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1

Shift cipher or ceasar cipher attains perfect secrecy only in the special case with the assumption that $26$ keys are used in equal probability in the shift cipher, and to encrypt each symbol we use a different key which is choosen equiprobably (i.e. perfectly random) from the key space. It is easy to check all keys given a plaintext when the key is fixed ...


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Does adding more characters to the (Enigma) rotors improve crypto strength? Not really… since the weaknesses of Enigma go well beyond the count of chars per wheel! Of course you could – theoretically – think of using very big (read: huge) wheels with an insane char range. But, when looking at the “time” and “resources” you would need to invest each time ...


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You ask: As a more general question, can other older ciphers be "re-invented" for computers and be viable in today's world? Yes, some can. Consider a Vigenère cipher where the key is as long as the plaintext, is generated using a TRNG and is only used once. That is a One Time Pad, and is very secure, though its viability will be as restricted as ...


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There is imprecision in what is stated in your notes. The Kasisky test only works if the corresponding letters in the two segments are separated by a distance that is a multiple of the key length (in other words they are encrypted by the same letter in the key). For example (source Wikipedia): [abcdea]bcdeabcdeabcde[abcdea]bcdeabc [crypto] is short for [...


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One possibility is that the cipher might be a combination of a transposition cipher and a (probably monoalphabetic) substitution cipher. Since the transposition step will not affect letter frequencies, you cannot tell this combination from a simple substitution cipher based on simple frequency analysis alone. One way to check whether this might be the case ...



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