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This question may seem related to this one, but it is not the same.

Let's suppose I take 1GiB of data, encrypt the data with a (secure) block cipher and apply the Vigenère key, being the key much smaller than the ciphertext data (let's say, 512KiB), repeating the key hundreds/thousands of times over the plaintext.

In this case, will the adversary be able to recover the key and decrypt all the ciphertext?

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    $\begingroup$ The Vigenere cipher was shown to be insecure, e.g., it cannot be against "Finding Repeated Sequences" attack. $\endgroup$
    – X.H. Yue
    Commented Mar 18 at 6:17
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    $\begingroup$ You use double encryption where the first one is assumed to be enough and the ciphertext is indistinguishable from a random string. The Kasiski's test will fail and there is no way to determine the Vigenere key without the double decryption.... What is the origin of this question? What is the usage of this? $\endgroup$
    – kelalaka
    Commented Mar 18 at 17:24
  • $\begingroup$ @kelalaka Thanks a lot for the answer: "The Kasiski's test will fail and there is no way to determine the Vigenere key without the double decryption..." =) I've studying about Kasiski's test. "What is the origin of this question? What is the usage of this?" Two years from now I will have to make a backup of 20TB of data and I want to encrypt it with an encryption that no one can break, never, I would like OTP, but a 20TB key is non-sense, I would like to use a 10MiB Vigénere key, I'm already writing a CLI app for doing that, so I need to know a way of using a OTP-like scheme without huge keys. $\endgroup$
    – alpominth
    Commented Mar 18 at 17:59
  • $\begingroup$ AES-256 is secure for quantum and post-question adversaries and it has been withstand attack for more than 20 years. If you are worried use double encryption with ChaCha20? $\endgroup$
    – kelalaka
    Commented Mar 18 at 18:04
  • $\begingroup$ I want to encrypt an industrial secret (related to nuclear physics) and all my studies for some years, I want no one to decrypt after my departure of this world. And also, I'm a little paranoid. $\endgroup$
    – alpominth
    Commented Mar 18 at 18:15

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