code: async function _hkdfSalt(rawSalt, rawEmail) {
    return new Promise(resolve => {
      const hkdf = new HKDF('sha256', rawSalt, rawEmail);
      hkdf.derive('', DEFAULT_BYTE_LENGTH, buffer => resolve(buffer.toString('hex')));
    });
  }
docs: ~~~~~~~~~~~~~~~~ // Helper Functions // ~~~~~~~~~~~~~~~~ // 
  Combine email and raw salt into usable salt
  
  @param rawSalt
  @param rawEmail
  @returns {Promise}