# Tag Info

5

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 ...

3

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 ...

3

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 ...

2

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) ...

2

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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