Hi,
I'd like to propose an alternative to BIP-118 [1] that is both safer and more
powerful. The proposal is called Inherited IDs (IIDs) and is described in a
paper that can be found here [2]. The paper presents IIDs and Layer 2 protocols
using IIDs that are far more scalable and usable than those proposed for BIP-118
(including eltoo [3]).
Like BIP-118, IIDs are a proposal for a softfork that changes the rules for
calculating certain signatures. BIP-118 supports signatures that do not
commit to the transaction ID of the parent transaction, thus allowing "floating
transactions". In contrast, the IID proposal does not allow floating
transactions, but it does allow an output to specify that child transaction
signatures commit to the parent transaction's IID, rather than its transaction
ID.
IID Definitions
===============
* If T is a transaction, TXID(T) is the transaction ID of T.
* An output is an "IID output" if it is a native SegWit output with version 2
and a 32-byte witness program, and is a "non-IID output" otherwise.
* A transaction is an "IID transaction" if it has at least one IID output.
* If T is a non-IID transaction, or a coinbase transaction, IID(T) = TXID(T).
* If T is a non-coinbase IID transaction, first_parent(T) = F is the transaction
referenced by the OutPoint in T's input 0, and IID(T) = hash(IID(F) || F_idx)
where F_idx is the index field in the OutPoint in T's input 0 (that is, T's
input 0 spends F's output F_idx).
IID Signature Validation
========================
* Signatures that spend IID outputs commit to signature messages in which IIDs
replace transaction IDs in all OutPoints of the child transaction that spend
IID outputs.
Note that IID(T) can be calculated from T (if it is a non-IID or a coinbase
transaction) or from T and F (otherwise). Therefore, as long as nodes store (or
calculate) the IID of each transaction in the UTXO set, they can validate
signatures of transactions that spend IID outputs. Thus, the IID proposal fits
Bitcoin's existing UTXO model, at the small cost of adding a 32-byte IID value
for certain unspent outputs. Also, note that the IID of a transaction may not
commit to the exact contents of the transaction, but it does commit to how the
transaction is related to some exactly-specified transaction (such as being the
first child of the second child of a specific transaction). As a result, a
transaction that is signed using IIDs cannot be used more than once or in an
unanticipated location, thus making it much safer than a floating transaction.
2-Party Channel Protocols
=========================
BIP-118 supports the eltoo protocol [3] for 2-party channels, which improves
upon the Lightning protocol for 2-party channels [4] by:
1) simplifying the protocol,
2) eliminating penalty transactions, and
3) supporting late determination of transaction fees [1, Sec. 4.1.5].
The IID proposal does not support the eltoo protocol. However, the IID proposal
does support a 2-party channel protocol, called 2Stage [2, Sec. 3.3], that is
arguably better than eltoo. Specifically, 2Stage achieves eltoo's 3 improvements
listed above, plus it:
4) eliminates the need for watchtowers [2, Sec. 3.6], and
5) has constant (rather than linear) worst-case on-chain costs [2, Sec. 3.4].
Channel Factories
=================
In general, an on-chain transaction is required to create or close a 2-party
channel. Multi-party channel factories have been proposed in order to allow a
fixed set of parties to create and close numerous 2-party channels between them,
thus amortizing the on-channel costs of those channels [5]. BIP-118 also
supports simple and efficient multi-party channel factories via the eltoo
protocol [1, Sec. 5.2] (which are called "multi-party channels" in that paper).
While the IID proposal does not support the eltoo protocol, it does support
channel factories that are far more scalable and powerful than any previously-
proposed channel factories (including eltoo factories). Specifically, IIDs
support a simple factory protocol in which not all parties need to sign the
factory's funding transaction [2, Sec. 5.3], thus greatly improving the scale
of the factory (at the expense of requiring an on-chain transaction to update
the set of channels created by the factory). These channel factories can be
combined with the 2Stage protocol to create trust-free and watchtower-free
channels including very large numbers of casual users.
Furthermore, IIDs support channel factories with an unbounded number of parties
that allow all of the channels in the factory to be bought and sold by anyone
(including parties not originally in the factory) with a single on-chain
transaction in a trust-free manner [2, Secs. 6 and 7]. As a result, a single
on-chain transaction can be used in place of thousands, or even millions, of
Lightning or eltoo on-chain transactions. These channel factory protocols make
critical use of IIDs and do not appear to be possible with BIP-118.
Next Steps
==========
If IIDs sounds interesting, please take a look at the IID paper [2]. It contains
many results not listed above, including rules for SVP nodes, protocols for
off-chain channel networks, Layer 2 protocol extensions, support for covenants
(including vaults), and nearly matching lower and upper bounds on multi-party
channels.
The paper also includes 3 options for how IIDs could be added to Bitcoin via a
softfork [2, Appendix A]. I'm new to Bitcoin and am not sure which of these 3
options is best. If anyone finds the IID proposal valuable, I would greatly
appreciate it if they were willing to pick the best option (or invent an even
better option) for adding IIDs to Bitcoin and create a BIP for that option.
Hopefully, IIDs will provide a safe way to dramatically scale Bitcoin while
improving its usability.
Thanks,
John
References
==========
[1] BIP-118:
https://anyprevout.xyz and
https://github.com/bitcoin/bips/pull/943[2] Scaling Bitcoing with Inherited IDs, by John Law:
iids13.pdf at
https://github.com/JohnLaw2/btc-iids[3] eltoo: A Simple Layer2 Protocol for Bitcoin, by Decker, Russell & Osuntokun:
https://blockstream.com/eltoo.pdf[4] The Bitcoin Lightning Network, by Poon & Dryja:
https://lightning.network/lightning-network-paper.pdf[5] Scalable Funding of Bitcoin Micropayment Channel Networks, by Burchert,
Decker & Wattenhofer:
http://dx.doi.org/10.1098/rsos.180089Acknowledgments
===============
Thanks to Ruben Somsen and Jeremy Rubin for their helpful comments.
Also, thanks to Bob McElrath for his original brainstorm that led to the
creation of the IID concept:
https://diyhpl.us/wiki/transcripts/2019-02-09-mcelrath-on-chain-defense-in-depth