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Essentially any IND-CPA-secure lattice-based cryptosystem offers additive homomorphism, up to a predetermined number of operations. I don't know of any IND-CCA1-secure post-quantum candidate that offers any homomorphic property, except Loftus-May-Smart-Vercauteren SAC'11, which is based on a nonstandard "knowledge of error" lattice assumption.


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The idea behind these models is to model an adversaries capabilities. To get reliable security the worst case for a capability is modelled. Let's start with chosen plaintext attacks (CPA): In this game the adversary is given access to an encryption oracle. This models the case where an attacker knows (parts of) the message. For example, the British knew ...


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"What prevents an attacker from just sending the received ciphertext to the recipient who will think that this is the legitimate message?" Nothing. $\:$ (In that case, the recipient will be correct.) Why "in the definition" is the attacker "only allowed to send another" message? If he knows before seeing the ciphertext that it will be an encryption of ...


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Let's try to simplify and abstract your protocol a bit. Instead of your server and client, we just have two parties, let's call them Sally and Charlie. Charlie has a key pair $K = (K_i, K_u)$ for a suitable asymmetric cryptosystem $\mathcal E$. We assume that this cryptosystem is partially homomorphic, such that $\mathcal E_K(a) \otimes \mathcal E_K(b) = ...


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There should be plenty of them. Off the top of my head, I'm thinking of the provable secure version of NTRU by Stehlé and Steinfeld [1], which is IND-CPA secure. In this scheme, ciphertexts are of the form: \begin{equation} c = pk \cdot s + p\cdot e + \operatorname{encode}(m) \end{equation} where $s$ and $e$ are random polynomials, $p$ is a small prime, ...



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