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From: Gregory Maxwell <greg@xiph.org>
Date: Mon, 2 Jul 2018 18:23:48 +0000
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To: Johnson Lau <jl2012@xbt.hk>, 
	Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
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Subject: Re: [bitcoin-dev] SIGHASH2 for version 1 witness programme
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On Thu, May 31, 2018 at 6:35 PM, Johnson Lau via bitcoin-dev
<bitcoin-dev@lists.linuxfoundation.org> wrote:
> The bit 0 to 3 of hashtype denotes a value between 0 and 15:
>
>         =E2=80=A2 If the value is 1, the signature is invalid.
>         =E2=80=A2 If the value is 3 or below, hashPrevouts is the hash of=
 all input, same as defined in BIP143. Otherwise, it is 32-byte of 0x0000..=
....0000.
>         =E2=80=A2 If the value is 7 or below, outpoint is the COutPoint o=
f the current input. Otherwise, it is 36-byte of 0x0000......0000.
>         =E2=80=A2 If the value is 0, hashSequence is the hash of all sequ=
ence, same as defined in BIP143. Otherwise, it is 32-byte of 0x0000......00=
00.
>         =E2=80=A2 If the value is even (including 0), nSequence is the nS=
equence of the current input. Otherwise, it is 0x00000000.
>         =E2=80=A2 If the value is 6, 7, 10, 11, 14, or 15, nInputIndex is=
 0x00000000. Otherwise, it is the index of the current input.
>         =E2=80=A2 If the value is 11 or below, nAmount is the value of th=
e current input (same as BIP143). Otherwise, it is 0x0000000000000000.
>
> The bit 4 and 5 of hashtype denotes a value between 0 and 3:
>
>         =E2=80=A2 If the value is 0, hashOutputs is same as the SIGHASH_A=
LL case in BIP143 as a hash of all outputs.
>         =E2=80=A2 If the value is 1, the signature is invalid.
>         =E2=80=A2 If the value is 2, hashOutputs is same as the SIGHASH_S=
INGLE case in BIP143 as a hash of the matching output. If a matching output=
 does not exist, hashOutputs is 32-byte of 0x0000......0000.
>         =E2=80=A2 If the value is 3, hashOutputs is 32-byte of 0x0000....=
..0000.
> If bit 6 is set (SIGHASH2_NOFEE), nFees is 0x0000000000000000. Otherwise,=
 it is the fee paid by the transaction.
> If bit 7 is set (SIGHASH2_NOLOCKTIME), nLockTime is 0x00000000. Otherwise=
, it is the transaction nLockTime.
>
> If bit 8 is set (SIGHASH2_NOVERSION), nVersion is 0x00000000. Otherwise, =
it is the transaction nVersion.
>
> If bit 9 is set (SIGHASH2_NOSCRIPTCODE), scriptCode is an empty script. O=
therwise, it is same as described in BIP143.
>
> Bits 10 to 15 are reserved and ignored, but the signature still commits t=
o their value as hashtype.
>
> hashtype of 0 is also known as SIGHASH2_ALL, which covers all the availab=
le options. In this case the singnature MUST be exactly 64-byte.
>
> hashtype of 0x3ff is also known as SIGHASH2_NONE, which covers nothing an=
d is effectively forfeiting the right related to this public key to anyone.


This seems fairly complicated and yet-- if I don't misunderstand-- it
doesn't capture the one special output masking case that I've seen
actual applications want (which itself, is one out of the only two
special sighash cases I've seen applications want-- the other being
no-input).

The case I think this is missing is SIGHASH_SINGLE |
SIGHASH_LAST_OUTPUT   e.g. "Sign the matching output, and the last
output regardless of its index". The application for this style is
"kickstarter" joint-payment transactions where you wish to sign both
your change output (SIGHASH_SINGLE)  and the joint-payment output
(SIGHASH_LAST_OUTPUT).  Without it, this kind of usage requires
usually a chain of depth two for each input to split off the change.

I came back around to your post at Sipa's recommendation because I was
musing on is there a _simple_ set of enhanced sighash flags that
capture real useful behaviour without falling down a rathole of
specifying a totally general behaviour.