Table of Contents

RFC-0074: Stateful Multisig Pallet

RFC-0074: Stateful Multisig Pallet


Start Date	15 February 2024
Description	Add Enhanced Multisig Pallet to Collectives chain
Authors	Abdelrahman Soliman (Boda)

Summary

A pallet to facilitate enhanced multisig accounts. The main enhancement is that we store a multisig account in the state with related info (owners, threshold,..etc). The module affords enhanced control over administrative operations such as adding/removing owners, changing the threshold, account deletion, canceling an existing proposal. Each owner can approve/revoke a proposal while still exists. The proposal is not intended for migrating or getting rid of existing multisig. It's to allow both options to coexist.

For the rest of the RFC We use the following terms:

proposal to refer to an extrinsic that is to be dispatched from a multisig account after getting enough approvals.
Stateful Multisig to refer to the proposed pallet.
Stateless Multisig to refer to the current multisig pallet in polkadot-sdk.

Motivation

Problem

Entities in the Polkadot ecosystem need to have a way to manage their funds and other operations in a secure and efficient way. Multisig accounts are a common way to achieve this. Entities by definition change over time, members of the entity may change, threshold requirements may change, and the multisig account may need to be deleted. For even more enhanced hierarchical control, the multisig account may need to be controlled by other multisig accounts.

Current native solutions for multisig operations are less optimal, performance-wise (as we'll explain later in the RFC), and lack fine-grained control over the multisig account.

Stateless Multisig

We refer to current multisig pallet in polkadot-sdk because the multisig account is only derived and not stored in the state. Although deriving the account is determinsitc as it relies on exact users (sorted) and thershold to derive it. This does not allow for control over the multisig account. It's also tightly coupled to exact users and threshold. This makes it hard for an organization to manage existing accounts and to change the threshold or add/remove owners.

We believe as well that the stateless multisig is not efficient in terms of block footprint as we'll show in the performance section.

Pure Proxy

Pure proxy can achieve having a stored and determinstic multisig account from different users but it's unneeded complexity as a way around the limitations of the current multisig pallet. It doesn't also have the same fine grained control over the multisig account.

Requirements

Basic requirements for the Stateful Multisig are:

The ability to have concrete and permanent (unless deleted) multisig accounts in the state.
The ability to add/remove owners from an existing multisig account by the multisig itself.
The ability to change the threshold of an existing multisig account by the multisig itself.
The ability to delete an existing multisig account by the multisig itself.
The ability to cancel an existing proposal by the multisig itself.
Owners of multisig account can start a proposal on behalf of the multisig account which will be dispatched after getting enough approvals.
Owners of multisig account can approve/revoke a proposal while still exists.

Use Cases

Corporate Governance: In a corporate setting, multisig accounts can be employed for decision-making processes. For example, a company may require the approval of multiple executives to initiate significant financial transactions.
Joint Accounts: Multisig accounts can be used for joint accounts where multiple individuals need to authorize transactions. This is particularly useful in family finances or shared business accounts.
Decentralized Autonomous Organizations (DAOs): DAOs can utilize multisig accounts to ensure that decisions are made collectively. Multiple key holders can be required to approve changes to the organization's rules or the allocation of funds.

and much more...

Stakeholders

Polkadot holders
Polkadot developers

Explanation

I've created the stateful multisig pallet during my studies in Polkadot Blockchain Academy under supervision from @shawntabrizi and @ank4n. After that, I've enhanced it to be fully functional and this is a draft PR#3300 in polkadot-sdk. I'll list all the details and design decisions in the following sections.

Let's start with a sequence diagram to illustrate the main operations of the Stateful Multisig.

multisig operations

Notes on above diagram:

It's a 3 step process to execute a proposal. (Start Proposal --> Approvals --> Execute Proposal)
Execute is an explicit extrinsic for a simpler API. It can be optimized to be executed automatically after getting enough approvals.
Any user can create a multisig account and they don't need to be part of it. (Alice in the diagram)
A proposal is any extrinsic including control extrinsics (e.g. add/remove owner, change threshold,..etc).
Any multisig account owner can start a proposal on behalf of the multisig account. (Bob in the diagram)
Any multisig account owener can execute proposal if it's approved by enough owners. (Dave in the diagram)

State Transition Functions

All functions have detailed rustdoc in PR#3300. Here is a brief overview of the functions:

create_multisig - Create a multisig account with a given threshold and initial owners. (Needs Deposit)
start_proposal - Start a multisig proposal. (Needs Deposit)
approve - Approve a multisig proposal.
revoke - Revoke a multisig approval from an existing proposal.
execute_proposal - Execute a multisig proposal. (Releases Deposit)
cancel_own_proposal - Cancel a multisig proposal started by the caller in case no other owners approved it yet. (Releases Deposit)

Note: Next functions need to be called from the multisig account itself. Deposits are reserved from the multisig account as well.

add_owner - Add a new owner to a multisig account. (Needs Deposit)
remove_owner - Remove an owner from a multisig account. (Releases Deposit)
set_threshold - Change the threshold of a multisig account.
cancel_proposal - Cancel a multisig proposal. (Releases Deposit)
delete_multisig - Delete a multisig account. (Releases Deposit)

Storage/State

Use 2 main storage maps to store mutlisig accounts and proposals.

#![allow(unused)]
fn main() {
#[pallet::storage]
  pub type MultisigAccount<T: Config> = StorageMap<_, Twox64Concat, T::AccountId, MultisigAccountDetails<T>>;

/// The set of open multisig proposals. A proposal is uniquely identified by the multisig account and the call hash.
/// (maybe a nonce as well in the future)
#[pallet::storage]
pub type PendingProposals<T: Config> = StorageDoubleMap<
    _,
    Twox64Concat,
    T::AccountId, // Multisig Account
    Blake2_128Concat,
    T::Hash, // Call Hash
    MultisigProposal<T>,
>;
}

As for the values:

#![allow(unused)]
fn main() {
pub struct MultisigAccountDetails<T: Config> {
	/// The owners of the multisig account. This is a BoundedBTreeSet to ensure faster operations (add, remove).
	/// As well as lookups and faster set operations to ensure approvers is always a subset from owners. (e.g. in case of removal of an owner during an active proposal)
	pub owners: BoundedBTreeSet<T::AccountId, T::MaxSignatories>,
	/// The threshold of approvers required for the multisig account to be able to execute a call.
	pub threshold: u32,
	pub creator: T::AccountId,
	pub deposit: BalanceOf<T>,
}
}

#![allow(unused)]
fn main() {
pub struct MultisigProposal<T: Config> {
    /// Proposal creator.
    pub creator: T::AccountId,
    pub creation_deposit: BalanceOf<T>,
    /// The extrinsic when the multisig operation was opened.
    pub when: Timepoint<BlockNumberFor<T>>,
    /// The approvers achieved so far, including the depositor.
    /// The approvers are stored in a BoundedBTreeSet to ensure faster lookup and operations (approve, revoke).
    /// It's also bounded to ensure that the size don't go over the required limit by the Runtime.
    pub approvers: BoundedBTreeSet<T::AccountId, T::MaxSignatories>,
    /// The block number until which this multisig operation is valid. None means no expiry.
    pub expire_after: Option<BlockNumberFor<T>>,
}
}

For optimization we're using BoundedBTreeSet to allow for efficient lookups and removals. Especially in the case of approvers, we need to be able to remove an approver from the list when they revoke their approval. (which we do lazily when execute_proposal is called).

There's an extra storage map for the deposits of the multisig accounts per owner added. This is to ensure that we can release the deposits when the multisig removes them even if the constant deposit per owner changed in the runtime later on.

Considerations & Edge cases

Removing an owner from the multisig account during an active proposal

We need to ensure that the approvers are always a subset from owners. This is also partially why we're using BoundedBTreeSet for owners and approvers. Once execute proposal is called we ensure that the proposal is still valid and the approvers are still a subset from current owners.

Multisig account deletion and cleaning up existing proposals

Once the last owner of a multisig account is removed or the multisig approved the account deletion we delete the multisig accound from the state and keep the proposals until someone calls cleanup_proposals multiple times which iterates over a max limit per extrinsic. This is to ensure we don't have unbounded iteration over the proposals. Users are already incentivized to call cleanup_proposals to get their deposits back.

Multisig account deletion and existing deposits

We currently just delete the account without checking for deposits (Would like to hear your thoughts here). We can either

Don't make deposits to begin with and make it a fee.
Transfer to treasury.
Error on deletion. (don't like this)

Approving a proposal after the threshold is changed

We always use latest threshold and don't store each proposal with different threshold. This allows the following:

In case threshold is lower than the number of approvers then the proposal is still valid.
In case threshold is higher than the number of approvers then we catch it during execute proposal and error.

Drawbacks

New pallet to maintain.

Testing, Security, and Privacy

Standard audit/review requirements apply.

Performance, Ergonomics, and Compatibility

Performance

Doing back of the envelop calculation to proof that the stateful multisig is more efficient than the stateless multisig given it's smaller footprint size on blocks.

Quick review over the extrinsics for both as it affects the block size:

Stateless Multisig: Both as_multi and approve_as_multi has a similar parameters:

#![allow(unused)]
fn main() {
origin: OriginFor<T>,
threshold: u16,
other_signatories: Vec<T::AccountId>,
maybe_timepoint: Option<Timepoint<BlockNumberFor<T>>>,
call_hash: [u8; 32],
max_weight: Weight,
}

Stateful Multisig: We have the following extrinsics:

#![allow(unused)]
fn main() {
pub fn start_proposal(
			origin: OriginFor<T>,
			multisig_account: T::AccountId,
			call_hash: T::Hash,
		)
}

#![allow(unused)]
fn main() {
pub fn approve(
			origin: OriginFor<T>,
			multisig_account: T::AccountId,
			call_hash: T::Hash,
		)
}

#![allow(unused)]
fn main() {
pub fn execute_proposal(
			origin: OriginFor<T>,
			multisig_account: T::AccountId,
			call: Box<<T as Config>::RuntimeCall>,
		)
}

The main takeway is that we don't need to pass the threshold and other signatories in the extrinsics. This is because we already have the threshold and signatories in the state (only once).

So now for the caclulations, given the following:

K is the number of multisig accounts.
N is number of owners in each multisig account.
For each proposal we need to have 2N/3 approvals.

The table calculates if each of the K multisig accounts has one proposal and it gets approved by the 2N/3 and then executed. How much did the total Blocks and States sizes increased by the end of the day.

Note: We're not calculating the cost of proposal as both in statefull and stateless multisig they're almost the same and gets cleaned up from the state once the proposal is executed or canceled.

Stateless effect on blocksizes = 2/3KN^2 (as each user of the 2/3 users will need to call approve_as_multi with all the other signatories(N) in extrinsic body)

Stateful effect on blocksizes = K * N (as each user will need to call approve with the multisig account only in extrinsic body)

Stateless effect on statesizes = Nil (as the multisig account is not stored in the state)

Stateful effect on statesizes = K*N (as each multisig account (K) will be stored with all the owners (K) in the state)

Pallet	Block Size	State Size
Stateless	2/3KN^2	Nil
Stateful	K*N	K*N

Simplified table removing K from the equation: | Pallet | Block Size | State Size | |----------------|:-------------:|-----------:| | Stateless | N^2 | Nil | | Stateful | N | N |

So even though the stateful multisig has a larger state size, it's still more efficient in terms of block size and total footprint on the blockchain.

On account deletion, should we transfer remaining deposits to treasury or remove owners' addition deposits completely and consider it as fees to start with?

Batch proposals. The ability to batch multiple calls into one proposal.
Batch addition/removal of owners.
Add expiry to proposals. After a certain time, proposals will not accept any more approvals or executions and will be deleted.
Add extra identifier other than call_hash to proposals (e.g. nonce). This will allow same call to be proposed multiple times and be in pending state.
Implement call filters. This will allow multisig accounts to only accept certain calls.

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