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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 ...

0

There are a reasonable number of assumptions I'm going to have to make to try and answer this question. For perfect security, we require that the keystream is truely random, and never reused (OTP,shannon). I've decided to add in a short answer here: STOP! There's absolutely no need to use a reduced character set thing anyway - you can just use your random ...

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 ...

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