|Proof Expiration & PoRep Security Policy
|Jakub Sztandera, Irene Giacomelli, Alex North, Luca Nizzardo
|Superseded by FIP-0067
FIP-0047: Proof Expiration & PoRep Security Policy
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.
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
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,
RefreshProofExpirationmethod 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
ProofExprirationepoch 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).
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.
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.
- The existing sector
Expirationproperty is renamed to
CommitmentExpirationwith a duration range from
- A new sector property is introduced called
ProofExpiration. The value of
ProofExpirationis 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)
- During activation, It is calculated as:
- If a sector’s
ProofExpirationepoch 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 - ProofRefreshWindowand is disallowed past the
ProofExpiration. It will increase the
ProofExpirationof each of the requested sectors by
MaxProofDuration - ProofRefreshWindow.
- When extending a sector’s commitment expiration, the existing
ExtendSectorExpirationmethod 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.
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
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).
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.
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.
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;
RefreshProofExpirationmethod is disallowed for old sectors (ie, created with the vulnerable PoRep algorithm), in particular, all old sectors will terminate at
- 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
ProveCommitSector). Note that:
- The sealing procedure and replacement must be completed before the
ProofExpirationepoch 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
- The sealing procedure and replacement must be completed before the
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.
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.
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.
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.
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.
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.
Will be included with implementation to be presented.
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:
ProofRefreshWindowof 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
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.
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.
ProofExpiration separation implementation is available at
The replacement sealing implementation is deferred until needed.
Copyright and related rights waived via CC0.
Please cite this document as:
Jakub Sztandera, Irene Giacomelli, Alex North, Luca Nizzardo, "FIP-0047: Proof Expiration & PoRep Security Policy," Filecoin Improvement Proposals, no. 0047, August 2022. [Online serial]. Available: https://fips.filecoin.io/fips/fip-0047.