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From: vjudeu@gazeta.pl
To: ZmnSCPxj <ZmnSCPxj@protonmail.com>, "yanmaani@cock.li" <yanmaani@cock.li>, 
 Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
In-Reply-To: <nSiUl71p9JyISxvRJ3Jq71zNahe-rpanbFFv1MSHSk7rUKjq36yD7vmrJQ5Pnh5oUdDAFflgSzbCE5KK7RacFRepvjqFc9xp9qT7hU-twXA=@protonmail.com>
Date: Sat, 16 Oct 2021 11:06:17 +0200
Message-Id: <143903239-0c7634127ba6ddee7e69b14740b993cd@pmq3v.m5r2.onet>
Subject: Re: [bitcoin-dev] Year 2038 problem and year 2106 chain halting
Precedence: list

> What happens if a series of blocks has a timestamp of 0xFFFFFFFF at the a=
ppropriate time?

The chain will halt for all old clients, because there is no 32-bit value g=
reater than 0xffffffff.

> 1. Is not violated, since "not lower than" means "greater than or equal t=
o"

No, because it has to be strictly "greater than" in the Bitcoin Core source=
 code, it is rejected when it is "lower or equal to", see: https://github.c=
om/bitcoin/bitcoin/blob/6f0cbc75be7644c276650fd98bfdb6358b827399/src/valida=
tion.cpp#L3089-L3094

> 2. Is not violated, since it would be a past actual real time.

If the current time is 0x0000000100000000, then the lowest 32 bits will poi=
nt to some time around 1970, so for old clients two rules are violated at t=
he same time.

> 3. Is not violated since 0xFFFFFFFF < 0x100000000.

This is hard to change, because 32-bit timestamps are included in block hea=
ders, so using any wider data type here will make it hardware-incompatible =
and will cause a hard-fork. That's why I think new timestamps should be pla=
ced in the coinbase transaction. But that still does not solve chain haltin=
g problem.

To test chain halting, all that is needed is starting regtest and producing=
 one block with 0xffffffff timestamp, just after the Genesis Block. Then, m=
edian time is equal to 0xffffffff and adding any new blocks is no longer po=
ssible. The only soft-fork solution I can see require overwriting that bloc=
k.

Example from https://bitcointalk.org/index.php?topic=3D5365359.0

submitblock 0100000006226e46111a0b59caaf126043eb5bbf28c34f3a5e332a1fc7b2b73=
cf188910f3663c0de115e2239e05df4df9c4bfa01b8e843aaf5dae590cac1d9bac0d44c0fff=
ffffffffff7f200100000001020000000001010000000000000000000000000000000000000=
000000000000000000000000000ffffffff03510101ffffffff0200f2052a010000001976a9=
1462e907b15cbf27d5425399ebf6f0fb50ebb88f1888ac0000000000000000266a24aa21a9e=
de2f61c3f71d1defd3fa999dfa36953755c690689799962b48bebd836974e8cf90120000000=
000000000000000000000000000000000000000000000000000000000000000000
null
generatetoaddress 1 mpXwg4jMtRhuSpVq4xS3HFHmCmWp9NyGKt
CreateNewBlock: TestBlockValidity failed: time-too-old, block's timestamp i=
s too early (code -1)

I don't know any timestamp that can be used in any next block and accepted =
by old nodes.

On 2021-10-16 01:01:54 user ZmnSCPxj <ZmnSCPxj@protonmail.com> wrote:
> Good morning yanmaani,


> It's well-known. Nobody really cares, because it's so far off. Not
> possible to do by softfork, no.

I think it is possible by softfork if we try hard enough?


> 1.  The block timestamp may not be lower than the median of the last 11
>     blocks'
>
> 2.  The block timestamp may not be greater than the current time plus two
>     hours
>
> 3.  The block timestamp may not be greater than 2^32 (Sun, 07 Feb 2106
>     06:28:16 +0000)

What happens if a series of blocks has a timestamp of 0xFFFFFFFF at the app=
ropriate time?

In that case:

1.  Is not violated, since "not lower than" means "greater than or equal to=
", and after a while the median becomes 0xFFFFFFFF and 0xFFFFFFFF =3D=3D 0x=
FFFFFFFF
2.  Is not violated, since it would be a past actual real time.
3.  Is not violated since 0xFFFFFFFF < 0x100000000.

In that case, we could then add an additional rule, which is that a 64-bit =
(or 128-bit, or 256-bit) timestamp has to be present in the coinbase transa=
ction, with similar rules except translated to 64-bit/128-bit/256-bit.

Possibly a similar scheme could be used for `nLockTime`; we could put a 64-=
bit `nLockTime64` in that additional signed block in Taproot SegWit v1 if t=
he legacy v`nLockTime` is at the maximum seconds-timelock possible.

Regards,
ZmnSCPxj