> For the complete documentation index, see [llms.txt](https://unirep.gitbook.io/unirep/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://unirep.gitbook.io/unirep/getting-started/start-with-typescript/4.-epoch-key-proof.md).

# 4. Epoch key proof

Before generating an epoch key proof, we should generate a current user state to know the current [global state tree](/unirep/protocol/glossary/trees.md#global-state-tree)s and the attestation histories.

### DB

We should initialize a storage to save the state.

```typescript
import { DB, SQLiteConnector } from 'anondb/node'
import { schema } from '@unirep/core'

// construct a memory db
const db = await SQLiteConnector.create(schema, ':memory:')
// or construct a SQLite db
// const db = await SQLiteConnector.create(schema, 'test.sqlite')
```

### Prover

Also we have to initialize a prover to generate proofs and verify proofs.

```typescript
const snarkjs = require('snarkjs')
import { Circuit, Prover } from '@unirep/circuits'
import { SnarkProof, SnarkPublicSignals } from '@unirep/crypto'
const buildPath = 'PATH/TO/THE/KEYS/'

const prover: Prover = {
    genProofAndPublicSignals: async (
        proofType: string | Circuit,
        inputs: any
    ): Promise<{
        proof: any,
        publicSignals: any
    }> => {
        const circuitWasmPath = buildPath + `${proofType}.wasm`
        const zkeyPath = buildPath + `${proofType}.zkey`
        const { proof, publicSignals } = await snarkjs.groth16.fullProve(
            inputs,
            circuitWasmPath,
            zkeyPath
        )
        return { proof, publicSignals }
    },
    verifyProof: async (
        name: string | Circuit,
        publicSignals: SnarkPublicSignals,
        proof: SnarkProof
    ): Promise<boolean> => {
        const vkey = require(buildPath + `${name}.vkey.json`)
        return snarkjs.groth16.verify(vkey, publicSignals, proof)
    },
}
```

{% hint style="warning" %}
After clone the [Unirep repository](https://github.com/Unirep/Unirep) and run

`yarn && yarn build`

The `buildPath` will be found at `packages/circuits/zksnarkBuild`

In the future it will be uploaded to a server to be downloaded.
{% endhint %}

### Identity

Also, the `ZkIdentity` can be unserialized with a serialized identity, for example:

```typescript
import { ZkIdentity, Strategy } from '@unirep/core'

const identity = new ZkIdentity(
    Strategy.SERIALIZED, 
    `{"identityNullifier":"27d1ae5c98aab64b851a9c668a7eec0d835867a17d4b9454a8bf9824836271d6","identityTrapdoor":"2596ecc2a1e1f6a8f279e097464e6edc3b18b946d934398dfe52a34c4e414e67","secret":["27d1ae5c98aab64b851a9c668a7eec0d835867a17d4b9454a8bf9824836271d6","2596ecc2a1e1f6a8f279e097464e6edc3b18b946d934398dfe52a34c4e414e67"]}`
)
```

### Generate current user state

And then we can use a `genUserState` to perform synchronization.

```typescript
const genUserState = async (
    provider: ethers.providers.Provider,
    address: string,
    userIdentity: ZkIdentity,
    db: DB
) => {
    const contract = getUnirepContract(address, provider)
    const userState = new UserState(
        db,
        prover,
        contract,
        userIdentity
    )
    await userState.start()
    await userState.waitForSync()
    return userState
}
```

```typescript
const userState = await genUserState(
    provider,
    UNIREP_CONTRACT_ADDRESS,
    identity,
    db
)
```

### Generate epoch key proof

Use the user state to generate epoch key proof.

```typescript
// genearte epoch key proof
const epochKeyNonce = 0
const proof = await userState.genVerifyEpochKeyProof(epochKeyNonce)
```

### Submit epoch key proof

```typescript
const tx = await contract.submitEpochKeyProof(
    proof.publicSignals,
    proof.proof
)
```

### Get proof index

Then get the proof index with the proof hash

```typescript
const proofHash = proof.hash()
const proofIdx = await contract.getProofIndex(proofHash)
console.log(proofIdx)
```

### Verify epoch key proof&#x20;

Verify the proof with UniRep smart contract

```typescript
const isValid = await contract.verifyEpochKeyValidity(
    proof.publicSignals,
    proof.proof
)
console.log(isValid)
```

Verify the proof with local prover

```typescript
const isValid = await proof.verify()
console.log(isValid)
```

### Verify UniRep state

Check if the global state tree exists in the current UniRep state.

It can be verified by either a `Synchronizer` or a `UserState` object. But the `Synchronizer` doesn't take `ZkIdentity` as an input. For example

```typescript
const genUnirepState = async (
    provider: ethers.providers.Provider,
    address: string,
    db: DB
) => {
    const contract = getUnirepContract(address, provider)
    const unirepState = new Synchronizer(
        db,
        prover,
        contract,
    )
    await unirepState.start()
    await unirepState.waitForSync()
    return unirepState
}

```

```typescript
const unirepState = await genUnirepState(
    provider,
    UNIREP_CONTRACT_ADDRESS,
    db
)
```

Then we can use the identity-free object to verify the global state tree root.

```typescript
const isGSTRootExisted = await unirepState.GSTRootExists(
    proof.globalStateTree,
    proof.epoch
)
console.log(isGSTRootExisted) // false then the proof will be invalid
```