---
order: 1
parent:
  title: Light Client
  order: 5
---

# Light Client Specification

This directory contains work-in-progress English and TLA+ specifications for the Light Client
protocol. Implementations of the light client can be found in
[Rust](https://github.com/informalsystems/tendermint-rs/tree/master/light-client) and
[Go](https://github.com/cometbft/cometbft/tree/v0.38.x/light).

Light clients are assumed to be initialized once from a trusted source
with a trusted header and validator set. The light client
protocol allows a client to then securely update its trusted state by requesting and
verifying a minimal set of data from a network of full nodes (at least one of which is correct).

The light client is decomposed into two main components:

- [Commit Verification](#commit-verification) - verify signed headers and associated validator
  set changes from a single full node, called primary
- [Attack Detection](#attack-detection) -  verify commits across multiple full nodes (called secondaries) and detect conflicts (ie. the existence of a lightclient attack)

In case a lightclient attack is detected, the lightclient submits evidence to a full node which is responsible for "accountability", that is, punishing attackers:

- [Accountability](#accountability) - given evidence for an attack, compute a set of validators that are responsible for it.

## Commit Verification

The [English specification](verification/verification_001_published.md) describes the light client
commit verification problem in terms of the temporal properties
[LCV-DIST-SAFE.1](https://github.com/cometbft/cometbft/blob/v0.38.x/spec/light-client/verification/verification_001_published.md#lcv-dist-safe1) and
[LCV-DIST-LIVE.1](https://github.com/cometbft/cometbft/blob/v0.38.x/spec/light-client/verification/verification_001_published.md#lcv-dist-live1).
Commit verification is assumed to operate within the Cosmos Failure Model, where +2/3 of validators are correct for some time period and
validator sets can change arbitrarily at each height.

A light client protocol is also provided, including all checks that
need to be performed on headers, commits, and validator sets
to satisfy the temporal properties - so a light client can continuously
synchronize with a blockchain. Clients can skip possibly
many intermediate headers by exploiting overlap in trusted and untrusted validator sets.
When there is not enough overlap, a bisection routine can be used to find a
minimal set of headers that do provide the required overlap.

The [TLA+ specification ver. 001](verification/Lightclient_A_1.tla)
is a formal description of the
commit verification protocol executed by a client, including the safety and
termination, which can be model checked with Apalache.

A more detailed TLA+ specification of
[Light client verification ver. 003](verification/Lightclient_003_draft.tla)
is currently under peer review.

The `MC*.tla` files contain concrete parameters for the
[TLA+ specification](verification/Lightclient_A_1.tla), in order to do model checking.
For instance, [MC4_3_faulty.tla](verification/MC4_3_faulty.tla) contains the following parameters
for the nodes, heights, the trusting period, the clock drifts,
correctness of the primary node, and the ratio of the faulty processes:

```tla
AllNodes == {"n1", "n2", "n3", "n4"}
TRUSTED_HEIGHT == 1
TARGET_HEIGHT == 3
TRUSTING_PERIOD == 1400     \* the trusting period in some time units
CLOCK_DRIFT = 10            \* how much we assume the local clock is drifting
REAL_CLOCK_DRIFT = 3        \* how much the local clock is actually drifting
IS_PRIMARY_CORRECT == FALSE
FAULTY_RATIO == <<1, 3>>    \* < 1 / 3 faulty validators
```

To run a complete set of experiments, clone [apalache](https://github.com/informalsystems/apalache) and [apalache-tests](https://github.com/informalsystems/apalache-tests) into a directory `$DIR` and run the following commands:

```sh
$DIR/apalache-tests/scripts/mk-run.py --memlimit 28 002bmc-apalache-ok.csv $DIR/apalache . out
./out/run-all.sh
```

After the experiments have finished, you can collect the logs by executing the following command:

```sh
cd ./out
$DIR/apalache-tests/scripts/parse-logs.py --human .
```

All lines in `results.csv` should report `Deadlock`, which means that the algorithm
has terminated and no invariant violation was found.

Similar to [002bmc-apalache-ok.csv](verification/002bmc-apalache-ok.csv),
file [003bmc-apalache-error.csv](verification/003bmc-apalache-error.csv) specifies
the set of experiments that should result in counterexamples:

```sh
$DIR/apalache-tests/scripts/mk-run.py --memlimit 28 003bmc-apalache-error.csv $DIR/apalache . out
./out/run-all.sh
```

All lines in `results.csv` should report `Error`.

The following table summarizes the experimental results for Light client verification
version 001. The TLA+ properties can be found in the
[TLA+ specification](verification/Lightclient_A_1.tla).
 The experiments were run in an AWS instance equipped with 32GB
RAM and a 4-core Intel® Xeon® CPU E5-2686 v4 @ 2.30GHz CPU.
We write “✗=k” when a bug is reported at depth k, and “✓<=k” when
no bug is reported up to depth k.

![Experimental results](experiments.png)

The experimental results for version 003 are to be added.

## Attack Detection

The [English specification](detection/detection_003_reviewed.md)
defines light client attacks (and how they differ from blockchain
forks), and describes the problem of a light client detecting
these attacks by communicating with a network of full nodes,
where at least one is correct.

The specification also contains a detection protocol that checks
whether the header obtained from the primary via the verification
protocol matches corresponding headers provided by the secondaries.
If this is not the case, the protocol analyses the verification traces
of the involved full nodes
and generates
[evidence](detection/detection_003_reviewed.md#cmbc-lc-evidence-data1)
of misbehavior that can be submitted to a full node so that
the faulty validators can be punished.

The [TLA+ specification](detection/LCDetector_003_draft.tla)
is a formal description of the
detection protocol for two peers, including the safety and
termination, which can be model checked with Apalache.

The `LCD_MC*.tla` files contain concrete parameters for the
[TLA+ specification](detection/LCDetector_003_draft.tla),
in order to run the model checker.
For instance, [LCD_MC4_4_faulty.tla](./verification/MC4_4_faulty.tla)
contains the following parameters
for the nodes, heights, the trusting period, the clock drifts,
correctness of the nodes, and the ratio of the faulty processes:

```tla
AllNodes == {"n1", "n2", "n3", "n4"}
TRUSTED_HEIGHT == 1
TARGET_HEIGHT == 3
TRUSTING_PERIOD == 1400     \* the trusting period in some time units
CLOCK_DRIFT = 10            \* how much we assume the local clock is drifting
REAL_CLOCK_DRIFT = 3        \* how much the local clock is actually drifting
IS_PRIMARY_CORRECT == FALSE
IS_SECONDARY_CORRECT == FALSE
FAULTY_RATIO == <<1, 3>>    \* < 1 / 3 faulty validators
```

To run a complete set of experiments, clone [apalache](https://github.com/informalsystems/apalache) and [apalache-tests](https://github.com/informalsystems/apalache-tests) into a directory `$DIR` and run the following commands:

```sh
$DIR/apalache-tests/scripts/mk-run.py --memlimit 28 004bmc-apalache-ok.csv $DIR/apalache . out
./out/run-all.sh
```

After the experiments have finished, you can collect the logs by executing the following command:

```sh
cd ./out
$DIR/apalache-tests/scripts/parse-logs.py --human .
```

All lines in `results.csv` should report `Deadlock`, which means that the algorithm
has terminated and no invariant violation was found.

Similar to [004bmc-apalache-ok.csv](verification/004bmc-apalache-ok.csv),
file [005bmc-apalache-error.csv](verification/005bmc-apalache-error.csv) specifies
the set of experiments that should result in counterexamples:

```sh
$DIR/apalache-tests/scripts/mk-run.py --memlimit 28 005bmc-apalache-error.csv $DIR/apalache . out
./out/run-all.sh
```

All lines in `results.csv` should report `Error`.

The detailed experimental results are to be added soon.

## Accountability

The [English specification](attacks/isolate-attackers_002_reviewed.md)
defines the protocol that is executed on a full node upon receiving attack [evidence](detection/detection_003_reviewed.md#cmbc-lc-evidence-data1) from a lightclient. In particular, the protocol handles three types of attacks

- lunatic
- equivocation
- amnesia

We discussed in the [last part](attacks/isolate-attackers_002_reviewed.md#Part-III---Completeness) of the English specification
that the non-lunatic cases are defined by having the same validator set in the conflicting blocks. For these cases,
computer-aided analysis of  [Tendermint Consensus in TLA+](./accountability/README.md) shows that equivocation and amnesia capture all non-lunatic attacks.

The [TLA+ specification](attacks/Isolation_001_draft.tla)
is a formal description of the
protocol, including the safety property, which can be model checked with Apalache.

Similar to the other specifications, [MC_5_3.tla](attacks/MC_5_3.tla) contains concrete parameters to run the model checker. The specification can be checked within seconds.

[tendermint-accountability](./accountability/README.md)
