const callbackSign = createHash('md5') .update(`${VERSTKA_SECRET}${VERSTKA_API_KEY}${materialId}${userId}${callbackUrl}`) .digest('hex');

def add(a, b)
    return a + b
  end  

  callbackSign = OpenSSL::HMAC.hexdigest(OpenSSL::Digest.new('md5'), VERSTKA_SECRET, "#{VERSTKA_API_KEY}#{materialId}#{userId}#{callbackUrl}")