Question: In the ElGamal cryptosystem, a plaintext message m is encrypted as E ( m ) = ( c 1 , c 2 ) where c

In the ElGamal cryptosystem, a plaintext message m is encrypted as E(m)=(c1, c2) where c1= gy and c2= m hy, with g being a generator of a group G of order q, y a random integer in {1,..., q1} new for each encryption, and hx the recipients public key. Show that ElGamal encryption is multiplicatively homomorphic, that is, given two messages m1 and m2, it holds that E(m1m2)= E(m1)E(m2). Show also that, in general, it is not additively homomorphic, that is, E(m1+m2)6= E(m1)+E(m2).

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