If the running key is truly random, never reused, and kept secret, the result is a one-time pad, a method that provides perfect secrecy (reveals no information about the plaintext). However, if (as usual) the running key is a block of text in a natural language, security actually becomes fairly poor, since that text will have non-random characteristics which can be used to aid cryptanalysis. As a result, the entropy per character of both plaintext and running key is low, and the combining operation is easily inverted.