Issue with decoding Messaging API secret key in Node JS

Hello all, I am trying to create a JWT token to sign and verify messages in Business Chat. To do this I need to decode the Messaging API secret key(Base64-encoded string) before using it to sign the JWT.

I am using the Buffer object to decode the secret key. However, it returns gibberish

Any help would be appreciated!

Here is my code:

        let base64string = process.env.API_SECRET_KEY;

        let bufferObj = Buffer.from(base64string, 'base64');
       
        let decodedStr = bufferObj.toString('utf-8');