[Bitcoin-development] SPV clients and relaying double spends

[Bitcoin-development] SPV clients and relaying double spends

[Aaron Voisine]

There was some discussion of having nodes relay double-spends in order
to alert the network about double spend attempts.

A lot more users will be using SPV wallets in the future, and one of
the techniques SPV clients use to judge how likely a transaction is to
be confirmed is if it propagates across the network. I wonder if and
when double-spend relaying is introduced, if nodes should also send
BIP61 reject messages or something along those lines to indicate which
transactions those nodes believe to be invalid, but are relaying
anyway.

This would be subject to sybil attacks, as is monitoring propagation,
however it does still increase the cost of performing a 0 confirmation
double spend attack on an SPV client above just relaying double-spends
without indicating if a node believes the transaction to be valid.

Aaron Voisine
breadwallet.com

Re: [Bitcoin-development] SPV clients and relaying double spends

[Matt Whitlock]

What's to stop an attacker from broadcasting millions of spends of the same output(s) and overwhelming nodes with slower connections? Might it be a better strategy not to relay the actual transactions (after the first) but rather only propagate (once) some kind of double-spend alert?


On Thursday, 25 September 2014, at 7:02 pm, Aaron Voisine wrote:
> There was some discussion of having nodes relay double-spends in order
> to alert the network about double spend attempts.
> 
> A lot more users will be using SPV wallets in the future, and one of
> the techniques SPV clients use to judge how likely a transaction is to
> be confirmed is if it propagates across the network. I wonder if and
> when double-spend relaying is introduced, if nodes should also send
> BIP61 reject messages or something along those lines to indicate which
> transactions those nodes believe to be invalid, but are relaying
> anyway.
> 
> This would be subject to sybil attacks, as is monitoring propagation,
> however it does still increase the cost of performing a 0 confirmation
> double spend attack on an SPV client above just relaying double-spends
> without indicating if a node believes the transaction to be valid.
> 
> Aaron Voisine
> breadwallet.com

Re: [Bitcoin-development] SPV clients and relaying double spends

[Aaron Voisine]

Something like that would be a great help for SPV clients that can't
detect double spends on their own. (still limited of course to sybil
attack concerns)

Aaron Voisine
breadwallet.com


On Thu, Sep 25, 2014 at 7:07 PM, Matt Whitlock <bip@mattwhitlock.name> wrote:
> What's to stop an attacker from broadcasting millions of spends of the same output(s) and overwhelming nodes with slower connections? Might it be a better strategy not to relay the actual transactions (after the first) but rather only propagate (once) some kind of double-spend alert?
>
>
> On Thursday, 25 September 2014, at 7:02 pm, Aaron Voisine wrote:
>> There was some discussion of having nodes relay double-spends in order
>> to alert the network about double spend attempts.
>>
>> A lot more users will be using SPV wallets in the future, and one of
>> the techniques SPV clients use to judge how likely a transaction is to
>> be confirmed is if it propagates across the network. I wonder if and
>> when double-spend relaying is introduced, if nodes should also send
>> BIP61 reject messages or something along those lines to indicate which
>> transactions those nodes believe to be invalid, but are relaying
>> anyway.
>>
>> This would be subject to sybil attacks, as is monitoring propagation,
>> however it does still increase the cost of performing a 0 confirmation
>> double spend attack on an SPV client above just relaying double-spends
>> without indicating if a node believes the transaction to be valid.
>>
>> Aaron Voisine
>> breadwallet.com
>

Re: [Bitcoin-development] SPV clients and relaying double spends

[Matt Whitlock]

Probably the first double-spend attempt (i.e., the second transaction to spend the same output(s) as another tx already in the mempool) would still need to be relayed. A simple "double-spend alert" wouldn't work because it could be forged. But after there have been two attempts to spend the same output, no further transactions spending that same output should be relayed, in order to prevent flooding the network.


On Thursday, 25 September 2014, at 7:12 pm, Aaron Voisine wrote:
> Something like that would be a great help for SPV clients that can't
> detect double spends on their own. (still limited of course to sybil
> attack concerns)
> 
> Aaron Voisine
> breadwallet.com
> 
> 
> On Thu, Sep 25, 2014 at 7:07 PM, Matt Whitlock <bip@mattwhitlock.name> wrote:
> > What's to stop an attacker from broadcasting millions of spends of the same output(s) and overwhelming nodes with slower connections? Might it be a better strategy not to relay the actual transactions (after the first) but rather only propagate (once) some kind of double-spend alert?
> >
> >
> > On Thursday, 25 September 2014, at 7:02 pm, Aaron Voisine wrote:
> >> There was some discussion of having nodes relay double-spends in order
> >> to alert the network about double spend attempts.
> >>
> >> A lot more users will be using SPV wallets in the future, and one of
> >> the techniques SPV clients use to judge how likely a transaction is to
> >> be confirmed is if it propagates across the network. I wonder if and
> >> when double-spend relaying is introduced, if nodes should also send
> >> BIP61 reject messages or something along those lines to indicate which
> >> transactions those nodes believe to be invalid, but are relaying
> >> anyway.
> >>
> >> This would be subject to sybil attacks, as is monitoring propagation,
> >> however it does still increase the cost of performing a 0 confirmation
> >> double spend attack on an SPV client above just relaying double-spends
> >> without indicating if a node believes the transaction to be valid.
> >>
> >> Aaron Voisine
> >> breadwallet.com
> >

Re: [Bitcoin-development] SPV clients and relaying double spends

[Aaron Voisine]

Of course you wouldn't want nodes to propagate alerts without
independently verifying them, otherwise anyone could just issue alerts
for every new transaction.

Aaron Voisine
breadwallet.com


On Thu, Sep 25, 2014 at 7:16 PM, Matt Whitlock <bip@mattwhitlock.name> wrote:
> Probably the first double-spend attempt (i.e., the second transaction to spend the same output(s) as another tx already in the mempool) would still need to be relayed. A simple "double-spend alert" wouldn't work because it could be forged. But after there have been two attempts to spend the same output, no further transactions spending that same output should be relayed, in order to prevent flooding the network.
>
>
> On Thursday, 25 September 2014, at 7:12 pm, Aaron Voisine wrote:
>> Something like that would be a great help for SPV clients that can't
>> detect double spends on their own. (still limited of course to sybil
>> attack concerns)
>>
>> Aaron Voisine
>> breadwallet.com
>>
>>
>> On Thu, Sep 25, 2014 at 7:07 PM, Matt Whitlock <bip@mattwhitlock.name> wrote:
>> > What's to stop an attacker from broadcasting millions of spends of the same output(s) and overwhelming nodes with slower connections? Might it be a better strategy not to relay the actual transactions (after the first) but rather only propagate (once) some kind of double-spend alert?
>> >
>> >
>> > On Thursday, 25 September 2014, at 7:02 pm, Aaron Voisine wrote:
>> >> There was some discussion of having nodes relay double-spends in order
>> >> to alert the network about double spend attempts.
>> >>
>> >> A lot more users will be using SPV wallets in the future, and one of
>> >> the techniques SPV clients use to judge how likely a transaction is to
>> >> be confirmed is if it propagates across the network. I wonder if and
>> >> when double-spend relaying is introduced, if nodes should also send
>> >> BIP61 reject messages or something along those lines to indicate which
>> >> transactions those nodes believe to be invalid, but are relaying
>> >> anyway.
>> >>
>> >> This would be subject to sybil attacks, as is monitoring propagation,
>> >> however it does still increase the cost of performing a 0 confirmation
>> >> double spend attack on an SPV client above just relaying double-spends
>> >> without indicating if a node believes the transaction to be valid.
>> >>
>> >> Aaron Voisine
>> >> breadwallet.com
>> >

Re: [Bitcoin-development] SPV clients and relaying double spends

[Christophe Biocca]

A lot of this discussion has already occured. Some code was even
merged into master, then reverted.

See:
https://github.com/bitcoin/bitcoin/issues/4550
https://github.com/bitcoin/bitcoin/pull/4570

It would probably be a good idea to start from that code, as it
addresses many of the possible pitfalls you've been discussing.

On Thu, Sep 25, 2014 at 10:37 PM, Aaron Voisine <voisine@gmail.com> wrote:
> Of course you wouldn't want nodes to propagate alerts without
> independently verifying them, otherwise anyone could just issue alerts
> for every new transaction.
>
> Aaron Voisine
> breadwallet.com
>
>
> On Thu, Sep 25, 2014 at 7:16 PM, Matt Whitlock <bip@mattwhitlock.name> wrote:
>> Probably the first double-spend attempt (i.e., the second transaction to spend the same output(s) as another tx already in the mempool) would still need to be relayed. A simple "double-spend alert" wouldn't work because it could be forged. But after there have been two attempts to spend the same output, no further transactions spending that same output should be relayed, in order to prevent flooding the network.
>>
>>
>> On Thursday, 25 September 2014, at 7:12 pm, Aaron Voisine wrote:
>>> Something like that would be a great help for SPV clients that can't
>>> detect double spends on their own. (still limited of course to sybil
>>> attack concerns)
>>>
>>> Aaron Voisine
>>> breadwallet.com
>>>
>>>
>>> On Thu, Sep 25, 2014 at 7:07 PM, Matt Whitlock <bip@mattwhitlock.name> wrote:
>>> > What's to stop an attacker from broadcasting millions of spends of the same output(s) and overwhelming nodes with slower connections? Might it be a better strategy not to relay the actual transactions (after the first) but rather only propagate (once) some kind of double-spend alert?
>>> >
>>> >
>>> > On Thursday, 25 September 2014, at 7:02 pm, Aaron Voisine wrote:
>>> >> There was some discussion of having nodes relay double-spends in order
>>> >> to alert the network about double spend attempts.
>>> >>
>>> >> A lot more users will be using SPV wallets in the future, and one of
>>> >> the techniques SPV clients use to judge how likely a transaction is to
>>> >> be confirmed is if it propagates across the network. I wonder if and
>>> >> when double-spend relaying is introduced, if nodes should also send
>>> >> BIP61 reject messages or something along those lines to indicate which
>>> >> transactions those nodes believe to be invalid, but are relaying
>>> >> anyway.
>>> >>
>>> >> This would be subject to sybil attacks, as is monitoring propagation,
>>> >> however it does still increase the cost of performing a 0 confirmation
>>> >> double spend attack on an SPV client above just relaying double-spends
>>> >> without indicating if a node believes the transaction to be valid.
>>> >>
>>> >> Aaron Voisine
>>> >> breadwallet.com
>>> >
>
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Re: [Bitcoin-development] SPV clients and relaying double spends

[Tom Harding]

On 9/25/2014 7:37 PM, Aaron Voisine wrote:
> Of course you wouldn't want nodes to propagate alerts without
> independently verifying them
How would a node independently verify a double-spend alert, other than 
by having access to an actual signed double-spend?

#4570 relays the first double-spend AS an alert.  Running this branch on 
mainnet, I have been keeping a live list of relayed double-spend 
transactions at http://respends.thinlink.com

[Bitcoin-development] replace-by-fee v0.9.3 release

[Peter Todd]

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On Sat, Sep 27, 2014 at 07:55:44PM -0700, Tom Harding wrote:
> On 9/25/2014 7:37 PM, Aaron Voisine wrote:
> > Of course you wouldn't want nodes to propagate alerts without
> > independently verifying them
> How would a node independently verify a double-spend alert, other than 
> by having access to an actual signed double-spend?
> 
> #4570 relays the first double-spend AS an alert.  Running this branch on 
> mainnet, I have been keeping a live list of relayed double-spend 
> transactions at http://respends.thinlink.com

Speaking of, I ported my replace-by-fee branch the recent v0.9.3
release: https://github.com/petertodd/bitcoin/tree/replace-by-fee-v0.9.3

I actually ported it a few days ago; that release has been running on a
half-dozen or so nodes right now for a few days with no issues.

The v0.9.3 release's scriptSig size limit increase adds a new category
of double-spending exploit. I'm not going to get time to add that
exploit to my replace-by-fee toolkit(1) for at least another week or so
though - pull-reqs accepted.

1) https://github.com/petertodd/replace-by-fee-tools

-- 
'peter'[:-1]@petertodd.org
0000000000000000084778cc7b7394a48d65c9451a59dcf98d0f1e1078f39c3a

[-- Attachment #2: Digital signature --]
[-- Type: application/pgp-signature, Size: 650 bytes --]

Re: [Bitcoin-development] replace-by-fee v0.9.3 release

[Luke Dashjr]

On Sunday, September 28, 2014 5:15:53 AM Peter Todd wrote:
> On Sat, Sep 27, 2014 at 07:55:44PM -0700, Tom Harding wrote:
> > On 9/25/2014 7:37 PM, Aaron Voisine wrote:
> > > Of course you wouldn't want nodes to propagate alerts without
> > > independently verifying them
> > 
> > How would a node independently verify a double-spend alert, other than
> > by having access to an actual signed double-spend?
> > 
> > #4570 relays the first double-spend AS an alert.  Running this branch on
> > mainnet, I have been keeping a live list of relayed double-spend
> > transactions at http://respends.thinlink.com
> 
> Speaking of, I ported my replace-by-fee branch the recent v0.9.3
> release: https://github.com/petertodd/bitcoin/tree/replace-by-fee-v0.9.3
> 
> I actually ported it a few days ago; that release has been running on a
> half-dozen or so nodes right now for a few days with no issues.
> 
> The v0.9.3 release's scriptSig size limit increase adds a new category
> of double-spending exploit. I'm not going to get time to add that
> exploit to my replace-by-fee toolkit(1) for at least another week or so
> though - pull-reqs accepted.
> 
> 1) https://github.com/petertodd/replace-by-fee-tools

Do you have or can you provide a version compatible with CPFP, such that a 
child paying a higher fee trumps the parent's replacement?

Preferably something that will merge cleanly into 0.9.x-ljr :)

Luke

Re: [Bitcoin-development] SPV clients and relaying double spends

[s7r]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1



On 9/26/2014 5:16 AM, Matt Whitlock wrote:
> Probably the first double-spend attempt (i.e., the second 
> transaction to spend the same output(s) as another tx already in 
> the mempool) would still need to be relayed. A simple
> "double-spend alert" wouldn't work because it could be forged. But
> after there have been two attempts to spend the same output, no
> further transactions spending that same output should be relayed,
> in order to prevent flooding the network.
> 
This sounds rational - is this already implemented nowadays or *SHOULD
BE* implemented to prevent this attack type in the future?
> 
> On Thursday, 25 September 2014, at 7:12 pm, Aaron Voisine wrote:
>> Something like that would be a great help for SPV clients that 
>> can't detect double spends on their own. (still limited of
>> course to sybil attack concerns)
>> 
>> Aaron Voisine breadwallet.com
>> 
>> 
>> On Thu, Sep 25, 2014 at 7:07 PM, Matt Whitlock 
>> <bip@mattwhitlock.name> wrote:
>>> What's to stop an attacker from broadcasting millions of
>>> spends of the same output(s) and overwhelming nodes with
>>> slower connections? Might it be a better strategy not to relay
>>> the actual transactions (after the first) but rather only
>>> propagate (once) some kind of double-spend alert?
>>> 
>>> 
>>> On Thursday, 25 September 2014, at 7:02 pm, Aaron Voisine 
>>> wrote:
>>>> There was some discussion of having nodes relay
>>>> double-spends in order to alert the network about double
>>>> spend attempts.
>>>> 
>>>> A lot more users will be using SPV wallets in the future,
>>>> and one of the techniques SPV clients use to judge how likely
>>>> a transaction is to be confirmed is if it propagates across
>>>> the network. I wonder if and when double-spend relaying is 
>>>> introduced, if nodes should also send BIP61 reject messages 
>>>> or something along those lines to indicate which
>>>> transactions those nodes believe to be invalid, but are
>>>> relaying anyway.
>>>> 
>>>> This would be subject to sybil attacks, as is monitoring 
>>>> propagation, however it does still increase the cost of 
>>>> performing a 0 confirmation double spend attack on an SPV 
>>>> client above just relaying double-spends without indicating 
>>>> if a node believes the transaction to be valid.
>>>> 
>>>> Aaron Voisine breadwallet.com
>>> 
> 
> ------------------------------------------------------------------------------
>
>
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s7r
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