 That should be true for all keys (there should be no "weak keys"), even if the attacker can "know" or "choose" some "plaintext" or "ciphertext".As with other attacks in cryptography, stream cipher attacks can be "certificational" so they are not necessarily practical ways to break the cipher but indicate that the cipher might have other weaknesses.Securely using a secure synchronous stream cipher requires that one never reuse the same keystream twice.