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Date: Fri, 4 Mar 2022 11:04:42 +1000
From: Anthony Towns <aj@erisian.com.au>
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Subject: [bitcoin-dev] bitcoin scripting and lisp
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On Sun, Feb 27, 2022 at 04:34:31PM +0000, ZmnSCPxj via bitcoin-dev wrote:
> In reaction to this, AJ Towns mailed me privately about some of his
> thoughts on this insane `OP_EVICT` proposal.
> He observed that we could generalize the `OP_EVICT` opcode by
> decomposing it into smaller parts, including an operation congruent
> to the Scheme/Haskell/Scala `map` operation.

At much the same time Zman was thinking about OP_FOLD and in exactly the
same context, I was wondering what the simplest possible language that
had some sort of map construction was -- I mean simplest in a "practical
engineering" sense; I think Simplicity already has the Euclidean/Peano
"least axioms" sense covered.

The thing that's most appealing to me about bitcoin script as it stands
(beyond "it works") is that it's really pretty simple in an engineering
sense: it's just a "forth" like system, where you put byte strings on a
stack and have a few operators to manipulate them.  The alt-stack, and
supporting "IF" and "CODESEPARATOR" add a little additional complexity,
but really not very much.

To level-up from that, instead of putting byte strings on a stack, you
could have some other data structure than a stack -- eg one that allows
nesting. Simple ones that come to mind are lists of (lists of) byte
strings, or a binary tree of byte strings [0]. Both those essentially
give you a lisp-like language -- lisp is obviously all about lists,
and a binary tree is just made of things or pairs of things, and pairs
of things are just another way of saying "car" and "cdr".

A particular advantage of lisp-like approaches is that they treat code
and data exactly the same -- so if we're trying to leave the option open
for a transaction to supply some unexpected code on the witness stack,
then lisp handles that really naturally: you were going to include data
on the stack anyway, and code and data are the same, so you don't have
to do anything special at all. And while I've never really coded in
lisp at all, my understanding is that its biggest problems are all about
doing things efficiently at large scales -- but script's problem space
is for very small scale things, so there's at least reason to hope that
any problems lisp might have won't actually show up for this use case.

So, to me, that seemed like something worth looking into...



After looking into it, I actually think chia lisp [1] gets pretty much all
the major design decisions pretty much right. There are obviously a few
changes needed given the differences in design between chia and bitcoin:

 - having secp256k1 signatures (and curve operations), instead of
   BLS12-381 ones

 - adding tx introspection instead of having bundle-oriented CREATE_COIN,
   and CREATE/ASSERT results [10]

and there are a couple of other things that could maybe be improved
upon:

 - serialization seems to be a bit verbose -- 100kB of serialized clvm
   code from a random block gzips to 60kB; optimising the serialization
   for small lists, and perhaps also for small literal numbers might be
   a feasible improvement; though it's not clear to me how frequently
   serialization size would be the limiting factor for cost versus
   execution time or memory usage.

 - I don't think execution costing takes into account how much memory
   is used at any one time, just how much was allocated in total; so
   the equivalent of (OP_DUP OP_DROP OP_DUP OP_DROP ..) only has the
   allocations accounted for, with no discount given for the immediate
   freeing, so it gets treated as having the same cost as (OP_DUP
   OP_DUP ..  OP_DROP OP_DROP ..). Doing it that way would be a worse
   than how bitcoin script is currently costed, but doing better might
   mean locking in an evaluation method at the consensus level. Seems
   worth looking into, at least.

But otherwise, it seems a pretty good match.



I think you'd need about 40 opcodes to match bitcoin script and (roughly)
chia lisp, something like:

   q                - quote
   a                - apply
   x                - exception / immediately fail (OP_RETURN style)
   i                - if/then/else
   softfork         - upgradability
   not, all, any    - boolean logic
   bitand, bitor, bitxor, bitnot, shift - bitwise logic
   =                - bitwise equality
   > - + * / divmod - (signed, bignum) arithmetic
   ashift           - arithmetic shift (sign extended)
   >s               - string comparison
   strlen, substr, concat - string ops
   f, r, c, l       - list ops (head, tail, make a list, is this a list?)
   sha256           - hashing

   numequal         - arithmetic equal, equivalent to (= (+ a 0) (+ b 0))
   ripemd160, hash160, hash256 - more hashing
   bip342-txmsg     - given a sighash byte, construct the bip342 message
   bip340-verify    - given a pubkey, message, and signature bip340 verify it
   tx               - get various information about the tx
   taproot          - get merkle path/internalpubkey/program/annex information
   ecdsa            - same as bip340-verify, except for traditional ecdsa?
   secp256k1-muladd - given (a B C) where B,C are points, calculate a*B+C?

That compares to about 60 (non-disabled) opcodes in current script.
Pretty much all the opcodes in the first section are directly from chia
lisp, while all the rest are to complete the "bitcoin" functionality.
The last two are extensions that are more food for thought than a real
proposal.



Using a lisp-style approach seems an improvement in general to me.
For example, rather than the streaming-sha256 approach in Elements,
where you could write:

  "a" SHA256INITIALIZE
  "b" SHA256UPDATE
  "c" SHA256UPDATE
  "d" SHA256FINALIZE

to get the sha256 of "abcd" without having to CAT them first (important
if they'd potentially overflow the 520B stack item limit), in chia lisp
you write:

  (sha256 "a" "b" "c" "d")

which still has the benefit of streaming the inputs into the function,
but only adds a single opcode, doesn't involve representing the internal
sha256 midstate on the stack, and generally seems easier to understand,
at least to me.

As another example, following the traditional functional "tail recursion
instead of for-loops" approach, doing CHECKMULTISIG might become
something like:

   (defun checksig (sig key)
          bip340-verify (f sig) (bip342-txmsg (r sig)) key)

   (defun checkmultisig (sigs keys k)
          if (= k 0) 
	     1
	     (if (l sigs)
	         (if (checksig (f sigs) (f keys))
	             (checkmultisig (r sigs) (r keys) (- k 1))
		     (checkmultisig sigs (r keys) k)
                 )
		 0
             )
   )

Here each "sig" is a pair of a 64B bip340 signature and a 1B sighash;
instead of a 65B string combining both, and sigs, keys are lists, and k
is the number of successful signature checks you're requiring for
success.

Of course, "defun" and "if" aren't listed as opcodes above; instead you
have a compiler that gives you nice macros like defun and translates them
into correct uses of the "a" opcode, etc. As I understand it, those sort
of macros and translations are pretty well understood across lisp-like
languages, and, of course, they're already implemented for chia lisp.



I think with the "tx" opcode defined similarly to how Rusty suggested it
[2] you could implement OP_CTV-like behaviours in similar way, and also
replace "bip342-txmsg" with your own code to generate SIGHASH_ANYPREVOUT
or SIGHASH_GROUP style messages to sign. (This would mean also being able
to pull information about the utxo being spent to obtain its amount and
scriptpubkey, which are committed to wit ANYPREVOUT. If it was also able
to obtain the "is_coinbase" flag, that might allow you a more accurate
covenant-based implementation of drivechains...)

Likewise, with the "taproot" opcode defined in a way that lets you extract
out the internal public key and merkle path, I think you could implement
OP_TLUV and OP_EVICT with a similar recursive approach.



There's two ways to think about upgradability here; if someday we want
to add new opcodes to the language -- perhaps something to validate zero
knowledge proofs or calculate sha3 or use a different ECC curve, or some
way to support cross-input signature aggregation, or perhaps it's just
that some snippets are very widely used and we'd like to code them in
C++ directly so they validate quicker and don't use up as much block
weight. One approach is to just define a new version of the language
via the tapleaf version, defining new opcodes however we like.

The other is to use the "softfork" opcode -- chia defines it as:

  (softfork cost code)

though I think it would probably be better if it were 

  (softfork cost version code)

where the idea is that "code" will use the "x" opcode if there's a
problem, and anyone supporting the "version" softfork can verify that
there aren't any problems at a cost of "cost". However, whether you
do or don't support that softfork, as far as the rest of the script is
concerned, the expression will either fail entirely or evaluate as zero;
so anyone who doesn't support the softfork can just replace it with zero
and continue on, treating it as if it had costed "cost" units.

One thing worth noting: "softfork" behaves more like OP_NOP than
tapscript's OP_SUCCESS -- I think it's just not possible in general to
have OP_SUCCESS-like behaviour if you're trying to allow accepting code
from the witness data -- otherwise as soon as you reveal that your script
does accept arbitrary code supplied by the spender, someone could stick
in an OP_SUCCESS code, and remove all the restrictions on spending and
steal your funds.



To me, it seems like chia lisp is a better answer to the problem here
than the Simplicity language. Simplicity is complicated in a few ways:

 - it defines over 100 jets (plus another 6 for sha3, and another 45 for
   individual libsecp256k1 functions) that need to be implemented
   natively to efficiently track consensus [3]

 - as far as I know, how to soft-fork in new jets isn't yet well
   established. I think the ideal is that you just write everything in
   raw simplicity, and either the interpreter has a jet and does things
   quickly, or doesn't, and gets the same result much more slowly [4]. But
   that approach doesn't seem compatible with maintaining consensus,
   when "slowly" can be slower by more than 6 orders of magnitude [5].

 - to understand what's going on with a smart contract, you need to
   understand both simplicity (to define the program) and the bit machine
   (to follow how it's computed), both of which are fairly novel --
   and if nobody's directly coding in simplicity which seems likely,
   you're adding a third layer on top of that: you want to understand
   what the programmer asked for (the source), what is included in the
   transaction (the simplicity code) and how that's executed by consensus
   code (via the bit machine).

There's a branch for enabling simplicity on elements/liquid [6]
to see what enabling simplicity might involve in concrete terms; it
just... doesn't seem at all simple in practice to me. I can't see how
you'd reasonably translate that simplicity code into a BIP series that
anyone could understand, or how you'd do a thorough review of all the
changes...

By contrast, chia lisp has fewer opcodes than Simplicity's jets, has
feasible approaches to low-impact soft forks to increase functionality,
can be used with only two levels of abstraction (lisp with macros and
the opcodes-only vm level) that seem not too bad to understand, and
(in my opinion) doesn't seem too hard to implement/maintain reasonably.

On the other hand, Simplicity's big advantage over *everything* else
is in formal verification. But I'm not really seeing why you couldn't
preserve that advantage by writing simplicity definitions for the "lisp"
opcodes [7], so that you can "compile" the lisp programs to simplicity,
and then verify them however you like.



One of the things people sometimes claim about bitcoin as an asset,
is that it's got both the advantage of having been first to market,
but also that if some altcoin comes along with great new ideas, then
those ideas can just be incorporated into bitcoin too, so bitcoin can
preserve it's lead even from innovators. Granted, I've only really been
looking at chia lisp for a bit over a week, but it really seems to me
like a case where it might be worth putting that philosophy into practice.



If we were to adopt this, obviously we shouldn't call it "chia lisp"
anymore, since it wouldn't work the same in important ways. But since
the program would be encoded as a binary-tree of car/cdr pairs, maybe
we could call it "binary-tree coded script", or "btc-script" for short...

PS: related tweets: [8]

Cheers,
aj

[0] You could also allow things to be pushed onto the stack that
    (recursively) can push things onto the stack -- the language "Joy"
    takes this approach. It seems to end up equivalent to doing things
    in a list oriented way to me.

[1] https://chialisp.com/docs/ref/clvm

[2] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-February/019871.html

[3] https://raw.githubusercontent.com/ElementsProject/simplicity/pdf/Simplicity-TR.pdf

[4] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-November/015244.html
    https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-October/015227.html
    https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-October/015228.html

[5] https://medium.com/blockstream/simplicity-jets-release-803db10fd589

[6] https://github.com/ElementsProject/elements/compare/simplicity

[7] At least I think so? chia lisp does multibyte math, that is "+"
    can accept "arbitrary" length bytestrings, which it interprets as
    numbers and adds together; whereas Simplicity requires finite types.
    I think you could decide "a program that only costs X can't have any
    bytestrings greater than length k*X" and construct a finite type up to
    that length, maybe? So long as you're only using it for verification,
    maybe that stays feasible? Or perhaps you could arbitrarily limit
    the strings to a max of 520 bytes at a consensus level, and the
    corresponding Simplicity types to 4160 bits and go from there?

[8] https://twitter.com/brian_trollz/status/1499048316956549123
    https://twitter.com/jb55/status/1499045998315724801

[9] Oops, out of order footnotes. Anyway...

[10] [9] The CREATE/ASSERT bundling stuff is interesting; and could be
    used to achieve functionality like the "transaction sponsorship"
    stuff. It doesn't magically solve the issues with maintaining the
    mempool and using that to speed up block acceptance, though, and
    the chia chain has apparently suffered from mempool-flooding attacks
    recently [11] so I don't think they've solved the broader problem,
    and thus I think it still makes more sense to stick with bitcoin's
    current model here.

[11] https://thechiaplot.net/2021/11/03/interview-with-the-chia-dust-stormer/
     https://github.com/Chia-Network/post-mortem/blob/main/post-mortem.md