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5

To answer your question: no this is not homomorphic encryption because one of the plaintexts is used unencrypted. There may be times when it is a useful property, but the only uses I know of it are to demonstrate the malleability of xor ciphers. To be a homomorphic encryption function, it should be possible to calculate the encryption of some function of ...

4

We typically refer to a homomorphic cipher if we can take two ciphertexts and combine them in a way that has a predictible result on the plaintexts. In your example you have taken one ciphertext and one plaintext. Using a stream cipher correctly you should never have 2 ciphertexts encrypted with the same portion of a keystream. So, combining two ciphertexts ...

3

This is called an Even-Mansour cipher. Actually, for the differential cryptanalysis it does not matter what sort of difference you use, you only need that it propagates deterministically through linear transformations (whatever linearity means). In this case you use a difference modulo $2^{32}$: $$A \boxminus B \equiv (A-B)\pmod{2^{32}}.$$ You compute ...

2

A better way to solve your problem is: on the server, encrypt the document under a document encryption key (a unique key that's specific to that document). The document remains encrypted in storage encrypted under this document key. When a client requests the document, send the client a copy of the encrypted document, as well as an encryption of the ...

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