Hashing function for session
I see that no one has put up an answer for quite a while, so I will put it up here. This has been correctly answered by RaisinBrainCrunch above.

Drupal 8 hashes its session ID before it puts it in the database. The hash function used is hashBase64($sid) under Drupal\Component\Utility\Crypt.

Therefore, I am guessing you will have to write the same function in Node. This is the full function:

public static function hashBase64($data) {
  $hash = base64_encode(hash('sha256', $data, TRUE));
  // Modify the hash so it's safe to use in URLs.
  return str_replace(['+', '/', '='], ['-', '_', ''], $hash);
}