Question: issue with RotateColumnsNew function in v5

[Yiping106283]

Hi Tuneinsight team,

Thanks for the great library! I have a question regarding the differences of the Rotate function between v4 and v5.

I was trying out the RotateColumnsNew function in v5 and the result seems incorrect, specifically, I did

galEls := 5
galk := kgen.GenGaloisKeyNew(galEls, Sk)
evk := rlwe.NewMemEvaluationKeySet(rlk, galk)
evaluator := heint.NewEvaluator(params, evk)

and it gave me an all-zero vector as the rotated result (for any input vectors).

However, the Rotate function is correct in v4, if I did the following:

galEls := params.GaloisElementsForMerge()
rtks := kgen.GenRotationKeys(galEls, Sk)
evaluator := bgv.NewEvaluator(params, rlwe.EvaluationKey{Rtks: rtks})

This gave me the correct rotated result.

It would be great to know if there is any steps I'm missing in v5 that causes the incorrect result. I also really appreciate if you can give an example related to rotations for bgv in v5!

Best,
Yiping

[Pro7ech]

Hi @Yiping106283, could you please provide a self contained main.go that reproduces the issue?

Also be sure to check the error that might be returned by the evaluator.

[Yiping106283]

Also be sure to check the error that might be returned by the evaluator.

Thanks for the hint! Just figured this out.
I have another question though about generating Galois keys, so pasting my main.go here:

package main

import (
	"fmt"

	"github.com/tuneinsight/lattigo/v5/core/rlwe"
	"github.com/tuneinsight/lattigo/v5/he/heint"
)

func pir() {
	// Creating encryption parameters from a default params with logN=14, logQP=438 with a plaintext modulus T=65929217
	params, err := heint.NewParametersFromLiteral(heint.ParametersLiteral{
		LogN:             14,
		LogQ:             []int{56, 55, 55, 54, 54, 54},
		LogP:             []int{55, 55},
		PlaintextModulus: 0x3ee0001,
	})
	if err != nil {
		panic(err)
	}

	encoder := heint.NewEncoder(params)
	kgen := rlwe.NewKeyGenerator(params)
	Sk, _ := kgen.GenKeyPairNew()

	decryptor := rlwe.NewDecryptor(params, Sk)
	encryptorSk := rlwe.NewEncryptor(params, Sk)

	// Relinearization Key
	rlk := kgen.GenRelinearizationKeyNew(Sk)

	// function from v4: 
        // galEls := params.GaloisElementsForMerge()
	// galEls := []uint64{5, 25, 625, 30177, 28609, 28545, 7937, 15873, 31745, 30721, 28673, 24577, 16385, 32767}
        // question: passing the argument galEls as a list doesn't seem to work here
	galk := kgen.GenGaloisKeyNew(5, Sk)
	evk := rlwe.NewMemEvaluationKeySet(rlk, galk)
	evaluator := heint.NewEvaluator(params, evk)

	// initialize an array
	lenArray := 1 << params.LogN()
	myArray := make([]uint64, 1<<params.LogN())
	for i := 0; i < lenArray/2; i++ {
		myArray[(i << 1)] = uint64((i << 1))
		myArray[(i<<1)+1] = uint64((i << 1) + 1)
	}

	myPlaintext := heint.NewPlaintext(params, params.MaxLevel())
	if err := encoder.Encode(myArray, myPlaintext); err != nil {
		panic(err)
	}

	myCiphertext, err := encryptorSk.EncryptNew(myPlaintext)
	if err != nil {
		panic(err)
	}

	// square
	myCiphertext, err = evaluator.MulRelinNew(myCiphertext, myCiphertext)
	if err != nil {
		panic(err)
	}

	// question: if wanting to rotate for any value, e.g., 1, not just 1
	myCiphertext, err = evaluator.RotateColumnsNew(myCiphertext, 3)
	if err != nil {
		panic(err)
	}

	evaluator.Rescale(myCiphertext, myCiphertext)

	result := make([]uint64, params.MaxSlots())
	if err := encoder.Decode(decryptor.DecryptNew(myCiphertext), result); err != nil {
		panic(err)
	}

	fmt.Println("my array = ", myArray[0:10])
	fmt.Println("result = ", result[0:10])

}

func main() {

	pir()
}

I marked the two question in comments, more specifically, passing the uint64 array galEls to the kgen.GenGaloisKeyNew function doesn't seem to work. Is there a way to do this in v5?

[Pro7ech]

The method GenGaloisKeysNew takes an array of Galois elements

[Yiping106283]

I see! One last question:

galEls := []uint64{5, 25, 625, 30177, 28609, 28545, 7937, 15873, 31745, 30721, 28673, 24577, 16385, 32767}
galk := kgen.GenGaloisKeysNew(galEls, Sk)
evk := rlwe.NewMemEvaluationKeySet(rlk, galk)
evaluator := heint.NewEvaluator(params, evk)

This doesn't allow the type []*rlwe.GaloisKey as the error message showed:

"cannot use galk (variable of type []*rlwe.GaloisKey) as *rlwe.GaloisKey value in argument to rlwe.NewMemEvaluationKeySet"

Should I change the way to set evk?

Just noticed that I can pass in each of the galks into the function.

Thanks for anwering!