feat: liveness module v2 (#320)

* feat: liveness module v2

Design document introducing an updated LivenessModule.

* comments from design review incorporated

* Clarify definition of simple majority

---------

Co-authored-by: alcueca <[email protected]>
Co-authored-by: Maurelian <[email protected]>

Author: 3 people

safe-modules/liveness-module-v2.md

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+
+# LivenessModule V2
+
+## Context and Problem Statement
+
+<!-- The Context and Problem Statement section is one of the most critical parts of the design
+document process. Use this section to clearly highlight the background context for the problem,
+the specific issues being faced by customers, and any constraints on a solution as defined either
+by customers or by technical limitations. Context and Problem Statement is an opportunity to tell
+the story that helps to motivate the rest of the design document. -->
+
+[Safe Accounts](https://safe.global) are configured as a set of owner addresses where some
+threshold of addresses can sign and ultimately execute a transaction through the account. We must
+solve for two safety cases, first that a threshold of signers do not maliciously execute a
+transaction (safety failure) and second that a blocking minority of signers
+(`total - threshold + 1`) don't become unable to sign (liveness failure). Either case can be
+equally catastrophic but we tend to focus on the safety failure case. Here we attempt to produce a
+solution for the liveness failure case that fits within certain constraints.
+
+We define multisig liveness as the ability for the multisig to reach consensus and execute a
+transaction within a bounded amount of time. Note that a multisig does not need to reach consensus
+over every possible transaction (because the members have the ability to reject transactions as
+needed). We instead say that a multisig is live as long as it is able to reach consensus over a
+transaction that states "this multisig is live".
+
+At a high level, a multisig fails to be live if at least a blocking minority of signers prevents
+the multisig from being able to reach consensus. This can happen if signers are honest but unable
+to sign (e.g., keys are lost) or if signers are malicious and refuse to sign. We must address both
+problems if we want to avoid multisig liveness failures. The existing `LivenessGuard` is
+insufficient in this regard because it does not handle malicious signers.
+
+Any system to address the above must also scale sufficiently such that a signer is not
+unnecessarily burdened by the liveness mechanism (this is a product constraint). The
+`LivenessGuard` is also insufficient here because it does not scale across multiple chains and it
+demands signatures on a regular basis (as opposed to only demanding signatures as needed).
+
+## Customer Description
+
+<!-- Provide a brief summary of the customers for this design document. -->
+
+The customers for this design doc are any participants in the multisig accounts that would utilize
+this module, as well as any 3rd parties who rely on the security properties of these accounts.
+
+### Customer Reviewers
+
+<!-- Identify at least one customer who should be involved in the review of this document. -->
+
+- Coinbase (potentially impacted 3rd party)
+- Uniswap (`LivenessModule` user)
+- Security Council (`LivenessModule` user)
+- Optimism Foundation
+
+## Requirements and Constraints
+
+<!-- Identify the solution requirements and any additional design constraints from the Context and
+Problem Statement section in a bulleted list. -->
+
+- Solve both the owner liveness and account liveness problems *securely*.
+- Keep code as minimal as possible to avoid complex logic in Safe modules.
+- Reduce mental overhead for Safe owners and multisig leads.
+
+## Proposed Solution
+
+<!-- Explain the solution that you believe best addresses the problem described above. -->
+
+We propose the introduction of a new module, `LivenessModuleV2` (name is unimportant) to replace
+the existing `LivenessGuard`/`LivenessModule`. `LivenessModuleV2` would employ a challenge/response
+mechanism as an alternative to the existing poke mechanism inside of `LivenessGuard`.
+
+### Singleton
+
+`LivenessModuleV2` is a singleton that exists at a known address on all chains and has no
+constructor parameters. Any Safe on any network can choose to configure the module and then enable
+the module within the Safe.
+
+If possible, this module should be integrated with all other safe modules and guards in a single
+contract that is enabled and configured in all Optimism Safes.
+
+### Configuration
+
+A Safe configures the module by setting a challenge period and a fallback owner.
+
+### Challenge Protocol
+
+The fallback owner may trigger an Account Liveness Challenge by targeting a specific Safe address.
+In response to this challenge, a threshold of owners (same threshold as the Safe itself) must 
+execute a regular transaction that confirms the liveness of the safe. If the Safe fails to execute
+this transaction, all owners are removed from the Safe and ownership is transferred to the fallback
+account. If the Safe successfully executes the transaction, the challenge fails and nothing happens.
+
+The Account Liveness Challenge addresses both of the ways in which a Safe can fail to be live. If
+sufficiently many honest users cannot sign that the account cannot reach a threshold, the account
+falls back to the fallback owner. If sufficiently many users are malicious that the account cannot
+reach a threshold, the honest users can refuse to sign and force a fallback.
+
+## Alternatives Considered
+
+<!-- Describe any alternatives that were considered during the development of this design. Explain
+why the alternative designs were ultimately not chosen and where they failed to meet the product
+requirements. -->
+
+### Owner Liveness Challenge
+
+We previously considered a model that would allow users to challenge either the entire Safe (as
+happens in the Account Liveness Challenge) or an individual owner (i.e., an Owner Liveness
+Challenge). An Owner Liveness Challenge would be more similar to the behavior of the existing
+`LivenessModule`, where a user could target a specific owner and require that owner respond or be
+removed from the Safe.
+
+The Owner Liveness Challenge faces a number of problems:
+
+- It creates a second challenge path.
+- It only solves the honest user case, whereas the Account Liveness Challenge solves both cases.
+- It requires that the Safe configure various new parameters like minimum owner count and threshold
+  percentage which creates more surface area for potential issues.
+
+Additionally, in most cases, the Owner Liveness Challenge would be unused because the Safe would
+prefer to simply remove an owner that is unable to sign.
+
+## Risks and Uncertainties
+
+<!-- Explain any risks and uncertainties that this design includes. Highlight aspects of the design
+that remain unclear and any potential issues we may face down the line. -->
+### Malicious Fallback
+While the fallback owner is chosen as a trusted actor, it can always happen that they are
+eventually compromised. In that case, they might issue a liveness challenge and if unnoticed
+gain control of the multisig enabling the LivenessModule.
+
+To mitigate this, we will require monitoring that will alert us of unexpected liveness challenges.
+
+### Blocking Minority is Below Quorum
+In order for the module to be effective for the stated scenario, a multisig must require at least a simple majority (defined as `threshold > (total + 1 )/ 2`)  as quorum, otherwise malicious actors will reach quorum before reaching a blocking minority.
+
+Instead of implementing a module that would address this scenario, we accept that this module
+is intended for multisigs where the quorum is greater than a simple majority.
+
+### Fallback Ownership
+
+Both the existing `LivenessModule` and this updated iteration rely on fallback ownership. In the
+case that the challenge fails, ownership of the account must be transferred to some fallback
+address. We believe this is acceptable and reasonable. Most multisig accounts can simply name the
+Optimism Foundation multisig as a fallback, and the Optimism Foundation multisig can name the
+Security Council as a fallback.
+
+One open question is what the Security Council should set as a fallback. We'd likely want this to
+be another multisig operated by an independent and well-trusted entity.
+
+### Blunt Instrument
+
+It should be noted that the updated `LivenessModule` is more of a blunt instrument than the
+existing module. We no longer have the ability to remove specific owners, we instead trigger a
+fallback immediately if a threshold cannot be reached. This is less surgical, but we believe it is
+equally effective and ultimately safer/simpler than the existing module because users don't need to
+account for as many parameters.