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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Generalised Replay Protection for Future Hard
Forks
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> But I like the 'old' idea of putting the hash of a block that MUST be =
on the chain that this txn can eventually be added to. If the hash is =
not a valid block on the chain, the txn can't be added.
>=20
> It means you can choose exactly which forks you want to allow your txn =
on, pre-fork for both, post-fork for only one, and gets round the issue =
of who gets to decide the nForkid value.. since you don't need one. =
Also, all the old outputs work fine, and LN not an issue.
>=20
> I'm missing why this scheme would be better ?
I do agree that solutions like `SIGHASH_BLOCKCOMMIT` are superior in the =
sense that they are very difficult to circumvent. However, a fork could =
also follow the original chain in SPV mode and allow transactions =
protected with these mechanism. Since it's fundamentally impossible to =
disallow transactions in future projects, the goal shouldn't be to make =
this overly complicated.
Furthermore, this schema is not just adding replay protection. It makes =
transacting safer overall (due to a dedicated address format per fork) =
and allows light clients to differentiate between multiple forks. In the =
past three months, at least $600k has been lost by users sending BCH to =
a BTC address [1].
> Thanks for the clarification. How would a tx specify a constraint =
like "nForkId>=3D1"? I was thinking of it just as a number set on the =
tx.
Whether the transaction is replay protected or not is specified by =
setting a bit in the `SigHashId`. If this bit is set, then the signature =
*preimage* MUST have `nForkId` appended. `nForkId` is not part of the =
final transaction, someone who wants to verify the transaction must know =
which `nForkId` it was created with.
If the bit isn't set, it means `nForkId=3D0`, which allows other forks =
to validate the signature.
> Also note that since forks form a partial order, but IDs (numbers) =
form a total order, ">=3D" will miss some cases. Eg, suppose BCH had =
forked with nForkId 2, and then you set up a LN funding tx on BCH with =
nForkId>=3D2, and then Segwit2x forked (from BTC!) with nForkId 3. The =
BCH funding tx would be valid on Segwit2x. This is more of a =
fundamental problem than a bug - to avoid it you'd have to get into =
stuff like making each fork reference its parent-fork's first block or =
something, someone has written about this...
Sorry, I was careless with the use of `>=3D` there. You are correct, =
forks form a tree. For this proposal, every leaf must be assigned a =
unique `nForkId`. The relationship between `nForkId` is irrelevant (e.g. =
which number is bigger), as long as they are unique. Transactions are =
only valid IFF `nForkId` matches exactly the `nForkId` of the software =
validating it. As described above, the transaction doesn't even contain =
`nForkId`, and the node surely is not starting to guess which one it =
could be.
[1]
https://twitter.com/khannib/status/930223617744437253 =
<https://twitter.com/khannib/status/930223617744437253>
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<html><head><meta http-equiv=3D"Content-Type" content=3D"text/html =
charset=3Dus-ascii"></head><body style=3D"word-wrap: break-word; =
-webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" =
class=3D""><div><br class=3D""><blockquote type=3D"cite" class=3D""><div =
class=3D""><div dir=3D"auto" class=3D""><div dir=3D"auto" class=3D"">But =
I like the 'old' idea of putting the hash of a block that MUST be on the =
chain that this txn can eventually be added to. If the hash is not a =
valid block on the chain, the txn can't be added.<br class=3D""></div><div=
dir=3D"auto" class=3D""><br class=3D""></div><div dir=3D"auto" =
class=3D"">It means you can choose exactly which forks you want to allow =
your txn on, pre-fork for both, post-fork for only one, and gets round =
the issue of who gets to decide the nForkid value.. since you don't need =
one. Also, all the old outputs work fine, and LN not an issue.</div><div =
dir=3D"auto" class=3D""><br class=3D""></div><div dir=3D"auto" =
class=3D"">I'm missing why this scheme would be better ?<br =
class=3D""></div></div></div></blockquote><div><br class=3D""></div><div>I=
do agree that solutions like `SIGHASH_BLOCKCOMMIT` are superior in the =
sense that they are very difficult to circumvent. However, a fork could =
also follow the original chain in SPV mode and allow transactions =
protected with these mechanism. Since it's fundamentally impossible to =
disallow transactions in future projects, the goal shouldn't be to make =
this overly complicated. </div><div><br =
class=3D""></div><div>Furthermore, this schema is not just adding replay =
protection. It makes transacting safer overall (due to a dedicated =
address format per fork) and allows light clients to differentiate =
between multiple forks. In the past three months, at least $600k has =
been lost by users sending BCH to a BTC address [1].</div><div><br =
class=3D""></div><blockquote type=3D"cite" class=3D""><div =
class=3D"gmail_extra"><div class=3D"gmail_quote"><blockquote =
class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc =
solid;padding-left:1ex"><div dir=3D"ltr" class=3D"">Thanks for the =
clarification. How would a tx specify a constraint like =
"nForkId>=3D1"? I was thinking of it just as a number set on =
the tx.</div></blockquote></div></div></blockquote><div><br =
class=3D""></div><div>Whether the transaction is replay protected or not =
is specified by setting a bit in the `SigHashId`. If this bit is set, =
then the signature *preimage* MUST have `nForkId` appended. `nForkId` is =
not part of the final transaction, someone who wants to verify the =
transaction must know which `nForkId` it was created =
with. </div><div><br class=3D""></div><div>If the bit isn't set, it =
means `nForkId=3D0`, which allows other forks to validate the =
signature.</div><div><br class=3D""></div><blockquote type=3D"cite" =
class=3D""><div class=3D"gmail_extra"><div =
class=3D"gmail_quote"><blockquote class=3D"gmail_quote" style=3D"margin:0 =
0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir=3D"ltr" =
class=3D""><div class=3D"">Also note that since forks form a partial =
order, but IDs (numbers) form a total order, ">=3D" will miss some =
cases. Eg, suppose BCH had forked with nForkId 2, and then you set =
up a LN funding tx on BCH with nForkId>=3D2, and then Segwit2x forked =
(from BTC!) with nForkId 3. The BCH funding tx would be valid on =
Segwit2x. This is more of a fundamental problem than a bug - to =
avoid it you'd have to get into stuff like making each fork reference =
its parent-fork's first block or something, someone has written about =
this...<br =
class=3D""></div></div></blockquote></div></div></blockquote><div><br =
class=3D""></div><div>Sorry, I was careless with the use of `>=3D` =
there. You are correct, forks form a tree. For this proposal, every leaf =
must be assigned a unique `nForkId`. The relationship between `nForkId` =
is irrelevant (e.g. which number is bigger), as long as they are unique. =
Transactions are only valid IFF `nForkId` matches exactly the `nForkId` =
of the software validating it. As described above, the transaction =
doesn't even contain `nForkId`, and the node surely is not starting to =
guess which one it could be. </div></div><br class=3D""><div =
class=3D"">[1]</div><div class=3D""><a =
href=3D"https://twitter.com/khannib/status/930223617744437253" =
class=3D"">https://twitter.com/khannib/status/930223617744437253</a></div>=
</body></html>=
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