MIN_STANDARD_TX_NONWITNESS_SIZE and OP_RETURN

MIN_STANDARD_TX_NONWITNESS_SIZE and OP_RETURN
2020-05-23 15:24 ` Greg Sanders
  
From: Thomas Voegtlin @ 2020-05-23 14:49 UTC (permalink / raw)
  To: bitcoin-dev

Hello list,

I have been trying to CPFP a transaction using OP_RETURN, because the
remaining output value would have been lower than the dust threshold.

The scriptPubkey of the output was OP_RETURN + OP_0, and there was a
single p2wsh input.

The result is a 60 bytes transaction (without witness), that gets
rejected because it is lower than MIN_STANDARD_TX_NONWITNESS_SIZE, which
is equal to 82 bytes.

Why is that value so high? Would it make sense to lower it to 60?


Thomas


 permalink raw reply	[|nested]
2020-05-23 14:49 [bitcoin-dev] MIN_STANDARD_TX_NONWITNESS_SIZE and OP_RETURN Thomas Voegtlin

  2020-05-23 15:25   ` Greg Sanders
  
From: Greg Sanders @ 2020-05-23 15:24 UTC (permalink / raw)
  To: Thomas Voegtlin, Bitcoin Protocol Discussion

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AFAIU the number was picked to protect against CVE-2017-12842 covertly.
See: https://github.com/bitcoin/bitcoin/pull/16885
<https://github.com/bitcoin/bitcoin/pull/16885/files> which updated the
text to explicitly mention this fact.

On Sat, May 23, 2020 at 11:20 AM Thomas Voegtlin via bitcoin-dev <
bitcoin-dev@lists•linuxfoundation.org> wrote:

> Hello list,
>
> I have been trying to CPFP a transaction using OP_RETURN, because the
> remaining output value would have been lower than the dust threshold.
>
> The scriptPubkey of the output was OP_RETURN + OP_0, and there was a
> single p2wsh input.
>
> The result is a 60 bytes transaction (without witness), that gets
> rejected because it is lower than MIN_STANDARD_TX_NONWITNESS_SIZE, which
> is equal to 82 bytes.
>
> Why is that value so high? Would it make sense to lower it to 60?
>
>
> Thomas
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists•linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>

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 permalink raw reply	[|nested]
2020-05-23 15:24 ` Greg Sanders

  2020-05-24  0:52     ` ZmnSCPxj
  
From: Greg Sanders @ 2020-05-23 15:25 UTC (permalink / raw)
  To: Thomas Voegtlin, Bitcoin Protocol Discussion

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So I think the question to ask would be "why can't we just make sure it's
not 64?"

On Sat, May 23, 2020 at 11:24 AM Greg Sanders <gsanders87@gmail•com> wrote:

> AFAIU the number was picked to protect against CVE-2017-12842 covertly.
> See: https://github.com/bitcoin/bitcoin/pull/16885
> <https://github.com/bitcoin/bitcoin/pull/16885/files> which updated the
> text to explicitly mention this fact.
>
> On Sat, May 23, 2020 at 11:20 AM Thomas Voegtlin via bitcoin-dev <
> bitcoin-dev@lists•linuxfoundation.org> wrote:
>
>> Hello list,
>>
>> I have been trying to CPFP a transaction using OP_RETURN, because the
>> remaining output value would have been lower than the dust threshold.
>>
>> The scriptPubkey of the output was OP_RETURN + OP_0, and there was a
>> single p2wsh input.
>>
>> The result is a 60 bytes transaction (without witness), that gets
>> rejected because it is lower than MIN_STANDARD_TX_NONWITNESS_SIZE, which
>> is equal to 82 bytes.
>>
>> Why is that value so high? Would it make sense to lower it to 60?
>>
>>
>> Thomas
>> _______________________________________________
>> bitcoin-dev mailing list
>> bitcoin-dev@lists•linuxfoundation.org
>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>
>

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 permalink raw reply	[|nested]
2020-05-23 15:25   ` Greg Sanders

  2020-05-27 15:15       ` Russell O'Connor
  
From: ZmnSCPxj @ 2020-05-24  0:52 UTC (permalink / raw)
  To: Greg Sanders, Bitcoin Protocol Discussion

Good morning Thomas,

> So I think the question to ask would be "why can't we just make sure it's not 64?"

If we accept a 60-byte tx, then SHA-256 will pad it to 64 bytes, and it may still be possible to mount CVE-2017-12842 attack with 32-bits of work.
Of course some other details will be changed from the standard SHA-256 in mounting this attack, but from my poor understanding it seems safer to just avoid the area around length 64.

It *might* be safe to accept 65-byte or larger (but do not believe me, I only play a cryptographer on the Internet), but that does not help your specific application, which uses 60 byte tx.

Regards,
ZmnSCPxj

>
> On Sat, May 23, 2020 at 11:24 AM Greg Sanders <gsanders87@gmail•com> wrote:
>
> > AFAIU the number was picked to protect against CVE-2017-12842 covertly. See: https://github.com/bitcoin/bitcoin/pull/16885 which updated the text to explicitly mention this fact.
> >
> > On Sat, May 23, 2020 at 11:20 AM Thomas Voegtlin via bitcoin-dev <bitcoin-dev@lists•linuxfoundation.org> wrote:
> >
> > > Hello list,
> > >
> > > I have been trying to CPFP a transaction using OP_RETURN, because the
> > > remaining output value would have been lower than the dust threshold.
> > >
> > > The scriptPubkey of the output was OP_RETURN + OP_0, and there was a
> > > single p2wsh input.
> > >
> > > The result is a 60 bytes transaction (without witness), that gets
> > > rejected because it is lower than MIN_STANDARD_TX_NONWITNESS_SIZE, which
> > > is equal to 82 bytes.
> > >
> > > Why is that value so high? Would it make sense to lower it to 60?
> > >
> > > Thomas
> > > _______________________________________________
> > > bitcoin-dev mailing list
> > > bitcoin-dev@lists•linuxfoundation.org
> > > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev


 permalink raw reply	[|nested]
2020-05-24  0:52     ` ZmnSCPxj

  
From: Russell O'Connor @ 2020-05-27 15:15 UTC ( / )
  To: ZmnSCPxj, Bitcoin Protocol Discussion;  Greg Sanders



I don't believe that 60 bytes is a problem here.  SHA256 padding includes a
length value of the original message data. Thus a padded non-64 byte
transaction can never be the same as any padded 64-byte value, and
therefore after applying the SHA256 compression function the resulting
hashes cannot be identical (unless SHA256 itself is broken).

P.S. SHA256 also includes 10* padding, which also suffices to ensure
messages of different lengths have different padding.

On Sat, May 23, 2020 at 8:52 PM ZmnSCPxj via bitcoin-dev <
bitcoin-dev@lists•linuxfoundation.org> wrote:

> Good morning Thomas,
>
> > So I think the question to ask would be "why can't we just make sure
> it's not 64?"
>
> If we accept a 60-byte tx, then SHA-256 will pad it to 64 bytes, and it
> may still be possible to mount CVE-2017-12842 attack with 32-bits of work.
> Of course some other details will be changed from the standard SHA-256 in
> mounting this attack, but from my poor understanding it seems safer to just
> avoid the area around length 64.
>
> It *might* be safe to accept 65-byte or larger (but do not believe me, I
> only play a cryptographer on the Internet), but that does not help your
> specific application, which uses 60 byte tx.
>
> Regards,
> ZmnSCPxj
>
> >
> > On Sat, May 23, 2020 at 11:24 AM Greg Sanders <gsanders87@gmail•com>
> wrote:
> >
> > > AFAIU the number was picked to protect against CVE-2017-12842
> covertly. See: https://github.com/bitcoin/bitcoin/pull/16885 which
> updated the text to explicitly mention this fact.
> > >
> > > On Sat, May 23, 2020 at 11:20 AM Thomas Voegtlin via bitcoin-dev <
> bitcoin-dev@lists•linuxfoundation.org> wrote:
> > >
> > > > Hello list,
> > > >
> > > > I have been trying to CPFP a transaction using OP_RETURN, because the
> > > > remaining output value would have been lower than the dust threshold.
> > > >
> > > > The scriptPubkey of the output was OP_RETURN + OP_0, and there was a
> > > > single p2wsh input.
> > > >
> > > > The result is a 60 bytes transaction (without witness), that gets
> > > > rejected because it is lower than MIN_STANDARD_TX_NONWITNESS_SIZE,
> which
> > > > is equal to 82 bytes.
> > > >
> > > > Why is that value so high? Would it make sense to lower it to 60?
> > > >
> > > > Thomas
> > > > _______________________________________________
> > > > bitcoin-dev mailing list
> > > > bitcoin-dev@lists•linuxfoundation.org
> > > > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists•linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>