Bug/Regression [<package>]: Limited number of levels supported by BFV/BGV

[carblu]

What version of Lattigo are you using?

v6.0.0

Does this issue persist with the latest release?

Yes

What were you trying to do?

In bfv/bgv, I was trying to use as many levels as possible. It seems that loqQ can be formed by at most 32 sizes of primes. Indeed, the following parameters do not work.

bgv.ParametersLiteral{
LogN: 16,
LogQ: []int{56, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30},
LogP: []int{60, 60, 60, 60, 60},
PlaintextModulus: 0xc0001,
}

See discussion #516

What were you expecting to happen?

What actually happened?

I can encrypt and decrypt, but when decoding the plaintext obtained as a result of the decryption, I got the runtime error "index out of range [32] with length 32" raised by ring.reconstructRNS.

Reproducibility

package main

import (
"fmt"
"math/rand"
"slices"

    "github.com/tuneinsight/lattigo/v6/core/rlwe"
    "github.com/tuneinsight/lattigo/v6/schemes/bgv"

)

func main() {
var err error
var params bgv.Parameters

    if params, err = bgv.NewParametersFromLiteral(
        bgv.ParametersLiteral{
            LogN:            16,
            LogQ:            []int{56, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30},
            LogP:            []int{60, 60, 60, 60, 60},
            PlaintextModulus: 0xc0001,       // It must be a prime number congruent to 1 modulo 2 * poly_modulus_degree
        }); err != nil {
            panic(err)
    }

    Slots := params.MaxSlots()

    fmt.Printf("(LogN, L): (%d, %d)\n",  params.LogN(), params.MaxLevel())
    fmt.Printf("Maximum number of slots: %d\n",  Slots)

    // Key Generator
    kgen := rlwe.NewKeyGenerator(params)

    // Generate a secret key (sk) and a corresponding public key (pk)
    sk := kgen.GenSecretKeyNew()
    pk := kgen.GenPublicKeyNew(sk)

    // Generate a relinearizaton key from the secret key (sk)
    rlk := kgen.GenRelinearizationKeyNew(sk)

    // Generate an evaluation key from the relinearizaton key (rlk)
    evk := rlwe.NewMemEvaluationKeySet(rlk)

    fmt.Printf("Secret Key Size:          %d\n", sk.BinarySize())
    fmt.Printf("Public Key Size:          %d\n", pk.BinarySize())
    fmt.Printf("Relinearization Key Size: %d\n", rlk.BinarySize())
    fmt.Printf("Evaluation Key Size:      %d\n", evk.BinarySize())

    // Encoder
    encoder := bgv.NewEncoder(params)

    // Encryptor-Decryptor
    encryptor := rlwe.NewEncryptor(params, sk)
    decryptor := rlwe.NewDecryptor(params, sk)

    // Vector of plaintext values
    values := make([]uint64, Slots)

    // Populates the vector of plaintext values
    r := rand.New(rand.NewSource(0))
    T := params.PlaintextModulus()
    for i := range values {
            values[i] = r.Uint64() % T
    }

    // Allocates a vector for the reference values
    want := make([]uint64, Slots)
    copy(want, values)

    // Allocates a plaintext at the max level.
    plaintext := bgv.NewPlaintext(params, params.MaxLevel())
    fmt.Printf("Plaintext Size: %d\n", plaintext.BinarySize())

    // Encodes the vector of plaintext values
    if err = encoder.Encode(values, plaintext); err != nil {
            panic(err)
    }

    // Encrypts the vector of plaintext values
    var ciphertext *rlwe.Ciphertext
    if ciphertext, err = encryptor.EncryptNew(plaintext); err != nil {
            panic(err)
    }
    fmt.Printf("Ciphertext Size:         %d\n", ciphertext.BinarySize())

    // Decrypts the vector of plaintext values
    plaintext = decryptor.DecryptNew(ciphertext)

    // Decodes the plaintext
    have := make([]uint64, Slots)
    if err = encoder.Decode(plaintext, have); err != nil {
            panic(err)
    }

    // Check correct decryption
    fmt.Printf("\tChecking decryption...")
    if !slices.Equal(want, have) {
            fmt.Printf("\t Wrong result\n")
    } else {
            fmt.Printf("\t No errors\n" )
    }

}