Complex sentence: In 2003, he independently rediscovered a serious vulnerability in "master key" security in physical locks that was an open secret among locksmiths; his decision to disclose it publicly provoked some controversy.
Split into simpler sentences: In 2003, he independently rediscovered, and published, a serious vulnerability in "master key" security in physical locks. Although apparantly an open secret among some professional locksmiths; his decision to disclose it publicly provoked some controversy.