(Reuters) - More than two dozen U.S. companies, including several big banks, have teamed up to establish shared principles that would allow them to better understand their cyber security ratings and to challenge them if necessary, the U.S. Chamber of Commerce said on Tuesday. Large corporations often use the ratings, the cyber equivalent of a FICO credit score, to assess how prepared the companies they work with are to withstand cyber attacks. Insurers also look at the ratings when they make