# SDK & Contract Reference \[SDK methods and contract functions for Newton Verifiable Credentials.]

This page covers the SDK methods and contract functions used to register, link, and manage user identity data with Newton.

## SDK Methods

The following methods are available on the wallet client actions extension (`newtonWalletClientActions`).

### `registerUserData`

Registers KYC data for a user, scoped to your application's identity domain. Opens the Newton identity popup to securely store the data.

**Signature:**

```typescript
registerUserData(args: {
  userData: KycUserData
  appIdentityDomain: Hex
  policyClient: Address
}): Promise<unknown>
```

**Parameters:**

| Parameter | Type | Description |
|-----------|------|-------------|
| `userData` | `KycUserData` | KYC data object from your identity vendor |
| `appIdentityDomain` | `Hex` | Output of `keccak256("kyc")` |
| `policyClient` | `Address` | Policy client contract address (used for HPKE encryption AAD binding) |

**Example:**

```typescript
await walletClient.registerUserData({
  userData: kycDataFromVendor,
  appIdentityDomain: '0xa29cad97d4110e8822dedef32879f84a8e8889bf965ff9c57998689e2fdafbc4',
  policyClient: '0xYourPolicyClientAddress',
})
```

***

### `linkApp`

Links a user's registered identity to your policy client contract. The user must confirm the link in the Newton identity popup, which triggers a signed transaction from their wallet.

**Signature:**

```typescript
linkApp(args: {
  appWalletAddress: Address
  appClientAddress: Address
  appIdentityDomain: Hex
}): Promise<unknown>
```

**Parameters:**

| Parameter | Type | Description |
|-----------|------|-------------|
| `appWalletAddress` | `Address` | The user's dapp wallet address |
| `appClientAddress` | `Address` | Your deployed policy client contract address |
| `appIdentityDomain` | `Hex` | Output of `keccak256("kyc")` |

**Example:**

```typescript
await walletClient.linkApp({
  appWalletAddress: '0xUserWallet...',
  appClientAddress: '0xYourPolicyClient...',
  appIdentityDomain: '0xabc123...',
})
```

***

### `unlinkApp`

Removes a user's identity link from your policy client contract.

**Signature:**

```typescript
unlinkApp(args: {
  appWalletAddress: Address
  appClientAddress: Address
  appIdentityDomain: Hex
}): Promise<unknown>
```

**Parameters:**

| Parameter | Type | Description |
|-----------|------|-------------|
| `appWalletAddress` | `Address` | The user's dapp wallet address |
| `appClientAddress` | `Address` | Your deployed policy client contract address |
| `appIdentityDomain` | `Hex` | Output of `keccak256("kyc")` |

**Example:**

```typescript
await walletClient.unlinkApp({
  appWalletAddress: '0xUserWallet...',
  appClientAddress: '0xYourPolicyClient...',
  appIdentityDomain: '0xabc123...',
})
```

***

## Contract Functions

### `linkIdentityAsUser`

Called on the `IdentityRegistry` contract to link a user's identity to a policy client. This is invoked under the hood when the user confirms the link via `linkApp`. See [Contract Addresses](/developers/reference/contract-addresses) for the `IdentityRegistry` address.

**Signature:**

```solidity
function linkIdentityAsUser(
    address _identityOwner,
    address _policyClient,
    bytes32[] calldata _identityDomains,
    bytes calldata _signature,
    uint256 _nonce,
    uint256 _deadline
)
```

**Parameters:**

The following are the output of the link SDK method.

| Parameter | Type | Description |
|-----------|------|-------------|
| `_identityOwner` | `address` | The identity owner being used to authorize access to the existing identity |
| `_policyClient` | `address` | The policy client where the data is to be associated |
| `_identityDomains` | `bytes32[]` | The identity domains specifying what type of data is associated |
| `_signature` | `bytes` | Signature by the `_identityOwner` to authorize the link |
| `_nonce` | `uint256` | Nonce for the signature |
| `_deadline` | `uint256` | Deadline for the signature |

***

## Sample Policy Client

Your policy client contract must inherit both `NewtonPolicyClient` and `EIP712`. Below is a complete example:

```solidity
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.27;

import {NewtonPolicyClient} from "newton-contracts/src/mixins/NewtonPolicyClient.sol";
import {INewtonProverTaskManager} from "newton-contracts/src/interfaces/INewtonProverTaskManager.sol";
import {EIP712} from "@openzeppelin/contracts/utils/cryptography/EIP712.sol";

contract NewtonPolicyWallet is NewtonPolicyClient, EIP712 {
    event Executed(address indexed to, uint256 value, bytes data, bytes32 taskId);
    error InvalidAttestation();
    error ExecutionFailed();

    constructor(
        address policyTaskManager,
        address owner
    ) payable EIP712("NewtonPolicyWallet", "1") {
        _initNewtonPolicyClient(policyTaskManager, owner);
    }

    function supportsInterface(bytes4 interfaceId) public view override returns (bool) {
        return interfaceId == 0xdbdcaa9c || super.supportsInterface(interfaceId);
    }

    function validateAndExecuteDirect(
        address to,
        uint256 value,
        bytes calldata data,
        INewtonProverTaskManager.Task calldata task,
        INewtonProverTaskManager.TaskResponse calldata taskResponse,
        bytes calldata signatureData
    ) external payable returns (bytes memory) {
        require(
            _validateAttestationDirect(task, taskResponse, signatureData),
            InvalidAttestation()
        );

        (bool success, bytes memory result) = to.call{value: value}(data);
        if (!success) revert ExecutionFailed();

        emit Executed(to, value, data, task.taskId);
        return result;
    }

    receive() external payable {}
}
```

:::note
After deploying, register this contract with the `PolicyClientRegistry`. See [Smart Contract Integration](/developers/guides/smart-contract-integration#identityregistry-integration) for instructions.
:::

## Next Steps

<Cards>
  <Card icon="brackets-curly" to="/developers/verified-credential/identity-policy-reference" title="Identity Policy Reference">
    Rego built-in functions for writing identity-aware policies
  </Card>

  <Card icon="map" to="/developers/reference/contract-addresses" title="Contract Addresses">
    IdentityRegistry and other deployed contract addresses
  </Card>
</Cards>