Hi, I'm a little late to the party, but I'm running into a similar issue. First off I want to point out that about halfway through this thread, the answers start pointing to using JWT for SSO and not for chat authenticate. For example the first answer to the OP says to remove the jti param from the payload, but the articles linked in this thread are for SSO and definitively show that you need to have a jti param in the payload making this really confusing to follow along with.

I have followed the instructions on this guide: Enabling authenticated visitors in Web Widget (Classic) https://support.zendesk.com/hc/en-us/articles/4408838925082.

When my web widget loads on the screen I am getting this 400 error: "Zendesk Chat Web SDK: Error: init: Failed to verify token: jwt verification error"

But if I take the resultant JWT to an online JWT debugger it says the signature is verified.

My secret is not base64 encoded. I am using the ruby-jwt gem as outlined in the Zendesk documentation linked above. The gem is required via rails, and thus is not explicitly required in my snippet below.

My Ruby code:

def zendesk_jwt_endpoint
   # This is and endpoint to call from the javascript portion to support JWT tokens in zendesk
   payload = {
      :name => "#{current_user.name.present? ? current_user.name : current_user.assumed_name_from_email}",
      :email => "#{current_user.email}",
      :iat => DateTime.now.to_i,
      :exp => (DateTime.now + 6.minutes).to_i,
      :external_id => current_user.id
   }
   # this syntax below is directly from the documentation in the ruby-jwt
   # gem for JWT.encode.
   token = JWT.encode payload, "#{ENV.fetch('ZENDESK_SHARED_SECRET','')}", 'HS256', {"typ": "JWT", "alg": 'HS256'}
   render json: {jwt:token}
end

My Javascript code (sample)

webWidget: {
  authenticate: {
    chat: {
      jwtFn: function(callback) {
        fetch('/zendesk_jwt_endpoint').then(function(res) {
          res.text().then(function(jwt) {
            callback(jwt);
          });
        });
      }
    }
  }
}