# Construct Signature

## Option 1

If `userId` is sent in `InitConfig` then the callback signature will be constructed the following way:

Data required to generate signature:

1. `transactionId`
2. `userId`
3. `secretKey`

The signature variable is calculated by taking the MD5 hash of `transactionId, userId, secretKey` separated by colons. In Python, it can be calculated using the code snippet below:

{% code overflow="wrap" %}

```python
import hashlib
signature = hashlib.md5(f"{transactionId}:{userId}:{secretKey}".encode('utf-8')).hexdigest())
```

{% endcode %}

## Option 2

If  `clientId` is sent in `InitConfig` then the callback signature will be constructed the following way:

Data required to generate signature:

1. `transactionId`
2. `clientId`
3. `secretKey`

The signature variable is calculated by taking the MD5 hash of `transactionId, clientId, secretKey` separated by colons. In Python, it can be calculated using the code snippet below:

{% code overflow="wrap" %}

```python
import hashlib
signature = hashlib.md5(f"{transactionId}:{clientId}:{secretKey}".encode('utf-8')).hexdigest())
```

{% endcode %}

### NOTE:

ArcPlay will send a `signature` variable in the postback payload as an Auth token. The publisher server must recalculate the `signature` and match it to the `signature` sent to securely validate the transaction. The `secretKey` will be shared with by the ArcPlay Customer Success Team. Each application should have a unique secretKey. Please do not use the same secretKey for all of your applications, as this might lead to a situation where rewarding the user in one application will also reward the user in another application.


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