RC4 is known for being simple and quick , but attacks are likely to happen when the start of the output keystream is not removed , or one keystream is used twice ; some ways of using RC4 can turn into very insecure cryptosystems such as WEP . It is especially vulnerable when the beginning of the output keystream is not discarded , or nonrandom or related keys are used ; some ways of using RC4 can lead to very insecure cryptosystems such as WEP .