[Bitcoin-development] 32 vs 64-bit timestamp fields

[Bitcoin-development] 32 vs 64-bit timestamp fields

[Addy Yeow]

Hi list,

Can someone explain why do we have 32-bit and 64-bit timestamp fields
instead of all being 64-bit?

https://en.bitcoin.it/wiki/Protocol_specification

Cheers,
Addy

Re: [Bitcoin-development] 32 vs 64-bit timestamp fields

[Jeff Garzik]

On Wed, May 8, 2013 at 7:39 PM, Addy Yeow <ayeowch@gmail.com> wrote:
> Hi list,
>
> Can someone explain why do we have 32-bit and 64-bit timestamp fields
> instead of all being 64-bit?
>
> https://en.bitcoin.it/wiki/Protocol_specification

Hysterical raisins.

-- 
Jeff Garzik
exMULTI, Inc.
jgarzik@exmulti.com

Re: [Bitcoin-development] 32 vs 64-bit timestamp fields

[Peter Todd]

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On Thu, May 09, 2013 at 09:39:10AM +1000, Addy Yeow wrote:
> Hi list,
> 
> Can someone explain why do we have 32-bit and 64-bit timestamp fields
> instead of all being 64-bit?
> 
> https://en.bitcoin.it/wiki/Protocol_specification

Who knows?

Satoshi used 32-bits and those fields can't be changed now without every
single Bitcoin user changing all at once. (a "hard-fork" change)

We'll probably need to do one of those eventually for other reasons, so
we might as well leave fixing the timestamps until then.

-- 
'peter'[:-1]@petertodd.org
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Re: [Bitcoin-development] 32 vs 64-bit timestamp fields

[John Dillon]

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On Wed, May 8, 2013 at 11:44 PM, Peter Todd <pete@petertodd.org> wrote:
> Who knows?
>
> Satoshi used 32-bits and those fields can't be changed now without every
> single Bitcoin user changing all at once. (a "hard-fork" change)
>
> We'll probably need to do one of those eventually for other reasons, so
> we might as well leave fixing the timestamps until then.

Perhaps Satoshi did this delibrately, knowing that at some point a hard-fork
would be a good idea, so that we all would have a good excuse to do one?
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Re: [Bitcoin-development] 32 vs 64-bit timestamp fields

[Jeff Garzik]

On Wed, May 8, 2013 at 9:00 PM, John Dillon
<john.dillon892@googlemail.com> wrote:
> Perhaps Satoshi did this delibrately, knowing that at some point a hard-fork
> would be a good idea, so that we all would have a good excuse to do one?

Guffaw :)  The year 2038 is so far in the future that it is not really
relevant, from that angle.

We need a hard fork to break the 1MB limit, and Satoshi explicitly
presumed that would happen sometime in the future.

-- 
Jeff Garzik
exMULTI, Inc.
jgarzik@exmulti.com

Re: [Bitcoin-development] 32 vs 64-bit timestamp fields

[Pieter Wuille]

On Wed, May 08, 2013 at 09:08:34PM -0400, Jeff Garzik wrote:
> On Wed, May 8, 2013 at 9:00 PM, John Dillon
> <john.dillon892@googlemail.com> wrote:
> > Perhaps Satoshi did this delibrately, knowing that at some point a hard-fork
> > would be a good idea, so that we all would have a good excuse to do one?
> 
> Guffaw :)  The year 2038 is so far in the future that it is not really
> relevant, from that angle.

"Meh". I think it's highly unlikely we'll break the block header format, as it
pretty much means invalidating all mining hardware.

There's also no need: 32 bits is plenty of precision. Hell, even 16 bits would
do (assuming there's never more than a 65535s (about 18 hours) gap between two
blocks). Just assume the "full" 64-bit time is the smallest one that makes
sense, given its lower 32 bits.

-- 
Pieter

Re: [Bitcoin-development] 32 vs 64-bit timestamp fields

[John Dillon]

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On Thu, May 9, 2013 at 1:13 AM, Pieter Wuille <pieter.wuille@gmail.com> wrote:
> On Wed, May 08, 2013 at 09:08:34PM -0400, Jeff Garzik wrote:
>> Guffaw :)  The year 2038 is so far in the future that it is not really
>> relevant, from that angle.
>
> "Meh". I think it's highly unlikely we'll break the block header format, as it
> pretty much means invalidating all mining hardware.

Doesn't most mining hardware at the ASCI level start with a SHA256 midstate
given that the nonce is at the end?  Adding further information to the block
should be possible at the beginning of the block without major changes to the
mining hardware.

> There's also no need: 32 bits is plenty of precision. Hell, even 16 bits would
> do (assuming there's never more than a 65535s (about 18 hours) gap between two
> blocks). Just assume the "full" 64-bit time is the smallest one that makes
> sense, given its lower 32 bits.

I feel somewhat uncomfortable about the "after-the-fact" auditing possible in
this scenario. Besides the timestamping provided by the block headers appears
to be useful in some payment protocols, not to mention in general.
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Re: [Bitcoin-development] 32 vs 64-bit timestamp fields

[Peter Todd]

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On Thu, May 09, 2013 at 01:27:33AM +0000, John Dillon wrote:
> > There's also no need: 32 bits is plenty of precision. Hell, even 16 bits would
> > do (assuming there's never more than a 65535s (about 18 hours) gap between two
> > blocks). Just assume the "full" 64-bit time is the smallest one that makes
> > sense, given its lower 32 bits.
> 
> I feel somewhat uncomfortable about the "after-the-fact" auditing possible in
> this scenario. Besides the timestamping provided by the block headers appears
> to be useful in some payment protocols, not to mention in general.

Remember that interpreting the timestamp on a block for the purposes of
timestamping is a lot more subtle than it appears at first.

Any node will accept a block with a timestamp no more than two hours
ahead of what it thinks the current time is. That time is adjusted by
the median of the timestamps reported by your peers. For instance the
RPC call getinfo returns, among other things:

{
    "timeoffset" : -1,
}

That is saying my node's wall clock time is 1 second behind the median
reported by it's peers - pretty good!


Naively you might think this means block timestamps are accurate to
within 2 hours right? Well, it's not so simple. Nodes will accept any
block with any timestamp *after* the median of the last 11 blocks. From
CBlock::AcceptBlock():

    // Check timestamp against prev
    if (GetBlockTime() <= pindexPrev->GetMedianTimePast())
        return state.Invalid(error("AcceptBlock() : block's timestamp is too early"));

So in theory a miner could prevent that block from moving forward,
although if they do they drive up the difficulty, so all miners have an
incentive to set the timestamp accurately.

There are two types of timestamps possible: proofs that data existed
before a time, and proofs that data existed after. With the former type
the *later* the proof says the data existed, the more conservative the
assumptions behind the proof. So simply adding two hours to the block's
timestamp is relatively reasonable. (this assumes the attack managed to
mine a single block, and all nodes have accurate clocks)


The latter type, where you prove data existed after a given time, is a
much more tricky thing to apply. The genesis block is a great example
with that famous newspaper headline:

    The Times 03/Jan/2009 Chancellor on brink of second bailout for
    banks

As I mentioned in my other (private) email to you a few minutes ago, the
sig of my emails has the latest block hash in each one. The basic idea
is called a random beacon; NIST has a good description and a project to
create one:

http://www.nist.gov/itl/csd/ct/nist_beacon.cfm

Now technically speaking a random beacon is actually a more
sophisticated concept than just timestamping, the random beacon's value
is public and distributed widely, but for timestamping the idea is
basically to have an unpredictable number known to have been produced at
a certain time.

So you know this email was written after block #235235, timestamp
2013-05-09 01:21:52 right? Not so fast. All you actually know is the PGP
*signature* was created after that time, because the actual text of the
email is independent of the beacon nonce. (dunno if I have the correct
terminology here FWIW)

For a blockchain it's easy enough, the blocks naturally depend on a
genesis block, but applying the concept more generally is tricky and
application dependent; consider for example proving you created a
keypair after some data, which might be a useful thing to prove if the
secret key was created in some tamperproof hardware that you know has
left the factory and is in your possesion. It's easy to see how to do
this with ECC: just use the same techniques as in HD wallets to derive
keys.

To use the blockchain as a secure random beacon you need to make two
assumptions, 50% of the hashing power is controlled by honest miners,
and those honest miners have accurate clocks. With those assumptions you
can work out what is the minimum possible time the block could have been
accepted by the GetMedianTimePast() function and you are good to go.

What do people do in practice? Well look at
http://vog.github.io/bitcoinproof/, they just give the timestamp and
nothing else. Same for OpenTimestamps. (although I'm adding this email
to my notes, half the reason it's so detailed...)


Back to the block header time... Frankly, the easiest thing to do is
just have a flag day where blocks after a certain height are considered
to have a different epoch from the standard 1970 one when computing
their time. Boring, but it works fine and only needs to be updated every
few decades.


You're midstate idea is very clever though and could come in handy in
the future for different purposes. Eventually we should discuss this
with the ASIC manufacturers - if it can be implemented as a firmware or
FPGA upgrade in the field all the better.

-- 
'peter'[:-1]@petertodd.org
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Re: [Bitcoin-development] 32 vs 64-bit timestamp fields

[John Dillon]

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On Thu, May 9, 2013 at 1:57 AM, Peter Todd <pete@petertodd.org> wrote:
> Remember that interpreting the timestamp on a block for the purposes of
> timestamping is a lot more subtle than it appears at first.

I actually just meant how Pieter Wuille was talking about a blocktime accurate
to only within 18 hours. :) But it is a nice writeup!

In any case, for many things simple relative ordering is enough rather than
absolute time.

> There are two types of timestamps possible: proofs that data existed
> before a time, and proofs that data existed after. With the former type
> the *later* the proof says the data existed, the more conservative the
> assumptions behind the proof. So simply adding two hours to the block's
> timestamp is relatively reasonable. (this assumes the attack managed to
> mine a single block, and all nodes have accurate clocks)

Nope. The attacker can make the timestamp on the block they mine as little as
the minimum from GetMedianTimePast(), and adding two hours to that number could
easily be well before true time.

What you probably need to do is some sort of median time calculation for the
blocks around your timestamp. The proof becomes probabalistic based on the % of
hashing power the attacker controls and in turn depends on if the time they
created their timestamp was of their own choosing.

IE, if you just want to create an inaccurate timestamp, but don't care when,
you can just mine blocks and wait until you get lucky. If you need to create an
inaccurate timestamp *now* the problem is much harder.

But all this analysis can be developed later, and data timestamped now. :)

> Back to the block header time... Frankly, the easiest thing to do is
> just have a flag day where blocks after a certain height are considered
> to have a different epoch from the standard 1970 one when computing
> their time. Boring, but it works fine and only needs to be updated every
> few decades.

Oh, right, yes, that is a much more simple idea and far less prone to bugs.

Many SPV clients wouldn't even need upgrades if they don't acturally validate
the blocks they receive and just look for the biggest PoW.

>
> You're midstate idea is very clever though and could come in handy in
> the future for different purposes. Eventually we should discuss this
> with the ASIC manufacturers - if it can be implemented as a firmware or
> FPGA upgrade in the field all the better.

Yes


Anyway, you are being distracted from what we were talking about before, get
back to work!
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Re: [Bitcoin-development] 32 vs 64-bit timestamp fields

[Peter Todd]

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On Thu, May 09, 2013 at 02:33:11AM +0000, John Dillon wrote:
> On Thu, May 9, 2013 at 1:57 AM, Peter Todd <pete@petertodd.org> wrote:
> > Remember that interpreting the timestamp on a block for the purposes of
> > timestamping is a lot more subtle than it appears at first.
> 
> I actually just meant how Pieter Wuille was talking about a blocktime accurate
> to only within 18 hours. :) But it is a nice writeup!
> 
> In any case, for many things simple relative ordering is enough rather than
> absolute time.

Ah, shoot, I just realized we both got missed Pieter's point entirely:
he means to change the meaning of the header timestamp to be relative
time passed since the last block...

Well, it was a nice writeup! Thanks for the correction re:
probabalistic; you are absolutely correct.

-- 
'peter'[:-1]@petertodd.org
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Re: [Bitcoin-development] 32 vs 64-bit timestamp fields

[Pieter Wuille]

On Wed, May 08, 2013 at 10:42:44PM -0400, Peter Todd wrote:
> Ah, shoot, I just realized we both got missed Pieter's point entirely:
> he means to change the meaning of the header timestamp to be relative
> time passed since the last block...

No, though that's also a possibility, but a backward-incompatible one.

What I mean is have a well-defined 64-bit timestamp for each block, but
only put the lowest 32 bit in the header. Under the condition:

* There is never a gap of more than 136 years between two blocks.

The actual 64-bit timestamp can be deterministically derived from the
header, by prefixing it with the lowest 32-bit value that does not
cause the result to violate the
at-least-above-the-median-of-the-previous-11-blocks rule.

-- 
Pieter

Re: [Bitcoin-development] 32 vs 64-bit timestamp fields

[Mike Hearn]

2038 issues only apply to use of signed timestamps, I thought we treat
this field as unsigned? Is it really a big deal?

On Thu, May 9, 2013 at 1:12 PM, Pieter Wuille <pieter.wuille@gmail.com> wrote:
> On Wed, May 08, 2013 at 10:42:44PM -0400, Peter Todd wrote:
>> Ah, shoot, I just realized we both got missed Pieter's point entirely:
>> he means to change the meaning of the header timestamp to be relative
>> time passed since the last block...
>
> No, though that's also a possibility, but a backward-incompatible one.
>
> What I mean is have a well-defined 64-bit timestamp for each block, but
> only put the lowest 32 bit in the header. Under the condition:
>
> * There is never a gap of more than 136 years between two blocks.
>
> The actual 64-bit timestamp can be deterministically derived from the
> header, by prefixing it with the lowest 32-bit value that does not
> cause the result to violate the
> at-least-above-the-median-of-the-previous-11-blocks rule.
>
> --
> Pieter
>
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Re: [Bitcoin-development] 32 vs 64-bit timestamp fields

[Jeff Garzik]

On Thu, May 9, 2013 at 11:40 AM, Mike Hearn <mike@plan99.net> wrote:
> 2038 issues only apply to use of signed timestamps, I thought we treat
> this field as unsigned? Is it really a big deal?

Not a big deal at all, no.
-- 
Jeff Garzik
exMULTI, Inc.
jgarzik@exmulti.com