Bootstrapping error

[SetarehGhorshi]

Hello,

I am trying to use the comparison evaluator for operations such as Max, Sign etc. But I get the following error from the bootstrapper:

cannot BootstrapMany: cannot Evaluate: ct.Level() < Mod1Parameters.LevelQ

I tried increasing the Q level in the ckks params but then the Mod1Parameters.LevelQ was also increased which caused the error to happens again. I am fairly new to this so don't have comprehensive knowledge of the math.

Below is the part of my code where the issue happens:

	params, err := ckks.NewParametersFromLiteral(ckks.ParametersLiteral{
		LogN:            14,
		LogQ:            []int{45, 45, 45, 45, 45, 45, 45},
		LogP:            []int{61, 61, 61, 61, 61},
		LogDefaultScale: 45,
		Xs:              ring.Ternary{H: 192},
	})
	if err != nil {
		panic(err)
	}

	btpParametersLit := bootstrapping.ParametersLiteral{
		LogN: utils.Pointy(14),
		LogP: []int{61, 61, 61, 61, 61},
		Xs:   params.Xs(),
	}
	btpParams, err := bootstrapping.NewParametersFromLiteral(params, btpParametersLit)
	if err != nil {
		panic(err)
	}

// generating the keys ...

        encoder := ckks.NewEncoder(params)
	encryptor := ckks.NewEncryptor(params, pk)


	input := 0.1
	pt := ckks.NewPlaintext(params, params.MaxLevel())


	encoder.Encode([]complex128{complex(input, 0)}, pt)

	ct, _ := encryptor.EncryptNew(pt)

	btk := bootstrapping.EvaluationKeys{
		MemEvaluationKeySet: evkSet,
	}
	
	btEval, err := bootstrapping.NewEvaluator(btpParams, &btk)

	btEval.BootstrappingParameters = params

	evaluator := ckks.NewEvaluator(params, evkSet)
	mn := minimax.NewEvaluator(params, evaluator, btEval)
	polys := minimax.NewPolynomial(comparison.DefaultCompositePolynomialForSign)
	comp := comparison.NewEvaluator(params, mn, polys)

       resCipher,err := comp.Sign(ct)
	if err != nil {
		panic(err)
	}

Thanks for your help.

[Pro7ech]

I see that you are creating a custom instance of bootstrapping.EvaluationKeys with evkSet . You should not have to do that The bootstrapping keys should be generated using the bootstrapping package.

Try again by generating the keys using the bootstrapping package.

See https://github.com/tuneinsight/lattigo/blob/main/circuits/ckks/bootstrapping/keys.go

[SetarehGhorshi]

Thanks for your response.

In my program, the ideal sk is not supposed to be known at this point. Is there a way to generate the bootstrapping keys without using the sk? I am not sure what the skN1 is. (I am using the multiparty setup.)

[Pro7ech]

The API expects the ideal secret key to generate the bootstrapping keys. Non interactive bootstrapping is not supported in multi party. But you can use the interactive bootstrapping (refresh protocol)

[SetarehGhorshi]

Thanks. So if I understand correctly, in the multiparty setup, the interactive bootstrapping protocol can be used for refreshing the ciphertext but operations such as Max(), Sign() etc that require the bootstrapper are not available. Is that right?

[Pro7ech]

The minimax polynomial evaluator takes an interface as bootstrapper, so all you need to do is implement this interface for the refresh protocol