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I am interested in ElGamal due to the fact that you can achieve some degree of homomorphic properties. I became interested in applying ElGamal to elliptic curves, and found this other question with an answer that shows 2 different ways to apply ElGamal to elliptic curves, but from what I can tell neither approach has the additive homomorphic property.

So are there any other ways to apply ECEG that give us the additive homomorphic property?

The only thing I was able to come up with is using a message-point mapping function that encodes the message with scalar point multiplication, ie the cipher text $(kP, mkY)$

It seems to work because with $(\sum k_iP, \sum m_i k_i Y)$ we can solve $\sum m_i = \frac{\sum m_i k_i Y}{x \sum k_i P}$, but this is solving the ECDLP so is very difficult. Are there any other options?

EDIT: I'm looking for the additive homomorphic property over integers, not points.

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tolstikh
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1 Answers1

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There is a way to use ECEG that gives us an additive homomorphic property, however the addition in this case is Elliptic Curve point addition, not addition over the integers.

One way to use ECEG to encrypt a point $M$ is to have the encrypted value be the pair of points $(kP, M + kY)$

That way, if you have the encryption of the points $M, M'$ as $(kP, M + kY)$ and $(k'P, M' + k'Y)$, you can add each element in the pair to form $( kP + k'P, M + kY + M' + k'Y) = ((k + k')P, M + M' + (k + k')Y)$, which is a valid encryption of the point $M + M'$

poncho
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