Good morning John,
> > However, I believe that Lightning and similar offchain protocols are not
> > possible on MimbleWimble, at least if we want to retain its "magical
> > shrinking blockchain" property.
>
> MimbleWimble can easily incorporate relative lock heights, in addition
> to absolute lock heights. Grin and Beam have included the latter since
> launch.
>
> Grin's proposal for relative lock heights is at [1] with discussion at [2].
> Based on these, Grin also has a rough design for payment channels at [3].
>
> Beam included relative lock heights in its recent HardFork [4] and has
> a payment channel design at [5].
>
Thank you for this information.
I am aware that absolute locktimes were possible in MimbleWimble.
However, it does seem to imply that kernels are not compressible (unlike the
original MimbleWimble where the kernel is just an empty string and thus never
stored).
So at least for kernels of relative locktimes, are not pruneable and will
contribute to blockchain size.
(I believe I saw some proposal for absolute locktimes that allow some amount of
aggregation/pruning of absolute-locktime kernels from the mimblewimble.pdf by
andytoshi.)
Which I suppose is my point: you lose some of the "magic shrinking blockchain"
property in implementing relative locktimes, as you now increase the data you
have to store forever (i.e. the kernels).
It is not a *total* loss of the "magic shrinking blockchain", I see now,
however.
Still, it does see worth the cost of accepting having to store kernels forever
in exchange for being able to layer on top of a MimbleWimble blockchain.
It seems to me that Poon-Dryja and Decker-Wattenhofer can be "directly" ported
over to any MimbleWimble blockchain with relative locktimes.
Reference [5] seems to be Poon-Dryja ported over to using relative locktimes
for MimbleWimble.
Decker-Russell-Osuntokun ("eltoo") is harder due to the `SIGHASH_NOINPUT`
requirement.
I have tried to derive an equivalent to this `SIGHASH_NOINPUT` somehow by
considering that the "reference to previous kernel" as being akin to the
Bitcoin transaction input referring to a previous output, however it seems to
be not easy to create a retargatable "reference to previous kernel" in this way.
In any case, it seems to me that the loss of SCRIPT does not prevent a
MimbleWimble blockchain from using an offchain updateable cryptocurrency system.
Regards,
ZmnSCPxj
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