fip-0047.md

fip	title	author	discussions-to	status	type	category	created
0047	Proof Expiration & PoRep Security Policy	Jakub Sztandera (@Kubuxu), Irene Giacomelli (@irenegia), Alex North (@anorth), Luca Nizzardo (@lucaniz)	#415	Superseded by FIP-0067	Technical	Core	2022-08-31

FIP-0047: Proof Expiration & PoRep Security Policy

Simple Summary

Adds a ProofExpiration parameter to each sector, and describes a policy to be adopted in case a flaw is discovered in the theory or implementation of proof-of-replication.

Abstract

We are not aware of any PoRep issues at this time, but it is crucial to have a plan to deal with any possibility. This proposal introduces a concrete mechanism for processing all sectors, and describes (but does not implement) a policy that uses the mechanism.

• The mechanism is to decouple sector expiration (renamed to CommitmentExpiration) and proof expiration (a new sector property is introduced called ProofExpiration), and introduce a method RefreshProofExpiration() that can be used to request a refreshed proof expiration. A sector’s commitment may then be longer than its proof expiration.
• The policy is that, in case of a PoRep flaw, the RefreshProofExpiration method is disallowed for sectors sealed with the vulnerable code. In order to maintain power, a provider must seal a new replacement sector before each existing sector’s ProofExpriration epoch is reached. If not replaced, a sector with a proof expiration before its commitment expiration will incur a termination fee (for example, the current sector early-termination penalty).

Motivation

The Filecoin network uses cryptographic techniques to provide assurance of the physical uniqueness of sectors of data (proof of replication, or PoRep) and their ongoing availability (proof of space-time, PoSt). These mechanisms provide the proof of resources underlying the blockchain’s security (in addition to the security offered by pledge collateral stake). Some of the cryptography involved has been developed relatively recently. It is possible that there are errors in either the theory or implementation, or that errors may be introduced one day, that undermine the desired assurances. The result of such an error would most likely be that storage providers could “cheat” the network to claim they were maintaining more committed storage than they in fact possessed. This would reduce network security as consensus power could be gained without the expected physical infrastructure commitment. It is also unfair to non-cheating providers, assuming knowledge of the flaw was limited. In the case of an error in PoRep, it is likely that there would be no possible protocol change that could detect the cheating sectors after commitment.

This situation has already arisen once in the life of the Filecoin network. The v1.1 PoRep algorithm patched a bug in the v1 PoRep implementation that weakened the security assurances of sectors. The bug was responsibly reported to the Filecoin team and it is unknown if it was ever exploited by a provider. We are not aware of any similar bugs at this time. Filecoin storage is secure as far as we can ascertain.

Current policy

The network today has an implicit policy on what to do if another such bug is detected:

• Implement a new, fixed PoRep algorithm
• In a network upgrade, the old algorithm is disallowed and the new one mandated for new sectors
• Old sectors are prohibited extension beyond the 1.5-year maximum commitment
• The power attributed to old sectors decreases as they expire and is extinct after 18 months. As new sectors are sealed, the insecure proportion diminishes even faster.

The policy might thus be summarised as: put up with the potentially-insecure power for a limited period of time, but retain existing commitments of providers to the network and vice-versa. The 1.5-year window was selected as a compromise: a longer maximum commitment would be beneficial for storage stability, but a shorter bound improves the response time in case of a bug.

This policy depends on the 1.5y commitment expiration, and would lose effectiveness if the maximum commitment duration were raised.

Changing the maximum sector commitment duration

The sector commitment duration (currently 1.5 years) is the period availability a provider commits to when first proving a sector. When a sector is close to expiration, a provider can extend the sector for up to the same duration again. This can be repeated until the sector maximum lifetime (currently 5 years), after which further extensions are prohibited.

It would be desirable to allow storage providers to make longer commitments to sectors (e.g. as proposed in FIP-0036). Longer commitments support stability of network’s committed storage, could immediately support longer deals, and generally reduce the overheads of cycling sectors into the future. A higher possible maximum commitment duration would be beneficial regardless of any incentives toward longer commitments.

The authors assert that the implementation of a concrete mechanism, and rough consensus around policy, should be pre-requisite to increasing the maximum sector commitment duration.

Specification

Mechanism

• The existing sector Expiration property is renamed to CommitmentExpiration with a duration range from MinSectorLifetime to MaxSectorLifetime.
• A new sector property is introduced called ProofExpiration. The value of ProofExpiration is not freely chosen by the SP, but takes a value that is derived from and quantised to the sector’s activation epoch.
  • During activation, It is calculated as: ProofExpiration = SectorActiviation + MaxProofDuration
  • On refresh: ProofExpiration = ProofExpiration + (MaxProofDuration - ProofRefreshWindow)
• If a sector’s ProofExpiration epoch is reached before its CommitmentExpiration, the sector is terminated and the provider pays a termination fee (the same value as for manual termination).
• The storage provider can call a new method RefreshProofExpiration(SectorSelector) requesting a refreshed proof expiration. This call is only available after ProofExpiration - ProofRefreshWindow and is disallowed past the ProofExpiration. It will increase the ProofExpiration of each of the requested sectors by MaxProofDuration - ProofRefreshWindow.
• When extending a sector's commitment expiration, the existing ExtendSectorExpiration method also refreshes the sector's proof expiration, if it is within the proof expiration window.

This mechanism is implemented by adapting the existing expiration queue mechanism in the miner actor to schedule sectors according to the earlier of their proof expiration or commitment expiration.

Parameters

Two new protocol parameters are defined:

• MaxProofDuration: the maximum period from last commitment or refresh for which a PoRep is valid, set to 545 days (a little over 1.5 years) to match current behaviour;
• ProofRefreshWindow: the window of time before ProofExpiration epoch when the proof can be extended, set to 60 days.

These values together support a MaxSectorLifetime of 5 years with only 3 proof refreshes, which is the same number of commitment extensions currently required.

The amount of time ProofExpiration is increased by when refreshed, MaxProofDuration - ProofRefreshWindow is set to 428 days (1 year 63 days).

The existing MinSectorExpiration and MaxSectorExpirationExtension parameters now refer to the commitment expiration. A new, larger, value for MaxSectorExpirationExtension may be considered.

• If the current value of 1.5 years is retained, then a sector's initial commitment will always expire before its proof expiration is reached.
• If the value is increased, then a sector may take an initial commitment longer than the proof expiration, and so must be refreshed.

If MaxSectorExpirationExtension is increased, then the built-in storage market's maximum deal duration is increased to match. This could permit deals up to 5 years in duration.

Policy

If we were to discover a flaw in PoRep theory or implementation:

• A new, fixed PoRep algorithm would be implemented;
• In a network upgrade, the old algorithm is disallowed and the new one mandated for new sectors;
• The RefreshProofExpiration method is disallowed for old sectors (ie, created with the vulnerable PoRep algorithm), in particular, all old sectors will terminate at ProofExpiration epoch;
• A new method ReplaceSector(OldSector, NewSector) can be called by a provider to replace an old sector with a newly-sealed one. This requires a Storage Provider to run the sealing procedure with the new PoRep algorithm on the new sector (ie, successful call to PreCommitSector and ProveCommitSector). Note that:
  • The sealing procedure and replacement must be completed before the ProofExpiration epoch of the old sector;
  • If the replacement sealing procedure is successful, the old sector is removed with no termination fee;
  • If the replacement sealing is not performed for an old sector before its proof expires, that sector is terminated at the ProofExpiration.

This proposal requires implementing the proof expiration mechanism, but does not require implementing the sector replacement mechanism until such time as it’s needed. Details of that mechanism may depend on the new PoRep algorithm.

However, acceptance of this proposal should be considered as ratification of this policy in general, so that emergency-response implementers can immediately design towards it.

Migration

During migration at network upgrade, each sector’s ProofExpiration is set to that value it would have if it had initially been set to SectorActiviation + MaxProofDuration and then refreshed until the proof expiration is no less than the commitment expiration.

The ExpirationQueue that exists in each miner partition will have the field early_sectors renamed to faulty_sectors, and will have a new field proof_expiring_sectors that will consist of an empty bitfield at migraiton.

Design Rationale

Decoupling of sector commitment duration from proof duration allow providers to make long sector commitments (see FIP0036) without comprising the possibility of executing an “emergency” PoRep algorithm upgrade. The mechanism of explicit proof refresh must be implemented prior to recognizing any sectors with longer commitments, because it would be generally infeasible to schedule an orderly iteration over all sectors after the fact. This design implements a way to cycle over all sectors every 1.5 years. Note that, in response to a serious PoRep flaw, it would be possible to speed up the cycling by processing the proof expiration queue faster than 1 epoch per epoch.

A sector’s ProofExpiration epoch is strictly derived from its activation epoch in order to spread expirations out relatively evenly, and with limited opportunity for storage providers to overload the network by scheduling them all at once.

The cost of this approach is approximately the same as the existing process for extending sector expirations every 1.5 years.

This proposal also outlines the policy for a disaster-response upgrade in order to align the community on a carefully designed plan and establish a foundation for a smooth network upgrade in the event of discovery of an insecure PoRep. The proposed Policy disallows extending the proof duration for any sectors sealed with vulnerable code in order to have the power attributed to insecure sectors decreases gradually as their proof validities expire. On the other side, sealing new secure sectors in order to maintain power is allowed by the replacement sealing and incentivized by applying a fee for sectors that are not replaced.

We hope that broad policy agreement now can facilitate smooth governance in response to any future emergency.

Backwards Compatibility

This upgrade changes the schema for sectors and the miner expiration queue, and thus requires a network upgrade and sector state migration.

Impact on tooling should be minimal beyond changes to sector management software.

Test Cases

Will be included with implementation to be presented.

Security Considerations

Decoupling sector proof expiration from their commitment duration allows longer commitment durations without compromising the network’s ability to respond to flaws in PoRep.

If a flaw is discovered, an increased commitment duration would create the need for replacement sealing, or penalties for non-replaced sectors.

Analysis was conducted on the economic preference for a reseal vs terminate early and re-onboard strategy in the context of a PoRep bug requiring replacement sealing. Our aim was to understand the tradeoffs and identify values for the ProofRefreshWindow and Termination Fee Cap, such that the utility of a storage provider is maximised when they conduct replacement sealing for their sectors. Our results suggest that:

• A ProofRefreshWindow of approximately 30-60 days is optimal even in scenarios where the termination fee penalty is revised in the future, and SDM scaling of fee cap is implemented. A value of 60 days is thus adopted in order to guarantee operational convenience for storage providers during normal operations.
• A 50-100 day block reward per sector termination fee is sufficient to drive storage providers towards the intended behaviour of resealing their sector in the case of a PoRep bug. But note that in the near future termination fees will likely need to be revised upwards: the current termination fees were set in the context of 6 month sectors, and revision soon after sector duration multiplier implementation (if accepted) is high priority.

A more detailed summary of our analysis can be found here

Incentive Considerations

This proposal does not adjust incentives for providers or clients, but it does make longer sector commitments possible. Other proposals may then introduce incentives for such commitments.

In case of an emergency response, charging some termination fee when a sector proof is not refreshed creates an incentive to seal replacement sectors vs let old sectors expired un-replaced. Further analysis is warranted into the ideal termination penalty in this case. However, such analysis need not prevent us adopting this mechanism, for its other advantages, and policy, in order to provide greater clarity for storage provider business operations.

Product Considerations

If the maximum sector commitment duration is increased, then a sector committed for longer than its initial proof expiration must be refreshed, or it will be terminated with a penalty. This is a different operational position than a sector committed only as as long as the proof duration, and then extended. In the latter case, because the sector was not committed to a longer period, no termination penalty is applied if the extension is missed. Committing a sector for longer than the initial proof duration thereby introduces some operational risk to the provider, which must remember to and succeed in refreshing the proof within the refresh window.

Implementations

The CommitmentExpiration and ProofExpiration separation implementation is available at filecoin-project/builtin-actors#518.

The replacement sealing implementation is deferred until needed.

Copyright

Copyright and related rights waived via CC0.