For a project, I am using homomorphic encryption with the Paillier cryptosystem, and I have to compare two values...

  • Can this be done using homomorphic encryption?
  • And I know subtraction can be done using homomorphic encryption, but can anyone simplify the steps?

Any advice will be helpful.


Can [comparison] be done using homomorphic encryption?

Not without interaction with the person with the private key.

Suppose there was a possible way; given $E_k(a)$ and $E_k(b)$, one could determine whether $a < b$. If so, then one could use that to decrypt - given $E_k(a)$, one can encrypt various values of $b$ and then check whether $a < b$ or not - when we find a value $b$ such that $a < b$ and $a \not\lt b-1$, then we know the value of $a$.

Now, another possible meaning of comparison is, given $E_k(a), E_k(b)$ create $E_k(0)$ if $a < b$ and $E_k(1)$ if $a \ge b$. In that case, Pallier along with that comparison operation becomes fully homomorphic; for example, to compute the NAND of $E_k(a)$ and $E_k(b)$, one just compares $E_k(1-a)$ (easily computed with Pallier) to $E_k(b)$ - once we have a homomorphic NAND function, we can construct any circuit.

And I know subtraction can be done using homomorphic encryption, but can anyone simplify the steps?

Given $E_k(b)$, we can compute $E_k(-b)$ simply by computing the modular inverse of $E_k(b)$ - that's because $E_k(b) = g^b r^n$ and so $(E_k(b))^{-1} = g^{-b}(r^{-1})^n$, which is a valid encryption of $-b$.

Then, we can do the normal homomorphic addition to compute $E_k(a - b)$

  • $\begingroup$ Let's say I have a msg m1 and it is in encrypted form, and I have another predefined value say K. I want to compute how close m1 is within the predefined value K. How can it be done? Which keys would I be needing? I am a beginner $\endgroup$
    – dsaharia
    Jul 6 '19 at 13:45
  • $\begingroup$ @dsaharia: you can compute $E( m1 - K )$ and $E( -(m1 - K))$; however that's about the best you can do... $\endgroup$
    – poncho
    Jul 6 '19 at 13:47
  • $\begingroup$ I want to determine the closeness of m1 with regards to another value. How can this be done? $\endgroup$
    – dsaharia
    Jul 7 '19 at 7:30
  • $\begingroup$ Also you can use the ability to compute the difference between two ciphertexts to at least test for equality. Given a random group element $d$, you can compute $E_k(a-b)^d$ and send that off for decryption. Decrypting will reveal whether or not $a==b$ and nothing else since it is blinded by the additional multiplicative factor. Explicitly, the difference between $a$ and $b$ equals zero iff $a==b$. Any other value will be randomly altered by the multiplication while zero is untouched. $\endgroup$
    – Ken Goss
    Oct 2 '19 at 15:00

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