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4

Another, more indirect take on this: because of the semantic security requirement, we evaluate ciphers by their ability to resist an adaptive chosen-plaintext attack—where the attack not only sees some plaintext/ciphertext pairs, but also: Gets to choose which plaintexts they wish to see ciphertexts for; Gets to use the knowledge they gain from earlier ...


16

If the user changes the key for every message sent, then what use is a known-plaintext attack? Stop right there. This is not what we are trying to prove when conducting a known-plaintext attack. A known-plaintext attack is one where we are given a bunch of ciphertexts, all stemming from encryption using a fixed key. We are then given one plaintext/...


20

if a cipher has a known-plaintext attack, then it is considered completely broken. Yes, pretty much... [Paraphrased] But can't we come up with a case where this isn't true, such as a One Time Pad? Yes, we can come up with cases like that; however the requirements of such a case (key as long as the plaintext, no key reuse) make such a cipher ...


1

Here's an analogue of your question: What is the use of cars which can seat six people when there are only four people in my family? If a key is not used more than once, then you do not need security against chosen plaintext attacks. But other (most) people do need it.


1

What you are describing is called a transposition cipher. First of all, it would be easy to detect which method you are using since the frequency distribution of your cipher text will be the same as the language of the plain text. I think the biggest problem with using only transposition as opposed to a combination of both substitution and transposition ...



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