[drlwe]: CKS/PCKS does not properly resizes the level of the output ciphertext to the one of the input ciphertext

[macknight]

Hi,
Would you please have a look at below code, it's almost using example/dbfv/psi/main.go, I only changed a little in evalPhase function and add some print code in main. Youc could see evalPhase function.
When I use power function to calculate x -> x^2, the result is wrong.
But when I use mul&relinearize to calculate x -> x^2, the result is correct.
I don't understand why it's wrong when using power function.

BR

[Pro7ech]

@macknight Can you produce a small and self-contained main.go that reproduces the error?

Regardless, we plan to discontinue the .Power(*) method, because it's functionality is now provided by the PolynomialBasis struct. You can use it like this to get your powers :

pb := ckks.NewPolynomialBasis(ct, ckks.Monomial)
pb.GenPower(power, false, params.DefaultScale(), evaluator)
ctPower := pb.Value[power].CopyNew()

[macknight]

@macknight Can you produce a small and self-contained main.go that reproduces the error?

Regardless, we plan to discontinue the .Power(*) method, because it's functionality is now provided by the PolynomialBasis struct. You can use it like this to get your powers :

pb := ckks.NewPolynomialBasis(ct, ckks.Monomial)
pb.GenPower(power, false, params.DefaultScale(), evaluator)
ctPower := pb.Value[power].CopyNew()

@Pro7ech
OK, I'll try the new method.

main.zip

And I've updated my post with the zip file including the main.go (I put the file here: example/dckks/psi/main.go for run), would you try that for testing? you could toggle in evalPhase function with one of the two options.

BR

[Pro7ech]

@macknight I've found the issue, it doesn't come from the evaluation phase, it comes from the level of the ciphertext that you allocate during the pcks phase. You should allocate a ciphertext of the level of the input ciphertext. However, this mistake should have been caught by the protocol. I'll add checks to ensure that you'd get an appropriate error if the levels do not match.

To fix your code change it to

encOut = ckks.NewCiphertext(params, 1, encRes.Level(), params.DefaultScale())

[macknight]

@Pro7ech
Right!
tested with power function and encOut = ckks.NewCiphertext(params, 1, encRes.Level(), params.DefaultScale())
result looks good.
Thanks