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The answer may depend on your exact definitions of "homomorphic" and "malleable", but I'll give it a shot. Basically, homomorphic encryption denotes that, given encryptions $E_k(x)$ and $E_k(y)$ of some values $x$ and $y$, it is possible to obtain an encryption of $x\ast y$ under $k$ from $E_k(x)$ and $E_k(y)$, where $\ast$ is some binary operation, without ...

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Vigenere Cryptosystem is as follow: You chose a key $(K_0,...,K_{m-1})$ consisting of elements in $Z_{26}$. Then a ciphertext for the message $(M_0,...,M_{n-1})$ is $$(M_i+K_{i\mod m}\mod 26)_{i \in [0..n-1]}$$ It is easy to see that you can generate a ciphertext for the message $(M_0+1,...,M_{n-1}+1)$ by adding 1 to each letter. It is therefore by ...

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I believe it would match the relaxed RCCA security, but it looks like it wouldn't be of much use because reencryption would not be secure. You could generate reencryptions of any ciphertext, but they would not be indistinguishable from each other, i.e. given $c_1$ and $c_2$ you can determine easily whether $c_2$ is a reencryption of $c_1$.

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Depending on how malleability is defined, the question actually has some merit. Given to the Wikipedia definition of malleability, a cipher is malleable if there exists at least one function $g$ over the set of possible cipher texts, and one function $f$ over the set of possible plain texts, such that given any cipher text $c_0$, the cipher text $c_1 = ... 1 What you are referring to is the same weakness in regard to malleability that is also applicable to (non-hashed schoolbook) RSA. In Elgamal an attacker can (in practice) not decrypt the transferred and encrypted message, but he can modify (factor) it and is able to determine the effect of his modification. Let$y\$ be the original encrypted message of the ...

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