code: function deriveSecret(parent_secret, child_name, hash_algorithm) {
    if (!hash_algorithm) {
      hash_algorithm = 'sha512';
    }
  
    return crypto.createHmac(hash_algorithm, parent_secret).update(child_name).digest('hex');
  }
docs: Derive a "child" secret from a "parent" one using HMAC.
  
  @param {String} parent_secret
  @param {String} child_name
  @param {String} ['sha512'] hash_algorithm
  
  @returns {String} Note that the number of bits depends on the hash algorithm used