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ADR 047: Handling evidence from light client

Changelog

18-02-2020: Initial draft
24-02-2020: Second version
13-04-2020: Add PotentialAmnesiaEvidence and a few remarks

Context

If the light client is under attack, either directly -> lunatic/phantom validators (light fork) or indirectly -> full fork, it's supposed to halt and send evidence of misbehavior to a correct full node. Upon receiving an evidence, the full node should punish malicious validators (if possible).

Decision

When a light client sees two conflicting headers (H1.Hash() != H2.Hash(), H1.Height == H2.Height), both having 1/3+ of the voting power of the currently trusted validator set, it will submit a ConflictingHeadersEvidence to all full nodes it's connected to. Evidence needs to be submitted to all full nodes since there's no way to determine which full node is correct (honest).

type ConflictingHeadersEvidence struct {
  H1 types.SignedHeader
  H2 types.SignedHeader
}

Remark: Theoretically, only the header, which differs from what a full node has, needs to be sent. But sending two headers a) makes evidence easily verifiable b) simplifies the light client, which does not have query each witness as to which header it possesses.

When a full node receives the ConflictingHeadersEvidence evidence, it should a) validate it b) figure out if malicious behaviour is obvious (immediately slashable) or the fork accountability protocol needs to be started.

Validating headers

Check both headers are valid (ValidateBasic), have the same height, and signed by 1/3+ of the validator set that the full node had at height H1.Height.

Q: What if light client validator set is not equal to full node's validator set (i.e. from full node's point of view both headers are not properly signed; this includes the case where none of the two headers were committed on the main chain)

Reject the evidence. It means light client is following a fork, but, hey, at least it will halt.
Q: Don't we want to punish validators who signed something else even if they have less or equal than 1/3?

No consensus so far. Ethan said no, Zarko said yes. https://github.com/tendermint/spec/pull/71#discussion_r374210533

Figuring out if malicious behaviour is immediately slashable

Let's say H1 was committed from this full node's perspective (see Appendix A). If neither of the headers (H1 and H2) were committed from the full node's perspective, the evidence must be rejected.

Intersect validator sets of H1 and H2.

if there are signers(H2) that are not part of validators(H1), they misbehaved as they are signing protocol messages in heights they are not validators => immediately slashable (#F4).
if H1.Round == H2.Round, and some signers signed different precommit messages in both commits, then it is an equivocation misbehavior => immediately slashable (#F1).
if H1.Round != H2.Round we need to run full detection procedure => not immediately slashable.
if ValidatorsHash, NextValidatorsHash, ConsensusHash, AppHash, and LastResultsHash in H2 are different (incorrect application state transition), then it is a lunatic misbehavior => immediately slashable (#F5).

If evidence is not immediately slashable, fork accountability needs to invoked (ADR does not yet exist).

It's unclear if we should further break up ConflictingHeadersEvidence or gossip and commit it directly. See https://github.com/tendermint/tendermint/issues/4182#issuecomment-590339233

If we'd go without breaking evidence, all we'll need to do is to strip the committed header from ConflictingHeadersEvidence (H1) and leave only the uncommitted header (H2):

type ConflictingHeaderEvidence struct {
  H types.SignedHeader
}

If we'd go with breaking evidence, here are the types we'll need:

F1. Equivocation

Existing DuplicateVoteEvidence needs to be created and gossiped.

F4. Phantom validators

A new type of evidence needs to be created:

type PhantomValidatorEvidence struct {
	Header                      types.Header
	Vote                        types.Vote
	LastHeightValidatorWasInSet int64
}

It contains a validator's public key and a vote for a block, where this validator is not part of the validator set. LastHeightValidatorWasInSet indicates the last height validator was in the validator set.

F5. Lunatic validator

type LunaticValidatorEvidence struct {
	Header             types.Header
	Vote               types.Vote
	InvalidHeaderField string
}

To punish this attack, we need support for a new Evidence type - LunaticValidatorEvidence. This type includes a vote and a header. The header must contain fields that are invalid with respect to the previous block, and a vote for that header by a validator that was in a validator set within the unbonding period. While the attack is only possible if +1/3 of some validator set colludes, the evidence should be verifiable independently for each individual validator. This means the total evidence can be split into one piece of evidence per attacking validator and gossipped to nodes to be verified one piece at a time, reducing the DoS attack surface at the peer layer.

Note it is not sufficient to simply compare this header with that committed for the corresponding height, as an honest node may vote for a header that is not ultimately committed. Certain fields may also be variable, for instance the LastCommitHash and the Time may depend on which votes the proposer includes. Thus, the header must be explicitly checked for invalid data.

For the attack to succeed, VC must sign a header that changes the validator set to consist of something they control. Without doing this, they can not otherwise attack the light client, since the client verifies commits according to validator sets. Thus, it should be sufficient to check only that ValidatorsHash and NextValidatorsHash are correct with respect to the header that was committed at the corresponding height.

That said, if the attack is conducted by +2/3 of the validator set, they don't need to make an invalid change to the validator set, since they already control it. Instead they would make invalid changes to the AppHash, or possibly other fields. In order to punish them, then, we would have to check all header fields.

Note some header fields require the block itself to verify, which the light client, by definition, does not possess, so it may not be possible to check these fields. For now, then, LunaticValidatorEvidence must be checked against all header fields which are a function of the application at previous blocks. This includes ValidatorsHash, NextValidatorsHash, ConsensusHash, AppHash, and LastResultsHash. These should all match what's in the header for the block that was actually committed at the corresponding height, and should thus be easy to check.

InvalidHeaderField contains the invalid field name. Note it's very likely that multiple fields diverge, but it's faster to check just one. This field MUST NOT be used to determine equality of LunaticValidatorEvidence.

F2. Amnesia

type PotentialAmnesiaEvidence struct {
	VoteA types.Vote
	VoteB types.Vote
}

To punish this attack, votes under question needs to be sent. Fork accountability process should then use this evidence to request additional information from offended validators and construct a new type of evidence to punish those who conducted an amnesia attack.

See ADR-056 for the architecture of the fork accountability procedure.

Status

Proposed.

Consequences

Positive

Tendermint will be able to detect & punish new types of misbehavior
light clients connected to multiple full nodes can help full nodes notice a fork faster

Negative

Accepting ConflictingHeadersEvidence from light clients opens up a DDOS attack vector (same is fair for any RPC endpoint open to public; remember that RPC is not open by default).

Neutral

References

Fork accountability spec

Appendix A

If there is an actual fork (full fork), a full node may follow either one or another branch. So both H1 or H2 can be considered committed depending on which branch the full node is following. It's supposed to halt if it notices an actual fork, but there's a small chance it doesn't.

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