacker (no honest nodes). An
non-interactive SPV proof needs to prove that a transaction belongs to
the best chain because creating a counterfeit proof cost more than the
amount of money involved in the proof. Suppose that the proof consist at
least of a block header and a merkle branch to the claimed transaction.

Do the proof need connectivity with the last checkpoint known by the
verifier? (here checkpoint is any block known for sure to be in the best
chain)

Not much, because connectivity only proves that the proof was not
pre-computed before the checkpoint. Only if the checkpoint is very near
(say 10 blocks back) it brings some practical evidence that the attacker
did not have much time to prepare an attack.
 
Do the proof need to know the interleaving proof-of-work?

Not much. If the distance between blocks is less than 2016 blocks, then
the difficulty may have change only by a factor of 4. Currently
difficult adjustments are much lower (I suppose that about 1.1 or so).
Then one can assume that the proof block target difficulty is almost the
same as the last known difficulty if the block distance is less than
2016. If the distance is more, we just load all the interleaving
re-target blocks to detect the actual difficulty.

Do the proof need to have a certain number of  confirmations?

Yes and no, because this is the only evidence that the prover has either
spend money in creating the fake block or took a genuine block.
The cost of creating a fake block can be approximated as the minimum of:
- The current reward of the block (since currently fees are much lower
than the reward)
- The average block fees (when the reward goes to zero)
- The minimum electricity cost of mining the block.

As time passes one could assume that the electricity cost of mining will
approach the other two. 

But there is the problem of parallel synchronized attacks: if an
attacker can reuse the same fake SPV proof to attack several victims,
then the reward for cheating increases proportionally but the cost stays
the same.
For instance, if 6 confirmations are required, and each block can hold
2000 transactions, the attacker can find 2000 victims and re-use the
same 6 block chain to "prove" payments to 2000 victims. Also the cost of
mining 6 blocks can be amortized by mining 7, and attacking in the first
two 4000 victims, etc...

Any scheme than relies on non-interactive SPV proofs will fail if
Bitcoin will scale up-to a point where victims can be easily found and
synchronized.
So I think one should assume that at least one peer is honest...

But if we assume than during the attack least one peer is honest, then
we could directly ask every peer to give us the blocks of their
best-chains at the same heights of the presented proof.  No back-links
are necessary.  If any peer shows a different block, then we should
carefully detect which of the two nodes is the one attacking us and ban
it, by downloading the best-chain headers from the last checkpoint to
the block of the proof.  This would be rare so I don't see when the
back-links can help.

The use case should be:

==Use cases==

For SPV client that has just come online asks peers what is the last block height/time. 
If a peer replies with an old block, then that peer is still downloading the block-chain and it's ignored.
For the remaining peers, the client starts asking for parents blocks until all parents agree (this is the last common parent). 
If (U)TxO hash-tree commitments are available, then the wallet is updated using this data from the common parent block. 

At the same time the client retrieves compact non-interactive proofs-of-inclusion (possibly orphan) for its transactions 
without having to download every intervening block header.

Is there something wrong with this?
 
Best regards,
Sergio.




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