* [Bitcoin-development] From the forums: one-confirmation attack
@ 2011-08-18 14:00 Gavin Andresen
2011-08-18 15:36 ` Joel Joonatan Kaartinen
2011-08-18 16:47 ` theymos
0 siblings, 2 replies; 8+ messages in thread
From: Gavin Andresen @ 2011-08-18 14:00 UTC (permalink / raw)
To: Bitcoin Dev
vector76 on the Forums posted this interesting variation on a 'Finney attack' :
https://bitcointalk.org/index.php?topic=36788.msg463391#msg463391
"Let's say I observe the timing of when nodes are broadcasting
transactions and how they are propagating through the network. By
watching for which nodes are earliest to broadcast transactions from
my target, I manage to establish a direct connection to my target.
I use a similar method of watching block broadcasts to establish
connections to most of the mining pools.
Now I create a transaction making a valid, large deposit into my
target. I do not broadcast this transaction but I add it to a block
that I am attempting to mine. I mine solo, just like normal, except
that I have an extra non-broadcasted tx that I am including.
Eventually, I succeed in creating a valid block. I do not broadcast
it immediately, but instead I wait until someone else mines a block,
and when that happens, I immediately broadcast my block to my target.
If my target sees my block before the other block, they will accept
it, and my transaction will have one confirmation. The block chain
has forked, and my target (and possibly other nodes, if my target
relays quickly enough) will believe that my block is the correct one,
while other nodes will believe that the other fork is the correct one.
I immediately request a withdrawal, and my target generates a
transaction sending the large amount of coins to an address I control.
I also double-spend some of the inputs, sending the coins to myself.
The part of the network that did not receive my block first (which
hopefully is most of the miners) will accept this as valid and work to
include it in the next block.
If my block eventually "wins" because enough miners saw my block first
and added onto it first, then I have just made a deposit and
withdrawal, and I lose nothing.
If my block eventually "loses", then the deposit is invalidated. If
the deposit tx was not one of the inputs to the withdrawal
transaction, then the withdrawal is still valid."
-------------------------------
The lessons are "don't accept 1-confirmation transactions" and "try
to be well-connected."
But maybe the deeper lesson is "don't trust information you get from
only one peer." Or maybe "watch for peers that are trying to fool
you."
--
--
Gavin Andresen
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [Bitcoin-development] From the forums: one-confirmation attack
2011-08-18 14:00 [Bitcoin-development] From the forums: one-confirmation attack Gavin Andresen
@ 2011-08-18 15:36 ` Joel Joonatan Kaartinen
2011-08-18 15:52 ` Mike Hearn
2011-08-18 16:47 ` theymos
1 sibling, 1 reply; 8+ messages in thread
From: Joel Joonatan Kaartinen @ 2011-08-18 15:36 UTC (permalink / raw)
To: Gavin Andresen; +Cc: Bitcoin Dev
On Thu, 2011-08-18 at 10:00 -0400, Gavin Andresen wrote:
> The lessons are "don't accept 1-confirmation transactions" and "try
> to be well-connected."
>
> But maybe the deeper lesson is "don't trust information you get from
> only one peer." Or maybe "watch for peers that are trying to fool
> you."
This particular attack seems quite dependant on the target not using the
deposit Tx as input. I believe this whole class of attacks become
ineffective if each account has it's own separate wallet.
Also, receiving a block with a transaction that hasn't been broadcast to
the network is in itself quite suspect. Are there cases where that
happens legitimately? Perhaps in such a case, don't treat the block it
came in as a confirmation at all. Instead, start counting from the next
one.
- Joel
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [Bitcoin-development] From the forums: one-confirmation attack
2011-08-18 15:36 ` Joel Joonatan Kaartinen
@ 2011-08-18 15:52 ` Mike Hearn
2011-08-18 16:16 ` Gavin Andresen
0 siblings, 1 reply; 8+ messages in thread
From: Mike Hearn @ 2011-08-18 15:52 UTC (permalink / raw)
To: Joel Joonatan Kaartinen; +Cc: Bitcoin Dev
> Also, receiving a block with a transaction that hasn't been broadcast to
> the network is in itself quite suspect. Are there cases where that
> happens legitimately?
There's no way to obtain the memory pools of your peers today, so if
you're newly started up it can happen that you get blocks with unseen
transactions.
For vectors variant, I wonder if it'd be safe to report the number of
confirmations differently for the duration of a chain split. If you
have a block but a majority of peers relayed a block that split the
chain, subtract 1 from each confirmation reported via RPC.
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [Bitcoin-development] From the forums: one-confirmation attack
2011-08-18 15:52 ` Mike Hearn
@ 2011-08-18 16:16 ` Gavin Andresen
2011-08-18 16:46 ` Gregory Maxwell
0 siblings, 1 reply; 8+ messages in thread
From: Gavin Andresen @ 2011-08-18 16:16 UTC (permalink / raw)
To: Mike Hearn; +Cc: Bitcoin Dev
> For vectors variant, I wonder if it'd be safe to report the number of
> confirmations differently for the duration of a chain split. If you
> have a block but a majority of peers relayed a block that split the
> chain, subtract 1 from each confirmation reported via RPC.
Or maybe report them as 'suspicious.' Changing the meaning of
'confirmations' is likely to break code (e.g. code like block =
current_blockchain[blockcount-tx.confirmations] ... would give the
wrong block).
A floating-point 0.0-1.0 'confidence' measure might be a good idea to
go along with the integer confirmations. I can think of all sorts of
ways of gauging the reliability of transactions or blocks (did it come
from a trusted peer-- assuming we eventually have trusted peers. Does
it have a lot of confirmations? Are there no active block chain
forks? Have we been getting new blocks at the rate we expect? etc
etc etc)
We could start with an as simple-as-possible "confidence == 0 if
confirmations < 2, otherwise confidence = function(#confirmations)"
and improve it from there.
--
--
Gavin Andresen
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [Bitcoin-development] From the forums: one-confirmation attack
2011-08-18 16:16 ` Gavin Andresen
@ 2011-08-18 16:46 ` Gregory Maxwell
2011-08-18 17:36 ` Gavin Andresen
0 siblings, 1 reply; 8+ messages in thread
From: Gregory Maxwell @ 2011-08-18 16:46 UTC (permalink / raw)
To: Gavin Andresen; +Cc: Bitcoin Dev
On Thu, Aug 18, 2011 at 12:16 PM, Gavin Andresen
<gavinandresen@gmail•com> wrote:
> We could start with an as simple-as-possible "confidence == 0 if
> confirmations < 2, otherwise confidence = function(#confirmations)"
> and improve it from there.
At the same time, if this causes people to wait less than the 6 blocks
that the software currently waits for before leaving unconfirmed
status then that would be sad.
Simply waiting a number of blocks is an excellent metric and provides
robustness against almost all attack patterns in a way that various
one-off-heuristics can not as it equates to _real difficulty_ (and
real expense (hashing computation, loss of income on the orphaned
blocks)) in a way that can't be faked.
A few weeks back when there was some rumor going around that mybitcoin
lost coin based on some kind of one confirmation attack I described on
IRC a similar attack pattern which had a useful improvement:
* The attacker runs many widely distributed sybil nodes (e.g. using
botnet drones as simple tcp proxies to appear at many addresses). He
takes advantage of the fact the bitcoin won't connect to /16s that
have already connected to it to further isolate nodes.
* By creating normal looking probe transactions which his own nodes
won't forward he detects network partitions which he is able to
create. He searches for a cut which causes there to be at least two
partitions which contain significant mining power.
* He creates two accounts at MoronBank. He doesn't even bother
identifying MoronBank's node. MoronBank will be in one partition or
another. He makes deposits in both partitions, and conflicting
transactions in the opposite partitions, while carefully filtering out
these transactions from crossing the boundary.
(Notably, the network doesn't appear partitioned to everyone else now
because he's still forwarding blocks and transactions unrelated to his
attack— it only becomes visible once some of his evil transactions are
mined)
* After the funds show up in MoronBank he withdraws and drops the partitioning.
Only if he has difficulty getting MoronBank into the smaller partition
does he need to bother locating it and attacking it directly.
The bad thing about this attack is that it doesn't require the
attacker to have any hash power at all: he captures miners as
unwilling (or willing but plausibly deniable) participants. The lost
income from orphaned blocks is externalized to the victimized miners
(and since most pools don't pay orphaned blocks out of pocket a pool
operator would be inclined to help out).
The good thing about it is that it's killed dead by nodes adding in a
few manually configured peerings, they don't even really need to be
trusted: You just need to trust that they don't all go to a single
bad-guy conspiracy. At a minimum all major miners should be fully
meshed.
Unfortunately, We don't currently have software for this as addnode
doesn't worry about keeping the links up... and the major pools also
don't seem to be interested in participating.
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [Bitcoin-development] From the forums: one-confirmation attack
2011-08-18 16:46 ` Gregory Maxwell
@ 2011-08-18 17:36 ` Gavin Andresen
0 siblings, 0 replies; 8+ messages in thread
From: Gavin Andresen @ 2011-08-18 17:36 UTC (permalink / raw)
To: Gregory Maxwell; +Cc: Bitcoin Dev
Gregory said: "...if this causes people to wait less than the 6 blocks
that the software currently waits for before leaving unconfirmed
status then that would be sad."
People are already considering transactions 'confirmed enough' at less
than six blocks. I'm guilty, too-- 3 is/was the magic number for
ClearCoin.
And people are already experimenting with ways of safely accepting
0-confirmation transactions, like InstaWallet's "green" payments (sent
from a trusted-not-to-double-spend address).
Since there is definitely market demand for "as fast as possible"
confirmation, I'm thinking adding a placeholder to the RPC interface
might be a good idea. Although after thinking about it some more,
maybe a signed integer "trust" rating for blocks/transactions would be
a better way of doing it...
RE: miners connecting themselves together in a semi-trusted "bitcoin
backbone" : agreed.
Matt submitted a patch to connect and stay-connected to a set of
nodes, but I complained about the implementation. Seems to me the
networking code needs an overhaul, to implement a priority queue of
potential peers (trusted peers would be sorted to near the top of the
queue, peers you think are badly-behaved would be sorted to the
bottom, with lots of randomness so not everybody on the network is
trying to connect to the same set of peers). With peer rotation to
mitigate manipulate-time and other Sybil attacks.
--
--
Gavin Andresen
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [Bitcoin-development] From the forums: one-confirmation attack
2011-08-18 14:00 [Bitcoin-development] From the forums: one-confirmation attack Gavin Andresen
2011-08-18 15:36 ` Joel Joonatan Kaartinen
@ 2011-08-18 16:47 ` theymos
2011-08-18 17:27 ` Gregory Maxwell
1 sibling, 1 reply; 8+ messages in thread
From: theymos @ 2011-08-18 16:47 UTC (permalink / raw)
To: bitcoin-development
> The part of the network that did not receive my block first (which
> hopefully is most of the miners) will accept this as valid and work to
> include it in the next block.
To make this more difficult, nodes that receive new transactions via
blocks should relay the new transactions as if they had received them
normally. Then the double-spend transaction will be rejected by most of
the network because the other version is already widely-known.
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [Bitcoin-development] From the forums: one-confirmation attack
2011-08-18 16:47 ` theymos
@ 2011-08-18 17:27 ` Gregory Maxwell
0 siblings, 0 replies; 8+ messages in thread
From: Gregory Maxwell @ 2011-08-18 17:27 UTC (permalink / raw)
To: theymos; +Cc: bitcoin-development
On Thu, Aug 18, 2011 at 12:47 PM, theymos <theymos@mm•st> wrote:
> To make this more difficult, nodes that receive new transactions via
> blocks should relay the new transactions as if they had received them
> normally. Then the double-spend transaction will be rejected by most of
> the network because the other version is already widely-known.
Not that helpful. In that attack pattern the attacker can release the
block and the conflicting transaction at the same time (to different
nodes, of course). They can also inject the conflicting transaction
into many places in the network at once.
^ permalink raw reply [flat|nested] 8+ messages in thread
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2011-08-18 14:00 [Bitcoin-development] From the forums: one-confirmation attack Gavin Andresen
2011-08-18 15:36 ` Joel Joonatan Kaartinen
2011-08-18 15:52 ` Mike Hearn
2011-08-18 16:16 ` Gavin Andresen
2011-08-18 16:46 ` Gregory Maxwell
2011-08-18 17:36 ` Gavin Andresen
2011-08-18 16:47 ` theymos
2011-08-18 17:27 ` Gregory Maxwell
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