NEW YORK (Reuters) -New York's attorney general on Thursday sued KuCoin for failing to register with the state before letting investors buy and sell cryptocurrencies on its platform, as part of her effort to rein in what she calls "shadowy" cryptocurrency companies. Attorney General Letitia James said the fourth-largest cryptocurrency platform violated the Martin Act, a powerful state securities law, by transacting in cryptocurrencies, selling the product "KuCoin Earn" to generate income for itself and investors, and wrongfully calling itself an "exchange." In papers filed with a state court in Manhattan, James is seeking a permanent injunction to stop KuCoin from operating in New York until it complies with the law.