Question

What is the weakness about using the same key stream multiple times for encryption in a stream cipher? How can it be solved in practice?

Answer 1

ANSWER:-

Encrypting again can't make the cipher weaker , as long as the keys you decided to encrypt a second time are independent of the keys you use the first time . The logic behind this is straight-forward; the attacker is expected to have a deep understanding of the cipher aside from the keys. If he has some ciphertext that he wants to attack, there's nothing preventing him from picking his own "second time key", and then encrypting the ciphertext to form a doubly-encrypted ciphertext. If double-encrypting was inherently weaker, at that point he could attack that (and thus single-encryption would be equally weak). Then again, if the two keys you use during the first and the second use are the same, this logic doesn't apply. Also, actually, there are ciphers where this encrypting twice with the same key is a Bad Idea. For example, some ciphers will, if you run the ciphertext through a second time with the same key, will generate a 'doubly-encrypted ciphertext' that is exactly the same as the plaintext; this is obviously weaker than the 'singly-encrypted ciphertext'.

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