tendermint/docs/architecture/adr-056-proving-amnesia-attacks.md

# ADR 056: Proving amnesia attacks

## Changelog

- 02.04.20: Initial Draft
- 06.04.20: Second Draft
- 10.06.20: Post Implementation Revision

## Context

Whilst most created evidence of malicious behaviour is self evident such that any individual can verify them independently there are types of evidence, known collectively as global evidence, that require further collaboration from the network in order to accumulate enough information to create evidence that is individually verifiable and can therefore be processed through consensus. [Fork Accountability](https://github.com/tendermint/spec/blob/master/spec/consensus/light-client/accountability.md) has been coined to describe the entire process of detection, proving and punishing of malicious behaviour. This ADR addresses specifically how to prove an amnesia attack but also generally outlines how global evidence can be converted to individual evidence.

### Amnesia Attack

The currently only known form of global evidence stems from [flip flopping](https://github.com/tendermint/spec/blob/master/spec/consensus/light-client/accountability.md#flip-flopping) attacks. The schematic below explains one scenario where an amnesia attack, a form of flip flopping, can occur such that two sets of honest nodes, C1 and C2, commit different blocks.

![](../imgs/tm-amnesia-attack.png)

1. C1 and F send PREVOTE messages for block A.
2. C1 sends PRECOMMIT for round 1 for block A.
3. A new round is started, C2 and F send PREVOTE messages for a different block B.
4. C2 and F then send PRECOMMIT messages for block B.
5. F breaks the lock and goes back and sends PRECOMMIT messages in round 1 for block A.


This creates a fork on the main chain.  Back to the past, another form of flip flopping, creates a light fork (capable of fooling those not involved in consensus), in a similar way, with F taking the precommits from C1 and forging a commit from them.

## Decision

As the distinction between these two attacks (amnesia and back to the past) can only be distinguished by confirming with all validators (to see if it is a full fork or a light fork), for the purpose of simplicity, these attacks will be treated as the same.

Currently, the evidence reactor is used to simply broadcast and store evidence. The idea of creating a new reactor for the specific task of verifying these attacks was briefly discussed, but it is decided that the current evidence reactor will be extended.

The process begins with a light client receiving conflicting headers (in the future this could also be a full node during fast sync or state sync), which it sends to a full node to analyse. As part of [evidence handling](https://github.com/tendermint/tendermint/blob/master/docs/architecture/adr-047-handling-evidence-from-light-client.md), this is extracted into potential amnesia evidence when the validator voted in more than one round for a different block.

```golang
type PotentialAmnesiaEvidence struct {
	VoteA *types.Vote
	VoteB *types.Vote

	Heightstamp int64
}
```

*NOTE: There had been an earlier notion towards batching evidence against the entire set of validators all together but this has given way to individual processing predominantly to maintain consistency with the other forms of evidence. A more extensive breakdown can be found [here](https://github.com/tendermint/tendermint/issues/4729)*

The evidence will contain the precommit votes for a validator that voted for both rounds. If the validator voted in more than two rounds, then they will have multiple `PotentialAmnesiaEvidence` against them hence it is possible that there is multiple evidence for a validator in a single height but not for a single round. The votes should be all valid and the height and time that the infringement was made should be within:

`MaxEvidenceAge - ProofTrialPeriod`

This trial period will be discussed later.

Returning to the event of an amnesia attack, if we were to examine the behaviour of the honest nodes, C1 and C2, in the schematic, C2 will not PRECOMMIT an earlier round, but it is likely, if a node in C1 were to receive +2/3 PREVOTE's or PRECOMMIT's for a higher round, that it would remove the lock and PREVOTE and PRECOMMIT for the later round. Therefore, unfortunately it is not a case of simply punishing all nodes that have double voted in the `PotentialAmnesiaEvidence`.

Instead we use the Proof of Lock Change (PoLC) referred to in the [consensus spec](https://github.com/tendermint/spec/blob/master/spec/consensus/consensus.md#terms). When an honest node votes again for a different block in a later round
(which will only occur in very rare cases), it will generate the PoLC and store it in the evidence reactor for a time equal to the `MaxEvidenceAge`

```golang
type ProofOfLockChange struct {
	Votes []*types.Vote
	PubKey crypto.PubKey
}
```

This can be either evidence of +2/3 PREVOTES or PRECOMMITS (either warrants the honest node the right to vote) and is valid, among other checks, so long as the PRECOMMIT vote of the node in V2 came after all the votes in the `ProofOfLockChange` i.e. it received +2/3 votes for a block and then voted for that block thereafter (F is unable to prove this).

In the event that an honest node receives `PotentialAmnesiaEvidence` it will first `ValidateBasic()` and `Verify()` it and then will check if it is among the suspected nodes in the evidence. If so, it will retrieve the `ProofOfLockChange` and combine it with `PotentialAmensiaEvidence` to form `AmensiaEvidence`. All honest nodes that are part of the indicted group will have a time, measured in blocks, equal to `ProofTrialPeriod`, the aforementioned evidence paramter, to gossip their `AmnesiaEvidence` with their `ProofOfLockChange`

```golang
type AmnesiaEvidence struct {
	*types.PotentialAmnesiaEvidence
	Polc   *types.ProofOfLockChange
}
```

If the node is not required to submit any proof than it will simply broadcast the `PotentialAmnesiaEvidence`, stamp the height that it received the evidence and begin to wait out the trial period. It will ignore other `PotentialAmnesiaEvidence` gossiped at the same height and round.

If a node receives `AmnesiaEvidence` that contains a valid `ProofOfClockChange` it will add it to the evidence store and replace any PotentialAmnesiaEvidence of the same height and round. At this stage, an amnesia evidence with polc, it is ready to be submitted to the chin. If a node receives `AmnesiaEvidence` with an empty polc it will ignore it as each honest node will conduct their own trial period to be sure that time was given for any other honest nodes to respond.

There can only be one `AmnesiaEvidence` and one `PotentialAmneisaEvidence` stored for each attack (i.e. for each height).

When, `state.LastBlockHeight > PotentialAmnesiaEvidence.timestamp + ProofTrialPeriod`, nodes will upgrade the corresponding `PotentialAmnesiaEvidence` and attach an empty `ProofOfLockChange`. Then honest validators of the current validator set can begin proposing the block that contains the `AmnesiaEvidence`.

*NOTE: Even before the evidence is proposed and committed, the off-chain process of gossiping valid evidence could be
 enough for honest nodes to recognize the fork and halt.*

Other validators will vote <nil> if:

- The Amnesia Evidence is not valid
- The Amensia Evidence is not within their own trial period i.e. too soon.
- They don't have the Amnesia Evidence and it is has an empty polc (each validator needs to run their own trial period of the evidence)
- Is of an AmnesiaEvidence that has already been committed to the chain.

Finally it is important to stress that the protocol of having a trial period addresses attacks where a validator voted again for a different block at a later round and time. In the event, however, that the validator voted for an earlier round after voting for a later round i.e. `VoteA.Timestamp < VoteB.Timestamp && VoteA.Round > VoteB.Round` then this action is inexcusable and can be punished immediately without the need of a trial period. In this case, PotentialAmnesiaEvidence will be instantly upgraded to AmnesiaEvidence. 


## Status

Proposed

## Consequences

### Positive

Increasing fork detection and accountability makes the system more secure

### Negative

Non-responsive but honest nodes that are part of the suspect group that don't produce a proof will be punished

A delay between the detection of a fork and the punishment of one

### Neutral


## References

- [Fork accountability algorithm](https://docs.google.com/document/d/11ZhMsCj3y7zIZz4udO9l25xqb0kl7gmWqNpGVRzOeyY/edit)
- [Fork accountability spec](https://github.com/tendermint/spec/blob/master/spec/consensus/light-client/accountability.md)