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My question is rather practical and specific. I am trying to setup an efficient CKKS scheme in Palisade. To this end, the automatic choice for secure parameters has to be turned off and I rely on the LWE estimator which estimates the security of an LWE instance. Now, the estimator provides a security estimation for an LWE instance $\mathrm{LWE}(n,\alpha,q)$, where $n$ is the dimension/ring dimension, $\alpha$ depends on error distribution, and $q$ is the ciphertext modulus. However, in Palisade there seems to be no possibility to setup the error in CKKS (this is easily possible in BGV). This way, $\alpha$ remains unknown and the security can not be estimated.

Therefore, how is the error chosen in Palisade for CKKS and how can this value be accessed?

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  • $\begingroup$ Even if you cannot change the value of $\alpha$, I don't see how it can remain unknown, since you can always check the code and see which value is used... $\endgroup$ Nov 20, 2020 at 11:44
  • $\begingroup$ Yep. That's why the question is how I can access the value. Actually, I have no idea how to do that. I admit its a pretty specific question. $\endgroup$
    – opag
    Nov 20, 2020 at 12:14

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In PALISADE, CKKS is configured to use $\alpha = 8/q$ (using the definition of $\alpha$ in the LWE estimator). This corresponds to the standard deviation of $8/\sqrt{2\pi} \approx 3.19$. This value was chosen based on the HE security standard at https://homomorphicencryption.org/standard/

You can manually change this value from 3.19 to something else by modifying $\textbf{genCryptoContextCKKS}$ in https://gitlab.com/palisade/palisade-release/-/blob/master/src/pke/lib/cryptocontextfactory.cpp

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  • $\begingroup$ Perfect! Thanks alot. $\endgroup$
    – opag
    Jan 5, 2021 at 15:22

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