lattice/circuits/ckks/bootstrapping/sk_bootstrapper.go

package bootstrapping

import (
	"github.com/luxfi/lattice/v5/core/rlwe"
	"github.com/luxfi/lattice/v5/he/hefloat"
	"github.com/luxfi/lattice/v5/utils/bignum"
)

// SecretKeyBootstrapper is an implementation of the rlwe.Bootstrapping interface that
// uses the secret-key to decrypt and re-encrypt the bootstrapped ciphertext.
type SecretKeyBootstrapper struct {
	hefloat.Parameters
	*hefloat.Encoder
	*rlwe.Decryptor
	*rlwe.Encryptor
	sk       *rlwe.SecretKey
	Values   []*bignum.Complex
	Counter  int // records the number of bootstrapping
	MinLevel int
}

func NewSecretKeyBootstrapper(params hefloat.Parameters, sk *rlwe.SecretKey) *SecretKeyBootstrapper {
	return &SecretKeyBootstrapper{
		Parameters: params,
		Encoder:    hefloat.NewEncoder(params),
		Decryptor:  rlwe.NewDecryptor(params, sk),
		Encryptor:  rlwe.NewEncryptor(params, sk),
		sk:         sk,
		Values:     make([]*bignum.Complex, params.N())}
}

func (d *SecretKeyBootstrapper) Bootstrap(ct *rlwe.Ciphertext) (*rlwe.Ciphertext, error) {
	values := d.Values[:1<<ct.LogDimensions.Cols]
	if err := d.Decode(d.DecryptNew(ct), values); err != nil {
		return nil, err
	}
	pt := hefloat.NewPlaintext(d.Parameters, d.MaxLevel())
	pt.MetaData = ct.MetaData
	pt.Scale = d.Parameters.DefaultScale()
	if err := d.Encode(values, pt); err != nil {
		return nil, err
	}
	ct.Resize(1, d.MaxLevel())
	d.Counter++
	return ct, d.Encrypt(pt, ct)
}

func (d SecretKeyBootstrapper) BootstrapMany(cts []rlwe.Ciphertext) ([]rlwe.Ciphertext, error) {
	for i := range cts {
		ct, err := d.Bootstrap(&cts[i])
		if err != nil {
			return nil, err
		}
		cts[i] = *ct
	}
	return cts, nil
}

func (d SecretKeyBootstrapper) Depth() int {
	return 0
}

func (d SecretKeyBootstrapper) MinimumInputLevel() int {
	return d.MinLevel
}

func (d SecretKeyBootstrapper) OutputLevel() int {
	return d.MaxLevel()
}