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Assume Alice encrypts messages $m_1$,...,$m_n$ using secret keys $k_1$,...,$k_n$ on a homomorphic encryption scheme (BHHO), so she would get $c_1=Enc_{k_1}(m_1)$,...,$c_n=Enc_{k_n}(m_n)$. Then Alice sends a constant a , some random numbers $a_1$,...,$a_n$ and these ciphertexts to Bob. Can Bob do the following homomorphic operations? (The homomorphic encryption scheme requires that the ciphertext is multiplied by a random number) $$d_1=Enc_{k_1}(m_1)*{a_1}*a=Enc_{k_1}(m_1*{a_1}*a)$$ ... $$d_n=Enc_{k_n}(m_n)*{a_n}*a=Enc_{k_n}(m_n*{a_n}*a)$$

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  • $\begingroup$ What kind of homomorphic cryptosystem are we talking? Something like Paillier? $\endgroup$ – mikeazo Jan 11 '16 at 4:02
  • $\begingroup$ Yes. It likes Paillier. $\endgroup$ – sam zhao Jan 11 '16 at 5:48
  • $\begingroup$ BHHO (Crypto 2008) $\endgroup$ – sam zhao Jan 11 '16 at 6:30
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Yes. Additive homomorphic encryption can also be used to obtain multiplication by a scalar. The way that this is done is by repeated addition, in exactly the same way as you do efficient exponentiation.

Recursively, define:

  • If $a$ equals 1 then $mult(a,Enc_k(m)) = Enc_k(m)$

  • If $a$ is even then $mult(a,Enc_k(m)) = mult\left(\frac{a}{2},Enc_k(m)\right)+mult\left(\frac{a}{2},Enc_k(m)\right)$

  • If $a$ is odd then $mult(a,Enc_k(m)) = mult\left(\frac{a-1}{2},Enc_k(m)\right)+mult\left(\frac{a-1}{2},Enc_k(m)\right)+Enc_k(m)$

Thus, you only need addition.

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  • $\begingroup$ The ciphertext is multiplied by a random matrix(not a random number) on BHHO homomorphic encryption scheme(i.e. a additive homomorphic encryption).If we have a lot of ciphertexts, which need to be done homomorphic operations, can all the ciphertexts be multiplied by the same random matrix? $\endgroup$ – sam zhao Jan 11 '16 at 7:50
  • $\begingroup$ It's the same thing. To multiply by a matrix, you multiply by scalars and then add. $\endgroup$ – Yehuda Lindell Jan 11 '16 at 7:58

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