Re: [bitcoin-dev] MAST/Schnorr related soft-forks

["Russell O'Connor" <roconnor@blockstream.io>]

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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@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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