The following specification covers the RLN construct as well as some auxiliary libraries useful for interacting with it.
Rate limiting nullifier (RLN) is a construct based on zero-knowledge proofs that provides an anonymous rate-limited signaling/messaging framework suitable for decentralized (and centralized) environments.
Anonymity refers to the unlinkability of messages to their owner.
For registration, the user needs to submit their `identity_commitment` (along with any additional registration requirements) to the registry.
Upon registration, they should receive `leaf_index` value which represents their position in the Merkle tree.
Receiving a `leaf_index` is not a hard requirement and is application specific.
The other way around is the users calculating the `leaf_index` themselves upon successful registration.
## Signaling
After registration,
the users can participate in the application by sending signals to the other participants in a decentralised manner or to a centralised server.
Along with their signal,
they need to generate a ZK-Proof by using the circuit with the specification described above.
For generating a proof,
the users need to obtain the required parameters or compute them themselves,
depending on the application implementation and client libraries supported by the application.
For example the users can store the membership Merkle tree on their end and
generate a Merkle proof whenever they want to generate a signal.
### Implementation notes
#### Signal hash
The signal hash can be generated by hashing the raw signal (or content) using the `keccak256` hash function.
#### External nullifier
The external nullifier MUST be computed as the Poseidon hash of the current epoch (e.g. a value equal to or derived from the current UNIX timestamp divided by the epoch length) and the RLN identifier.
The Merkle proof should be obtained locally or from a trusted third party.
By using the [incremental Merkle tree algorithm](https://github.com/appliedzkp/incrementalquintree/blob/master/ts/IncrementalQuinTree.ts),
the Merkle can be obtained by providing the `leaf_index` of the `identity_commitment`.
The proof (`Merkle_proof`) is composed of the following fields:
```
{
root: bigint
indices: number[]
path_elements: bigint[][]
}
```
1.**root** - The root of membership group Merkle tree at the time of publishing the message
2.**indices** - The index fields of the leafs in the Merkle tree - used by the Merkle tree algorithm for verification
3.**path_elements** - Auxiliary data structure used for storing the path to the leaf - used by the Merkle proof algorithm for verificaton
#### Generating proof
For proof generation,
the user need to submit the following fields to the circuit:
```
{
identity_secret: identity_secret_hash,
path_elements: Merkle_proof.path_elements,
identity_path_index: Merkle_proof.indices,
x: signal_hash,
external_nullifier: external_nullifier
}
```
#### Calculating output
The proof output is calculated locally,
in order for the required fields for proof verification to be sent along with the proof.
The proof output is composed of the `y` share of the secret equation and the `internal_nullifier`.
The `internal_nullifier` represents a unique fingerprint of a user for a given `epoch` and app.
The following fields are needed for proof output calculation:
```
{
identity_secret_hash: bigint,
external_nullifier: bigint,
x: bigint,
}
```
The output `[y, internal_nullifier]` is calculated in the following way:
```
a_0 = identity_secret_hash
a_1 = poseidonHash([a0, external_nullifier])
y = a_0 + x * a_1
internal_nullifier = poseidonHash([a_1])
```
It relies on the properties of the [Shamir's Secret sharing scheme](https://en.wikipedia.org/wiki/Shamir%27s_Secret_Sharing).
#### Sending the output message
The user's output message (`output_message`),
containing the signal should contain the following fields at minimum:
```
{
signal: signal, # non-hashed signal
proof: zk_proof,
internal_nullifier: internal_nullifier,
x: x, # signal_hash
y: y,
rln_identifier: rln_identifier
}
```
Additionally depending on the application,
the following fields might be required:
```
{
root: Merkle_proof.root,
epoch: epoch
}
```
## Verification and slashing
The slashing implementation is dependent on the type of application.
If the application is implemented in a centralised manner,
and everything is stored on a single server,
the slashing will be implemented only on the server.
Otherwise if the application is distributed,
the slashing will be implemented on each user's client.
### Implementation notes
Each user of the protocol (server or otherwise) will need to store metadata for each message received by each user,
for the given `epoch`.
The data can be deleted when the `epoch` passes.
Storing metadata is required, so that if a user sends more than one unique signal per `epoch`,
they can be slashed and removed from the protocol.
The metadata stored contains the `x`, `y` shares and the `internal_nullifier` for the user for each message.
If enough such shares are present, the user's secret can be retreived.
One way of storing received metadata (`messaging_metadata`) is the following format:
```
{
[external_nullifier]: {
[internal_nullifier]: {
x_shares: [],
y_shares: []
}
}
}
```
#### Verification
The output message verification consists of the following steps:
-`external_nullifier` correctness
- non-duplicate message check
-`zk_proof` verification
- spam verification
**1. `external_nullifier` correctness**
Upon received `output_message`, first the `epoch` and `rln_identifier` fields are checked,
to ensure that the message matches the current `external_nullifier`.
If the `external_nullifier` is correct the verification continues, otherwise, the message is discarded.
**2. non-duplicate message check**
The received message is checked to ensure it is not duplicate.
The duplicate message check is performed by verifying that the `x` and `y` fields do not exist in the `messaging_metadata` object.
If the `x` and `y` fields exist in the `x_shares` and `y_shares` array for the `external_nullifier` and
the `internal_nullifier` the message can be considered as a duplicate.
Duplicate messages are discarded.
**3. `zk_proof` verification**
The `zk_proof` should be verified by providing the `zk_proof` field to the circuit verifier along with the `public_signal`:
```
[
y,
Merkle_proof.root,
internal_nullifier,
x, # signal_hash
external_nullifier
]
```
If the proof verification is correct,
the verification continues, otherwise the message is discarded.
**4. Double signaling verification**
After the proof is verified the `x`, and `y` fields are added to the `x_shares` and `y_shares` arrays of the `messaging_metadata``external_nullifier` and `internal_nullifier` object.
If the length of the arrays is equal to the signaling threshold (`limit`), the user can be slashed.
#### Slashing
After the verification, the user can be slashed if two different shares are present to reconstruct their `identity_secret_hash` from `x_shares` and `y_shares` fields,
for their `internal_nullifier`.
The secret can be retreived by the properties of the Shamir's secret sharing scheme.
In particular the secret (`a_0`) can be retrieved by computing [Lagrange polynomials](https://en.wikipedia.org/wiki/Lagrange_polynomial).
After the secret is retreived,
the user's `identity_commitment` can be generated from the secret and it can be used for removing the user from the membership Merkle tree (zeroing out the leaf that contains the user's `identity_commitment`).
Additionally, depending on the application the `identity_secret_hash` can be used for taking the user's provided stake.
| **Stake** | Financial or social stake required for registering in the RLN applications. Common stake examples are: locking cryptocurrency (financial), linking reputable social identity. |
| **Identity secret** | An array of two unique random components (identity nullifier and identity trapdoor), which must be kept private by the user. Secret hash and identity commitment are derived from this array. |
| **Identity nullifier** | Random 32 byte value used as component for identity secret generation. |
| **Identity trapdoor** | Random 32 byte value used as component for identity secret generation. |
| **Identity secret hash** | The hash of the identity secret, obtained using the Poseidon hash function. It is used for deriving the identity commitment of the user, and as a private input for zk proof generation. The secret hash should be kept private by the user. |
| **Identity commitment** | Hash obtained from the `Identity secret hash` by using the poseidon hash function. It is used by the users for registering in the protocol. |
| **Signal** | The message generated by a user. It is an arbitrary bit string that may represent a chat message, a URL request, protobuf message, etc. |
| **Signal hash** | Keccak256 hash of the signal modulo circuit's field characteristic, used as an input in the RLN circuit. |
| **RLN Identifier** | Random finite field value unique per RLN app. It is used for additional cross-application security. The role of the RLN identifier is protection of the user secrets from being compromised when signals are being generated with the same credentials in different apps. |
| **RLN membership tree** | Merkle tree data structure, filled with identity commitments of the users. Serves as a data structure that ensures user registrations. |
| **Merkle proof** | Proof that a user is member of the RLN membership tree. |
| **x** | Keccak hash of the signal, same as signal hash (Defined above). |
| **A0** | The identity secret hash. |
| **A1** | Poseidon hash of [A0, External nullifier] (see about External nullifier below). |
| **y** | The result of the polynomial equation (y = a0 + a1*x). The public output of the circuit. |
| **External nullifier** | Poseidon hash of [Epoch, RLN Identifier]. An identifier that groups signals and can be thought of as a voting booth. |
| **Internal nullifier** | Poseidon hash of [A1]. This field ensures that a user can send only one valid signal per external nullifier without risking being slashed. Public input of the circuit. |
Anonymous signaling with a controlled rate limit is enabled by proving that the user is part of a group which has high barriers to entry (form of stake) and
enabling secret reveal if more than 1 unique signal is produced per external nullifier.
The membership part is implemented using membership [Merkle trees](https://en.wikipedia.org/wiki/Merkle_tree) and Merkle proofs,
while the secret reveal part is enabled by using the Shamir's Secret Sharing scheme.
Essentially the protocol requires the users to generate zero-knowledge proof to be able to send signals and participate in the application.
The zero knowledge proof proves that the user is member of a group,
but also enforces the user to share part of their secret for each signal in an external nullifier.
The external nullifier is usually represented by timestamp or a time interval.
It can also be thought of as a voting booth in voting applications.
The ZK Circuit is implemented using a [Groth-16 ZK-SNARK](https://eprint.iacr.org/2016/260.pdf),
using the [circomlib](https://docs.circom.io/) library.
For a valid signal, a user's `identity_commitment` (more on identity commitments below) must exist in identity membership tree.
Membership is proven by providing a membership proof (witness).
The fields from the membership proof required for the verification are:
`path_elements` and `identity_path_index`.
[IncrementalQuinTree](https://github.com/appliedzkp/incrementalquintree) algorithm is used for constructing the Membership Merkle tree.
The circuits are reused from this repository.
You can find out more details about the IncrementalQuinTree algorithm [here](https://ethresear.ch/t/gas-and-circuit-constraint-benchmarks-of-binary-and-quinary-incremental-Merkle-trees-using-the-poseidon-hash-function/7446).
### Slashing and Shamir's Secret Sharing
Slashing is enabled by using polynomials and [Shamir's Secret sharing](https://en.wikipedia.org/wiki/Shamir%27s_Secret_Sharing).
In order to produce a valid proof, `identity_secret_hash` as a private input to the circuit.
Then a secret equation is created in the form of:
```
y = a_0 + x * a_1,
```
where `a_0` is the `identity_secret_hash` and `a_1 = hash(a_0, external nullifier)`.
Along with the generated proof,
the users need to provide a `(x, y)` share which satisfies the line equation,
in order for their proof to be verified.
`x` is the hashed signal, while the `y` is the circuit output.
With more than one pair of unique shares, anyone can derive `a_0`, i.e. the `identity_secret_hash` .
The hash of a signal will be the evaluation point `x`.
In this way, a member who sends more than one unique signal per `external_nullifier` risks their identity secret being revealed.
Note that shares used in different epochs and different RLN apps cannot be used to derive the identity secret hash.
Thanks to the `external_nullifier` definition, also shares computed from same secret within same epoch but in different RLN apps cannot be used to derive the identity secret hash.
The `rln_identifier` is a random value from a finite field,
unique per RLN app,
and is used for additional cross-application security - to protect the user secrets being compromised if they use the same credentials accross different RLN apps.
If `rln_identifier` is not present,
the user uses the same credentials and sends a different message for two different RLN apps using the same `external_nullifier`,
then their user signals can be grouped by the `internal_nullifier` which could lead the user's secret revealed.
This is because two separate signals under the same `internal_nullifier` can be treated as rate limiting violation.
With adding the `rln_identifier` field we obscure the `internal_nullifier`,
so this kind of attack can be hardened because we don't have the same `internal_nullifier` anymore.
Shamir-Secret Sharing requires polynomial coefficients to be independent of each other.
However, `a_1` depends on `a_0` through the Poseidon hash algorithm.
Due to the design of Poseidon, it is possible to [attack](https://github.com/Rate-Limiting-Nullifier/rln-circuits/pull/7#issuecomment-1416085627) the protocol.
It was decided *not* to change the circuits design, since at the moment the attack is infeasible. Therefore, implementers must be aware that the current version provides approximately 160-bit security and not 254.
Possible improvements:
* [change the circuit](https://github.com/Rate-Limiting-Nullifier/rln-circuits/pull/7#issuecomment-1416085627) to make coefficients independent;
There are few additional tools implemented for easier integrations and usage of the RLN protocol.
[`zerokit`](https://github.com/vacp2p/zerokit) is a set of Zero Knowledge modules, written in Rust and designed to be used in many different environments.
Among different modules, it supports `Semaphore` and `RLN`.
[`zk-kit`](https://github.com/appliedzkp/zk-kit) is a typescript library which exposes APIs for identity credentials generation,
as well as proof generation.
It supports various protocols (`Semaphore`, `RLN`).
[`zk-keeper`](https://github.com/akinovak/zk-keeper) is a browser plugin which allows for safe credential storing and proof generation.
