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1

For example, let say that you have a message: "100 dollars should be moved." and you encrypt it with OTP. Then everybody can just take the first character "1" and change it to a 9 by XOR the 1 from the cipher-text and then XOR a "9" with the key you got. What you are describing is what happens if an attacker introduces changes in the ciphertext by a ...

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Perfect secrecy is a security concept mathematically proved to be true for the one-time pad by Shannon in 1949. One-time pad is offering perfect secrecy if used with these three properties : The key space (number of possible keys) is greater than the number of possible plaintexts The key is truly random : each key of the key space has the same probability ...

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Is CBC mode in OTP more secure? No. If your one time pad satisfies the required properties (it's truly random, the attacker has no information about it, and it's only used once), then OTP already has perfect secrecy; playing around with how it works can't make things better. If your one time pad doesn't satisfy the required properties, then all bets ...

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You can generate a random string $s_1$ as long as the plaintext. Then XOR this value with the plaintext generating $s_2$. Now encrypt both parts using $\mathrm{Enc}_1$ and $\mathrm{Enc}_2$. You need to decrypt both to XOR the two parts together again. This is similar to secret sharing where you need two parts of a key to decrypt. If $\mathrm{Gen}_1$ and ...

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Faliure of indistinguishablity of encryptions under a eavesdropper does imply faliure of indistinguishablity of encryptions under a chosen-plaintext attack. But the converse is not necessarily true (ex. OTP) The aim of CPA-secure is not to decrypt previously unobserved ciphertext but to pass the distinguishability test after a set of (plaintext, ciphertext) ...

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Yes, if (and this is important) the keys for $E$ and $S$ are selected independently. Consider that we had two encryption methods $E$, $S$ for which their composition $E(S(x))$ is not CPA secure; that is, we have some distinguisher $D$ that had some advantage in distinguishing that from a random function. Then, we can build a distinguisher for $E$ (by ...

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No. This isn't secure by itself against chosen-plaintext attacks. This mode is known as plaintext-feedback mode (PFB) and referenced for example in here. The next point is this mode hasn't received much attention in the cryptographic literature, whereas other modes (CFB, OFB, CBC, CTR) have. Two notes: Don't roll your own crypto. Never use such modes if ...

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Note: In this answer, I stick to a definition of the One Time Pad where the random pad is used only One Time; at least, I've the name of it as support! Otherwise, it is well known that the OTP encryption scheme consisting of XOR with a repeated key is insecure by even the weakest standard (unknown plaintext with redundancy). INDistinguishability under ...

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Ciphertext indistinguishability under CPA is equivalent to semantic security. Semantic security is the computational complexity analogue to Shannon's concept of perfect secrecy. OTP is perfectly secure, therefor is CPA secure. Under CPA, the adversary will be presented with a ciphertext corresponding to a plaintext queried (the adversary chose the ...

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