Russell O'Connor [ARCHIVE] on Nostr: 📅 Original date posted:2018-05-10 📝 Original message:Thanks for writing this up ...

📅 Original date posted:2018-05-10
📝 Original message:Thanks for writing this up Anthony.

Do you think that a CHECKSIGFROMSTACK proposal should be included within
this discussion of signature soft-forks, or do you see it as an unrelated
issue?

CHECKSIGFROMSTACK enables some forms of (more) efficent MPC (See
http://people.csail.mit.edu/ranjit/papers/scd.pdf), enables poor-man's
covenants, and I believe the lightning folks are interested in it as well
for some constant space storage scheme.

On Thu, May 10, 2018 at 8:10 AM, Anthony Towns via bitcoin-dev <
bitcoin-dev at lists.linuxfoundation.org> wrote:

> Hello world,
>
> After the core dev meetup in March I wrote up some notes of where I
> think things stand for signing stuff post-Schnorr. It was mostly for my
> own benefit but maybe it's helpful for others too, so...
>
> They're just notes, so may assume a fair bit of background to be able to
> understand the meaning of the bullet points. In particular, note that I'm
> using "schnorr" just to describe the signature algorithm, and the terms
> "key aggregation" to describe turning an n-of-n key multisig setup into
> a single key setup, and "signature aggregation" to describe combining
> signatures from many inputs/transactions together: those are often all
> just called "schnorr signatures" in various places.
>
>
> Anyway! I think it's fair to split the ideas around up as follows:
>
> 1) Schnorr CHECKSIG
>
> Benefits:
> - opportunity to change signature encoding from DER to save a few
> bytes per signature, and have fixed size signatures making tx size
> calculations easier
>
> - enables n-of-n multisig key aggregation (a single pubkey and
> signature gives n-of-n security; setup non-interactively via muSig,
> or semi-interactively via proof of possession of private key;
> interactive signature protocol)
>
> - enables m-of-n multisig key aggregation with interactive setup and
> interactive signature protocol, and possibly substantial storage
> requirements for participating signers
>
> - enables scriptless scripts and discreet log contracts via
> key aggregation and interactive
>
> - enables payment decorrelation for lightning
>
> - enables batch validation of signatures, which substantially reduces
> computational cost of signature verification, provided a single
> "all sigs valid" or "some sig(s) invalid" output (rather than
> "sig number 5 is invalid") is sufficient
>
> - better than ecdsa due to reducing signature malleability
> (and possibly due to having a security proof that has had more
> review?)
>
> Approaches:
> - bump segwit version to replace P2WPKH
> - replace an existing OP_NOP with OP_CHECKSCHNORRVERIFY
> - hardfork to allowing existing addresses to be solved via Schnorr sig
> as alternative to ECDSA
>
> 2) Merkelized Abstract Syntax Trees
>
> Two main benefits for enabling MAST:
> - logarithmic scaling for scripts with many alternative paths
> - only reveals (approximate) number of alternative execution branches,
> not what they may have been
>
> Approaches:
> - replace an existing OP_NOP with OP_MERKLE_TREE_VERIFY, and treat an
> item remaining on the alt stack at the end of script exeution as a
> script and do tail-recursion into it (BIP 116, 117)
> - bump the segwit version and introduce a "pay-to-merkelized-script"
> address form (BIP 114)
>
> 3) Taproot
>
> Requirements:
> - only feasible if Schnorr is available (required in order to make the
> pubkey spend actually be a multisig spend)
> - andytoshi has written up a security proof at
> https://github.com/apoelstra/taproot
>
> Benefits:
> - combines pay-to-pubkey and pay-to-script in a single address,
> improving privacy
> - allows choice of whether to use pubkey or script at spend time,
> allowing for more efficient spends (via pubkey) without reducing
> flexibility (via script)
>
> Approaches:
> - bump segwit version and introduce a "pay-to-taproot" address form
>
> 4) Graftroot
>
> Requirements:
> - only really feasible if Schnorr is implemented first, so that
> multiple signers can be required via a single pubkey/signature
> - people seem to want a security proof for this; not sure if that's
> hard or straightforward
>
> Benefits:
> - allows delegation of authorisation to spend an output already
> on the blockchain
> - constant scaling for scripts with many alternative paths
> (better than MAST's logarithmic scaling)
> - only reveals the possibility of alternative execution branches,
> not what they may have been or if any actually existed
>
> Drawbacks:
> - requires signing keys to be online when constructing scripts (cannot
> do complicated pay to cold wallet without warming it up)
> - requires storing signatures for scripts (if you were able to
> reconstruct the sigs, you could just sign the tx directly and
> wouldn't
> use a script)
> - cannot prove that alternative methods of spending are not
> possible to anyone who doesn't exclusively hold (part of) the
> output address private key
> - adds an extra signature check on script spends
>
> Approaches:
> - bump segwit version and introduce a "pay-to-graftroot" address form
>
> 5) Interactive Signature Aggregation
>
> Requirements:
> - needs Schnorr
>
> Description:
> - allows signers to interactively collaborate when constructing a
> transaction to produce a single signature that covers multiple
> inputs and/or OP_CHECKSIG invocations that are resolved by Schnorr
> signatures
>
> Benefits:
> - reduces computational cost of additional signatures (i think?)
> - reduces witness storage needed for additional signatures to just the
> sighash flag byte (or bytes, if it's expanded)
> - transaction batching and coinjoins potentially become cheaper than
> independent transactions, indirectly improving on-chain privacy
>
> Drawbacks:
> - each soft-fork introduces a checkpoint, such that signatures that
> are not validated by versions prior to the soft-fork cannot be
> aggregated with signatures that are validated by versions prior to
> the soft-fork (see [0] for discussion about avoiding that drawback)
>
> Approaches:
> - crypto logic can be implemented either by Bellare-Neven or MuSig
> - needs a new p2wpkh output format, so likely warrants a segwit
> version bump
> - may warrant allowing multiple aggregation buckets
> - may warrant peer-to-peer changes and a new per-tx witness
>
> 6) Non-interactive half-signature aggregation within transaction
>
> Requirements:
> - needs Schnorr
> - needs a security proof before deployment
>
> Benefits:
> - can halve the size of non-aggregatable signatures in a transaction
> - in particular implies the size overhead of a graftroot script
> is just 32B, the same as a taproot script
>
> Drawbacks:
> - cannot be used with scriptless-script signatures
>
> Approaches:
> - ideally best combined with interactive aggregate signatures, as it
> has similar implementation requirements
>
> 7) New SIGHASH modes
>
> These will also need a new segwit version (for p2pk/p2pkh) and probably
> need to be considered at the same time.
>
> 8) p2pk versus p2pkh
>
> Whether to stick with a pubkeyhash for the address or just have a pubkey
> needs to be decided for any new segwit version.
>
> 9) Other new opcodes
>
> Should additional opcodes in new segwit versions be reserved as OP_NOP
> or
> as OP_RETURN_VALID, or something else?
>
> Should any meaningful new opcodes be supported or re-enabled?
>
> 10) Hard-fork automatic upgrade of p2pkh to be spendable via segwit
>
> Making existing p2pk or p2pkh outputs spendable via Schnorr with
> interactive signature aggregation would likely be a big win for people
> with old UTXOs, without any decrease in security, especially if done
> a significant time after those features were supported for new outputs.
>
> 11) Should addresses be hashes or scripts?
>
> maaku's arguments for general opcodes for MAST make me wonder a bit
> if the "p2pkh" approach isn't better than the "p2wpkh" approach; ie
> should we have script opcodes as the top level way to write addresses,
> rather than picking the "best" form of address everyone should use,
> and having people have to opt-out of that. probably already too late
> to actually have that debate though.
>
> Anyway, I think what that adds up to is:
>
> - Everything other than MAST and maybe some misc new CHECKVERIFY opcodes
> really needs to be done via new segwit versions
>
> - We can evaluate MAST in segwit v0 independently -- use the existing
> BIPs to deploy MAST for v0; and re-evaluate entirely for v1 and later
> segwit versions.
>
> - There is no point deploying any of this for non-segwit scripts
>
> - Having the taproot script be a MAST root probably makes sense. If so,
> a separate OP_MERKLE_MEMBERSHIP_CHECK opcode still probably makes
> sense at some point.
>
> So I think that adds up to:
>
> a) soft-fork for MAST in segwit v0 anytime if there's community/economic
> support for it?
>
> b) soft-fork for OP_CHECK_SCHNORR_SIG_VERIFY in segwit v0 anytime
>
> c) soft-fork for segwit v1 providing Schnorr p2pk(h) addresses and
> taproot+mast addresses in not too much time
>
> d) soft-fork for segwit v2 introducing further upgrades, particularly
> graftroot
>
> e) soft-fork for segwit v2 to support interactive signature aggregation
>
> f) soft-fork for segwit v3 including non-interactive sig aggregation
>
> The rationale there is:
>
> (a) and (b) are self-contained and we could do them now. My feeling is
> better to skip them and go straight to (c)
>
> (c) is the collection of stuff that would be a huge win, and seems
> "easily" technically feasible. signature aggregation seems too
> complicated to fit in here, and getting the other stuff done while we
> finish thinking about sigagg seems completely worthwhile.
>
> (d) is a followon for (c), in case signature aggregation takes a
> *really* long while. It could conceivably be done as a different
> variation of segwit v1, really. It might turn out that there's no
> urgency for graftroot and it should be delayed until non-interactive
> sig aggregation is implementable.
>
> (e) and (f) are separated just because I worry that non-interactive
> sig aggregation might not turn out to be possible; doing them as a
> single upgrade would be preferrable.
>
> Cheers,
> aj
>
> [0] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/
> 2018-March/015838.html
>
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