Sending Email with Dark

Posted 2021-05-14

In my last post on Dark, I created a self-destructing message app that lets you share a secret, like a password, with someone by sharing a link. When they view the message it will be deleted on the server. A nice feature would be to notify the sender via email when their message was viewed.

To start, we will modify the React client to pass the email address to the backend:

const response = await fetch(BASE_API + '/upload', {
  method: 'POST',
  headers: {'Content-Type': 'application/json'},
  body: JSON.stringify({text: cipherText, email}),
});

Then we can submit the form, now including an email, to create a trace in Dark. Recall from our last post that we have a Datastore named Messages, that has two fields, text and createdAt. In order to add a new field, we will need to clear the Datastore:

DB::deleteAllv1 Messages

Then we can add a new field email of type String. Next, as we process the message upload, we will include the email in the call to DB::set:

let text = request.body.text
if String::lengthv1 text > 1024
then
  Http::badRequest "Text too long: text is limited to 256 ch
                   aracters"
else
  let key = DB::generateKey
  let _ = DB::setv1
            {
              text : text
              createdAt : Date::now
              email : request.body.email
            }
            key
            Messages
  {
    id : key
  }

Finally we get to the part where we actually send an email. I used (sendinblue)[https://www.sendinblue.com/] because they have a generous free tier and good docs with an easy to use API. Once you've signed up, create a new Secret Key in Dark by clicking the + next to "Secret Keys". Call it SENDINBLUE_API and paste in your API key.

Then we'll create a function in Dark to keep the /download handler concise. Click the + next to "Functions" and call it emailMessageViewed. We will setup this function with one paramter email of type String.

In the body we will use sendinblue's REST API to send a confirmation email:

HttpClient::postv5
  "https://api.sendinblue.com/v3/smtp/email"
  {
    sender : {
               name : "Secure Message"
               email : "noreply@ianobermiller.com"
             }
    to : [{
            email : email
          }]
    subject : "Secure Message Viewed"
    htmlContent : "<html><head></head><body><p>Your secure
                  message has been viewed and deleted.</p>
                  <p><a href="https://ianobermiller.com/se
                  cure">Create a new message</a></p></body
                  ></html>"
  }
  {}
  {
    api-key : SENDINBLUE_API
  }

This is quite straightforward since Dark provides good helpers for making HTTP calls. In this case we use the sendinblue API endpoint https://api.sendinblue.com/v3/smtp/email, pass in a JSON blob with the sender, to, subject, and htmlContent, and finally set the api-key header to our Secret Key named SENDINBLUE_API.

Finally, let's call this new function from our `/download' handlers:

let entry = DB::getv2 request.body.id Messages
let _ = DB::deletev1 request.body.id Messages
let _ = if entry.email
        then
          emailMessageViewed entry.email
        else
          Nothing
{
  text : entry.text
}

Since the email is optional, we want to verify it is non-empty before trying to send a confirmation. And that's all there is to it!

Ian Obermiller

Part time hacker, full time dad.

Sending Email with Dark