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Homomorphic Encryption: a Toy Implementation in Python

Motivation: We made this blog post as self-contained as possible, even though it was initially thought as a follow-up of this tutorial given by OpenMined. The starting point of our Python implementation is this github gist, which follows the Homomorphic Encryption scheme from [FV12]. The motivation behind our implementation was for us to understand in detail the two techniques of [FV12] used for ciphertext multiplication, namely relinearization and modulus-switching. This essential operation of ciphertext multiplication was missing in the previous implementation. We thought we might share this understanding through a blog post as well since it may be of interest to anyone using the [FV12] scheme in TenSeal or Seal libraries.

Disclaimer: Our toy implementation is not meant to be secure or optimized for efficiency. We did it to better understand the inner workings of the [FV12] scheme, so you can use it as a learning tool. Our full implementation can be found here.

Curious about how to work with data you can't see? In the first part of this blog post we are going to broadly explain what Homomorphic Encryption is and some closely related concepts. In the second part we will follow an example of such a scheme, namely the [FV12] scheme, and discuss some of the details of our implementation.

1. What is Homomorphic Encryption?

Homomorphic Encryption (HE) enables a user to perform meaningful computations on sensitive data while ensuring the privacy of the data. This may sound paradoxical to anyone who has ever worked with encrypted data before: if you want to perform useful computations on the encrypted data (e.g. encrypted under classical algorithms like AES), you need to decrypt it first. But once decryption takes place, the privacy of the data is compromised. So how is it possible for HE to overcome this seeming contradiction? ๐Ÿ”ฎ Well, the solution is highly non-trivial, as it took the cryptographic community more than 30 years to come up with a construction. The first solution was proposed by Craig Gentry in 2009 and was of theoretical interest only. Since then, a lot of research has been done (e.g. [BGV11], [FV12], [CKKS16], [FHEW], [TFHE], [GSW13]) to make these ideas more practical. By the end of this post you should have a basic understanding of how such a construction may work.

Besides the traditional encryption (Enc\mathsf{Enc}), decryption (Dec\mathsf{Dec}) and key generation (Keygen\mathsf{Keygen}) algorithms, an HE scheme also uses an evaluation algorithm (Eval\mathsf{Eval}). This is the distinguishing feature that makes computations on encrypted data possible. ๐Ÿ’ฅ Let's consider the following example: Alice holds some personal information xx (e.g. her medical records and her family's medical history). There is also a company that makes very good predictions based on this kind of information, using a refined model, expressed as the functionality FF (e.g. a well chosen machine learning model). On one hand, Alice is very interested in these predictions but is also reluctant to trust the company with her sensitive information. On the other hand, the company can't just give their model to Alice to make the predictions herself. A solution using Homomorphic Encryption is given in the picture below. Some important things to notice are:

  • Alice sends her data encrypted, so the company never learns anything about xx.
  • Computing on the encrypted data CC does not involve Alice's secret key sksk. Only her public key pkpk is used.
  • To obtain Cโ€ฒC' as the encryption of F(x)F(x), the evaluation algorithm uses the description of FF to do computations on CC (which encrypts xx).
  • By using her secret key, sksk, Alice manages to recover the information that interests her, namely F(x)F(x).

alice_overview

A closer look at the Eval algorithm ๐Ÿ”Ž

All the existing HE constructions are homomorphic with respect to two basic operations: some kind of addition and some kind of multiplication (e.g. ++ and ร—\times over the integers or the binary operations XOR\mathsf{XOR} and AND\mathsf{AND}, etc.). What we mean is that the scheme allows the efficient computation of caddc_{add} from the individual ciphertexts c1=Enc(pk,m1)c_1=\mathsf{Enc}(pk,m_1) and c2=Enc(pk,m2)c_2=\mathsf{Enc}(pk,m_2) such that the decryption of caddc_{add} yields m1+m2m_1+m_2.

summation

Analogously, the ciphertext cmulc_{mul} corresponding to multiplication, that decrypts to m1ร—m2m_1\times m_2, is efficiently computable from the individual ciphertexts c1=Enc(pk,m1)c_1=\mathsf{Enc}(pk,m_1) and c2=Enc(pk,m2)c_2=\mathsf{Enc}(pk,m_2), respectively.

multiplication

There are classical examples of encryption schemes that are homomorphic with respect to only *one* operation.

For instance:

  • the RSA encryption: Ence,NRSA(m):=meโ€Šmodโ€ŠN\mathsf{Enc}^{\mathsf{RSA}}_{e,N}(m):=m^e \bmod N is multiplicatively homomorphic as m1eโ‹…m2eโ€Šmodโ€ŠN=(m1โ‹…m2)eโ€Šmodโ€ŠNm_1^e \cdot m_2^e \bmod N = (m_1 \cdot m_2)^{e} \bmod N
  • the El-Gamal encryption: Encg,hEG(m)=(gr,hrโ‹…m)\mathsf{Enc}^{\mathsf{EG}}_{g,h}(m)=(g^r,h^r\cdot m). It is easy to verify that the multiplicative property holds:

(gr1,hr1โ‹…m1)โ‹…(gr2,hr2โ‹…m2)=(gr1+r2,hr1+r2โ‹…(m1โ‹…m2)). (g^{r_1},h^{r_1}\cdot m_1)\cdot (g^{r_2},h^{r_2}\cdot m_2) =(g^{r_1+r_2},h^{r_1+r_2}\cdot (m_1\cdot m_2)).


All the existing HE schemes support only two types of computations on the encrypted data: some forms of addition and multiplication. This means that the Eval\mathsf{Eval} algorithm works only for functionalities FF that can be expressed using additions (++) and multiplications (ร—\times). Another way of saying this is that HE schemes support only arithmetic circuits with addition/multiplication gates. Below we can view as an arithmetic circuit the functionality F(m1,m2,m3,m4)=m1ร—m2ร—m4+m3ร—m4F(m_1,m_2,m_3,m_4)= m_1\times m_2\times m_4 + m_3\times m_4.

arithmetic_circuit

Why focus on homomorphisms with respect to two operations?

In principle, any functionality can be expressed using only two basic operations. For example, any binary circuit can be obtained using NAND gates exclusively. In turn, a NAND operation consists of one addition and one multiplication: NAND(a,b)=aร—b+1โ€Šmodโ€Š2\mathsf{NAND}(a,b)=a\times b + 1\bmod 2, for any bits a,bโˆˆ{0,1}a,b \in \{0,1\}.

๐Ÿ’ก Therefore it is enough to have an HE scheme that supports an unlimited number of additions and multiplications to be able to make any efficient computation we can think of on encrypted data.

Homomorphic Encryption and "noisy" ciphertexts

The most practical HE constructions rely on the hardness of the Ring Learning With Errors (RLWE) problem for their security, as is the case with many lattice-based cryptographic constructions. The inherent "noise" of the RLWE problem is inherited by all the schemes that are based on it. In particular, this "noise" element is present in every HE ciphertext and has a great impact on the parameters of the scheme.

noisy_cyphertext

The noise grows with every addition or multiplication we do on the ciphertexts. This is very relevant as decryption stops working correctly once the noise exceeds a certain threshold.

Because of this phenomenon, the number of multiplications and additions that can be carried out correctly on the ciphertext is limited. ๐Ÿ’ฅ The parameters of such a scheme can be generated such that it can handle a minimum number of operations. But this minimum number must be decided in advance to set the parameters accordingly. We usually call such a scheme Somewhat Homomorphic Encryption (SHE) scheme. When the construction allows an unbounded number of operations, we call such a scheme Fully Homomorphic Encryption (FHE). Even though we are not going to discuss it any further, we have to mention that it's possible to obtain FHE from SHE. In fact, Gentry showed how to transform any SHE (that can homomorphically evaluate its own decryption circuit) to FHE, through a computationally expensive process called bootstrapping. For applications that don't require many homomorphic evaluations SHE is preferred, as we want to avoid the computational overhead of the boostrapping.

2. A SHE scheme example

For the remaining of this blog post we will try to make the concepts that we have already presented more concrete, by discussing a toy implementation of the SHE scheme construction of [FV12]. Our main goal is to understand how relinearization and modulus-switching are used to obtain ciphertext multiplication.

Notations: For an integer qq, by Zq\mathbb{Z}_q we mean the set {0,1,โ€ฆ,qโˆ’1}\{0,1,\ldots,q-1\}. We denote by [a]q[a]_q the remainder of aa modulo qq. For example [18]7=4[18]_7 = 4. When rounding to the nearest integer, we use โŒŠโ‹…โŒ‰\lfloor \cdot \rceil. The basic elements we work with will not be integers, but merely polynomials with integer coefficients. We also work with noisy polynomials, whose coefficients are sampled according to some error distribution ฯ‡\chi. We bound such errors by their largest absolute value of their coefficients, denoted as โˆฅโ‹…โˆฅ\|\cdot\|.

Quick recap on working with polynomials

The HE scheme we are going to describe deals with adding and multiplying polynomials. Here we present a quick example of how to work with polynomials, so that we all have the same starting point. If you already know how to do this, you can skip it.

First thing, let's add and multiply polynomials modulo some polynomial ff. This "modulo ff" thing simply means that we add and multiply the polynomials in the usual way, but we take the remainders of the results when divided by ff. When we do these additions and multiplications โ€Šmodโ€Šf\bmod f, we sometimes say in a fancy way that we are working in the ring Z[x]/(f)\mathbb{Z}[x]/(f) of reduced polynomials.๐Ÿ’

Let's take f=x4+1f = x^4 +1. If we look at p(x)=x5p(x) =x^5, then p(x)=xโ‹…(x4+1)โˆ’xp(x) = x \cdot (x^4 + 1) - x. Therefore, when taking the reminder we get p(x)=โˆ’xโ€Šmodโ€Šfp(x) = -x \bmod f. For faster computations โ€Šmodโ€Šf\bmod f you can use this trick: when making โ€Šmodโ€Šf\bmod f, simply replace x4x^4 by โˆ’1-1, x5x^5 by โˆ’x-x and so on.

Let's consider two polynomials a(x)=x3+x2+7a(x) = x^3 + x^2 + 7 and b(x)=x2+11xb(x) = x^2 + 11x. Then:

a(x)+b(x)โ€Šmodโ€Šf=x3+2x2+11x+7ย modย f.a(x)+b(x) \bmod f = x^3 + 2x^2 + 11x + 7 \text { mod }f.

Here nothing special happened. Let's multiply them:

a(x)โ‹…b(x)โ€Šmodโ€Šf=(x3+x2+7)โ‹…(x2+11x)ย modย f=x5+11x4+x4+11x3+7x2+77xย modย f=โˆ’xโˆ’11โˆ’1+11x3+7x2+77xย modย f=11x3+7x2+76xโˆ’12ย modย f.\begin{aligned} a(x) \cdot b(x) \bmod f &= (x^3 + x^2 + 7)\cdot (x^2 +11x) \text { mod }f \\ &= x^5 + 11x^4 + x^4 + 11x^3 + 7x^2 + 77x \text { mod }f\\ &= -x - 11 - 1 + 11x^3 + 7x^2 + 77x \text { mod }f \\ &= 11x^3 + 7x^2 + 76x - 12 \text { mod }f. \end{aligned}

These operations are implemented here and make use of the cool Numpy library:

import numpy as np
from numpy.polynomial import polynomial as poly

#------Functions for polynomial evaluations mod poly_mod only------
def polymul_wm(x, y, poly_mod):
    """Multiply two polynomials
    Args:
        x, y: two polynomials to be multiplied.
        poly_mod: polynomial modulus.
    Returns:
        A polynomial in Z[X]/(poly_mod).
    """ 
    return poly.polydiv(poly.polymul(x, y), poly_mod)[1] 
def polyadd_wm(x, y, poly_mod):
    """Add two polynomials
    Args:
        x, y: two polynomials to be added.
        poly_mod: polynomial modulus.
    Returns:
        A polynomial in Z[X]/(poly_mod).
    """ 
    return poly.polydiv(poly.polyadd(x, y), poly_mod)[1] 

Now let's go one step further and see how we perform these operations of polynomials not just modulo ff, but also modulo some integer qq. As you might expect, the coefficients of these polynomials are also reduced modulo qq, so they always take integer values from 00 to qโˆ’1.q-1. This time, we say that we are working the ring of reduced polynomials Zq[x]/(f)\mathbb{Z}_q[x]/(f). ๐Ÿ’

Let's take the previous example, f=x4+1f = x^4+1, a(x)=x3+x2+7a(x)=x^3+x^2+7, b(x)=x2+11xb(x)=x^2+11x and consider q=5q =5. We can think of aa and bb as already taken โ€Šmodโ€Šf\bmod f. If we take them further modulo qq, then [a(x)]q=x3+x2+2[a(x)]_q = x^3 + x^2 +2 and [b(x)]q=x2+x[b(x)]_q = x^2+x. Moreover,

[a(x)+b(x)]q=x3+x2+2+x2+x=x3+2x2+x+2[a(x) + b(x)]_q = x^3 + x^2 +2+ x^2 + x = x^3 + 2x^2 + x+2

and

[a(x)โ‹…b(x)]q=(x3+x2+2)โ‹…(x2+x)=x5+x4+x4+x3+2x2+2x=โˆ’xโˆ’1โˆ’1+x3+2x2+2x=x3+2x2+x+3\begin{aligned} [a(x) \cdot b(x)]_q &= (x^3 + x^2+2) \cdot (x^2 + x)\\ &= x^5 + x^4 + x^4 +x^3+2x^2 + 2x\\ &= -x -1 -1 + x^3 +2x^2+2x\\ &= x^3+2x^2+x+3 \end{aligned}

where at the last but one equality we performed modulo f=x4+1f = x^4+1 and at the last one, modulo q=5q=5.

These operations already mentioned are polyadd\texttt{polyadd} and polymul\texttt{polymul} implemented here.

The Fan-Vercauteren ([FV12]) scheme

Next, we recall the basic (Keygen\mathsf{Keygen}, Enc\mathsf{Enc}, Dec\mathsf{Dec}) algorithms of the [FV12] scheme. These (almost identical) algorithms have already been described here, but for the sake of completeness, we present them as well. Then we will explain in detail the core of the Eval\mathsf{Eval} algorithm: the addition and multiplication of the ciphertexts. Spoiler alert: We will primarily focus on the two Relinearization techniques that enable ciphertext multiplication.

Let nn be power of 2. We call a positive integer tt the plaintext modulus and a positive integer qq as the ciphertext modulus. We set ฮ”=โŒŠq/tโŒ‹\Delta = \lfloor q/t \rfloor. The scheme involves adding and multiplying polynomials in Rt=Zt[x]/(xn+1)R_t = \mathbb{Z}_t[x]/(x^n+1), on the plaintext side, and adding and multiplying polynomials in Rq=Zq[x]/(xn+1)R_q = \mathbb{Z}_q[x]/(x^n+1), on the ciphertext side. We also denote by RR the ring Z[x]/(xn+1).\mathbb{Z}[x]/(x^n+1).

Disclaimer: From now on all polynomial operations are assumed to be mod xn+1x^n+1, even if we don't mention it every time.

Here is the high level of the scheme:

he_scheme5

โžก๏ธ Keygen\mathsf{Keygen}: The secret key sksk is a secret binary polynomial ss in RR, i.e. its coefficients are either 0 or 1. The public key pkpk is created as follows: we sample aa uniformly over RqR_q and an error ee according to some error distribution ฯ‡\chi over RR and output pk=([โˆ’(aโ‹…s+e)]q,a)โˆˆRqร—Rqpk = ([-(a\cdot s+e)]_q,a) \in R_q \times R_q.

Notice that hardness of the RLWE problem prevents the computation of the secret ss from the public key.

The way we generate the uniform polynomials and the binary polynomials is implemented as gen_uniform_poly\texttt{gen\_uniform\_poly} and as gen_binary_poly\texttt{gen\_binary\_poly} respectively. The error distribution ฯ‡\chi is usually taken as a discretized variant of the Normal distribution, over Zn\mathbb{Z}^n, and is implemented as gen_normal_poly\texttt{gen\_normal\_poly}. They can be found here.

def keygen(size, modulus, poly_mod, std1):
    """Generate a public and secret keys.
    Args:
        size: size of the polynoms for the public and secret keys.
        modulus: coefficient modulus.
        poly_mod: polynomial modulus.
        std1: standard deviation of the error.
    Returns:
        Public and secret key.
    """
    s = gen_binary_poly(size)
    a = gen_uniform_poly(size, modulus)
    e = gen_normal_poly(size, 0, std1)
    b = polyadd(polymul(-a, s, modulus, poly_mod), -e, modulus, poly_mod)
    return (b, a), s

โžก๏ธ Enc\mathsf{Enc}: To encrypt a plaintext mโˆˆRtm \in R_t, we let pk=(pk0,pk1)pk = (pk_0, pk_1), sample u,e1,e2u, e_1, e_2 according to ฯ‡\chi over RR and output the ciphertext

Enc(pk,m)=([pk0โ‹…u+e1+ฮ”โ‹…m]q,[pk1โ‹…u+e2]q)โˆˆRqร—Rq\mathsf{Enc}(pk,m) = ([pk_0 \cdot u +e_1 + \Delta \cdot m]_q,[pk_1 \cdot u + e_2]_q) \in R_q \times R_q

Due to the RLWE assumption, the ciphertexts "look" uniformly random to a possible attacker, so they don't reveal any information about the plaintext.

In the piece of code below, the message we want to encrypt, mm, is an integer vector of length at most nn, with entries in the set {0,1,โ€ฆ,tโˆ’1}\{0, 1,\ldots, t-1\}. Before we encode it as a polynomial in mโˆˆRtm \in R_t, we pad it with enough zeros to make it a length nn vector.

def encrypt(pk, size, q, t, poly_mod, m, std1): 
    """Encrypt an integer vector pt.
    Args:
        pk: public-key.
        size: size of polynomials.
        q: ciphertext modulus.
        t: plaintext modulus.
        poly_mod: polynomial modulus.
        m: plaintext message, as an integer vector (of length <= size) with entries mod t.
    Returns:
        Tuple representing a ciphertext.
    """
    m = np.array(m + [0] * (size - len(m)), dtype=np.int64) % t
    delta = q // t
    scaled_m = delta * m
    e1 = gen_normal_poly(size, 0, std1)
    e2 = gen_normal_poly(size, 0, std1)
    u = gen_binary_poly(size)
    ct0 = polyadd(
        polyadd(
            polymul(pk[0], u, q, poly_mod),
            e1, q, poly_mod),
        scaled_m, q, poly_mod
    )
    ct1 = polyadd(
        polymul(pk[1], u, q, poly_mod),
        e2, q, poly_mod
    )
    return (ct0, ct1)

โžก๏ธ Dec\mathsf{Dec}: Given a ciphertext ct=Enc(pk,m)=(ct0,ct1)ct = \mathsf{Enc}(pk,m)=(ct_0, ct_1), we decrypt it using the secret key sk=ssk=s as follows:

Dec(sk,ct)=[โŒŠtโ‹…[ct0+ct1โ‹…s]qqโŒ‰]tโˆˆRt\mathsf{Dec}(sk,ct) = \Bigg[ \Bigg\lfloor \frac{t\cdot [ct_0+ct_1\cdot s]_q}{q} \Bigg\rceil \Bigg]_t \in R_t

โ›” Let's stop for a bit and check together how the Dec\mathsf{Dec} algorithm works. The intuition behind it is that pk0+pk1โ‹…skpk_0 + pk_1\cdot sk is "small". This implies that ct0+ct1โ‹…skct_0 + ct_1\cdot sk is "close" to the scaled message ฮ”โ‹…m\Delta \cdot m. To recover the message mm, we get rid of ฮ”\Delta (โ‰ˆq/t)(\approx q/t) and then apply rounding to shave off the "small" noise. Let's check that this intuition actually works.

First, we set the notation ct(s):=ct0+ct1โ‹…sct(s) := ct_0 + ct_1 \cdot s, which we'll frequently use for the rest of the post. If we perform this computation, we will end up getting a noisy scaled variant of the plaintext, namely ฮ”โ‹…m+v\Delta \cdot m+v!

You can click here to see why this happens.

If we go back to see how ctct and pkpk were defined, we get:

[ct(s)]q=[(pk0โ‹…u+e1+ฮ”โ‹…m)+(pk1โ‹…u+e2)โ‹…s]q=[โˆ’(aโ‹…s+e)โ‹…u+e1+ฮ”โ‹…m+aโ‹…uโ‹…s+e2โ‹…s]q=ฮ”โ‹…mโˆ’eโ‹…u+e1+e2โ‹…s=ฮ”โ‹…m+v,\begin{aligned} [ct(s)]_q &= [(pk_0 \cdot u + e_1 + \Delta \cdot m) + (pk_1 \cdot u + e_2) \cdot s]_q\\ &= [-(a\cdot s + e)\cdot u+e_1 + \Delta \cdot m + a \cdot u \cdot s + e_2 \cdot s]_q\\ &= \Delta \cdot m - e \cdot u + e_1 + e_2 \cdot s\\ &= \Delta \cdot m + v, \end{aligned}

which is nothing but the scaled plaintext mm with some "small" noise vv.

Because we always have โˆฅฮ”โ‹…m+vโˆฅ<q\|\Delta\cdot m + v\| < q, reducing it โ€Šmodโ€Šq\bmod q has no effect (e.g. [4]7=4[4]_{7} = 4). As long as the noise โˆฅvโˆฅ<ฮ”/2\|v\|<\Delta/2, we can always recover the correct mm.

decoder_details

For example, in the picture above, any green point will decrypt to 22 when we scale it by t/qt/q (โ‰ˆ1/ฮ”)(\approx 1/\Delta) and round it. Analogously, any dark-brown point will decrypt to 33.

We can implement this evaluation, as scaled_pt\texttt{scaled\_pt} below, by performing polynomial operations in RqR_q. You will see that this equation,

[ct(s)]q=[ct0+ct1โ‹…s]q=ฮ”โ‹…m+v,[ct(s)]_q = [ct_0 + ct_1 \cdot s]_q = \Delta \cdot m + v,

which we will call the decryption equation, becomes really useful in deriving ways of computing on ciphertexts. Here we provide the code for the Dec\mathsf{Dec} algorithm:

def decrypt(sk, q, t, poly_mod, ct):
    """Decrypt a ciphertext.
    Args:
        sk: secret-key.
        size: size of polynomials.
        q: ciphertext modulus.
        t: plaintext modulus.
        poly_mod: polynomial modulus.
        ct: ciphertext.
    Returns:
        Integer vector representing the plaintext.
    """
    scaled_pt = polyadd(
        polymul(ct[1], sk, q, poly_mod),
        ct[0], q, poly_mod
    )
    decrypted_poly = np.round(t * scaled_pt / q) % t
    return np.int64(decrypted_poly)

Homomorphic operations of [FV12]

As explained in the first part, the Eval\mathsf{Eval} algorithm works only for functionalities that can be expressed using addition (+)(+) or multiplication (ร—)(\times).

Let's take two ciphertexts ct=Enc(pk,m)ct = \mathsf{Enc}(pk,m) and ctโ€ฒ=Enc(pk,mโ€ฒ)ct' = \mathsf{Enc}(pk,m'). We want to see how to construct ciphertexts that decrypt both the addition, m+mโ€ฒm+m', and the multiplication, mโ‹…mโ€ฒm\cdot m'. Also, keep in mind that when performing operations on mm and mโ€ฒm', we are actually doing them modulo xn+1x^n+1 and modulo tt, since these are the plaintext operations from Rt=Zt[x]/(xn+1)R_t = \mathbb{Z}_t[x]/(x^n+1).

Let's write the decryption equations of ctct and ctโ€ฒct':

[ct(s)]q=ฮ”โ‹…m+vย andย [ctโ€ฒ(s)]q=ฮ”โ‹…mโ€ฒ+vโ€ฒ.[ct(s)]_q = \Delta \cdot m + v \text{ and } [ct'(s)]_q = \Delta \cdot m' + v'.

โžก๏ธ Addition: If we simply add the decryption equations, we get

[ct(s)]q+[ctโ€ฒ(s)]q=ฮ”โ‹…(m+mโ€ฒ)+v+vโ€ฒ.[ct(s)]_q + [ct'(s)]_q = \Delta \cdot (m+m') + v + v'.

But wait a sec, we need to decrypt to m1+m2m_1+m_2 modulo tt! Notice that m+mโ€ฒ=tโ‹…ฯต+[m+mโ€ฒ]tm + m' = t \cdot \epsilon + [m+m']_t, for some binary polynomial ฯต\epsilon. Using the notation rt(q):=qโˆ’ฮ”โ‹…tr_t(q):= q- \Delta \cdot t (this is just the remainder of qq divided by tt) we get:

[ct(s)+ctโ€ฒ(s)]q=ฮ”โ‹…[m+mโ€ฒ]t+vaddย modย q,\begin{aligned} [ct(s) + ct'(s)]_q = \Delta \cdot [m+m']_t + v_{add} \text{ mod }q, \end{aligned}

where vadd=v+vโ€ฒโˆ’rt(q)โ‹…ฯตv_{add} = v+v'-r_t(q) \cdot \epsilon.

Click here if you want to see why this follows.

[ct(s)+ctโ€ฒ(s)]q=ฮ”โ‹…[m+mโ€ฒ]t+ฮ”โ‹…tโ‹…ฯต+v+vโ€ฒ=ฮ”โ‹…[m+mโ€ฒ]t+(qโˆ’rt(q))โ‹…ฯต+v+vโ€ฒโ‰กฮ”โ‹…[m+mโ€ฒ]t+vaddย modย q.\begin{aligned} [ct(s) + ct'(s)]_q =& \Delta \cdot [m+m']_t + \Delta \cdot t \cdot \epsilon + v + v' \\ =& \Delta \cdot [m+m']_t + (q-r_t(q)) \cdot \epsilon + v+v' \\ \equiv& \Delta \cdot [m+m']_t + v_{add} \text{ mod }q. \end{aligned}

This suggests to set the new ciphertext as cadd=(ct0+ct0โ€ฒ,ย ct1+ct1โ€ฒ)c_{add}=(ct_0+ct'_0, \text{ }ct_1+ct'_1), which can be computed without the knowledge of the secret key ss. Therefore, cadd=ct+ctโ€ฒc_{add} = ct + ct' decrypts to the sum, [m+mโ€ฒ]t[m+m']_t, as long as the new "noise", vaddv_{add}, is smaller than ฮ”/2\Delta/2.

๐Ÿ’ก The noise growth for addition is quite slow as โˆฅvaddโˆฅ<โˆฅvโˆฅ+โˆฅvโ€ฒโˆฅ+t<2B+t\|v_{add}\|<\|v\|+\|v'\| + t < 2B+t, where BB is an upper bound on the "noise" of the ciphertexts that were added. This means we can probably do many additions before decryption stops working.

To add ciphertexts, it seems like we only need to add polynomials in RqR_q. So, ciphertext addition is a piece of cake ๐Ÿฐ.

def add_cipher(ct1, ct2, q, poly_mod):
    """Add a ciphertext and a ciphertext.
    Args:
        ct1, ct2: ciphertexts.
        q: ciphertext modulus.
        poly_mod: polynomial modulus.
    Returns:
        Tuple representing a ciphertext.
    """
    new_ct0 = polyadd(ct1[0], ct2[0], q, poly_mod)
    new_ct1 = polyadd(ct1[1], ct2[1], q, poly_mod)
    return (new_ct0, new_ct1)

โžก๏ธ Multiplication: Producing a new ciphertext that decrypts the product of the two messages is not that easy. But we should still try ๐Ÿคž. The first idea that comes to mind is to simply multiply the decryption equations. It worked for addition, so maybe it works here as well.

ct(s)โ‹…ctโ€ฒ(s)=ฮ”2โ‹…mmโ€ฒ+ฮ”โ‹…(mvโ€ฒ+mโ€ฒv)+vvโ€ฒ.ย ย (1)ct(s) \cdot ct'(s) = \Delta^2 \cdot mm' + \Delta\cdot (mv'+m'v) + vv'. \text{ } \ (1)

If we scale by t/qt/q to get rid of one ฮ”\Delta, we get something that looks like what we want.

tqโ‹…ct(s)โ‹…ctโ€ฒ(s)โ‰ˆฮ”โ‹…mmโ€ฒ+(mvโ€ฒ+mโ€ฒv)\frac{t}{q}\cdot ct(s) \cdot ct'(s) \approx \Delta \cdot mm' + (mv'+m'v)

It seems we are on the right track. Let's examine these expressions further. Recall the notations ct(s)=ct0+ct1โ‹…sct(s) = ct_0 + ct_1\cdot s and ctโ€ฒ(s)=ct0โ€ฒ+ct1โ€ฒโ‹…s.ct'(s) = ct'_0 + ct'_1\cdot s. Both of them are linear in ss, but their multiplication is quadratic in ss:

ctโ‹…ctโ€ฒ(s)=ct0โ‹…ct0โ€ฒ+(ct0โ‹…ct1โ€ฒ+ct1โ‹…ct0โ€ฒ)s+ct1โ‹…ct1โ€ฒs2,ct\cdot ct'(s) = ct_0\cdot ct'_0 + (ct_0\cdot ct'_1 + ct_1\cdot ct'_0)s +ct_1\cdot ct'_1s^2,

which we will write, in short, as ctโ‹…ctโ€ฒ(s)=c0ร—+c1ร—โ‹…s+c2ร—โ‹…s2.ct \cdot ct' (s) = c_0^\times + c_1^{\times}\cdot s + c_2^{\times} \cdot s^2.

But what about the right hand side? Keep in mind that we work with plaintexts m,mโ€ฒโˆˆRtm, m' \in R_t, so we should take mmโ€ฒmm' with coefficients modulo tt. Therefore, we can apply the same trick as we did for addition: we divide by tt and write mmโ€ฒ=trm+[mmโ€ฒ]tmm' = tr_m + [mm']_t, where rmr_m is an integer polynomial. Skipping a lot of details, we end up with:

t/qโ‹…ctโ‹…ctโ€ฒ(s)=ฮ”โ‹…[mmโ€ฒ]t+u2t/q \cdot ct \cdot ct'(s) = \Delta \cdot [mm']_t + u_2

for u2u_2 a polynomial with rational coefficients. Looks like we're getting closer to obtaining the decryption equation. We can now write the original expression (1)(1) as:

t/qโ‹…c0ร—+t/qโ‹…c1ร—โ‹…s+t/qโ‹…c2ร—โ‹…s2=ฮ”โ‹…[mmโ€ฒ]t+u2t/q \cdot c_0^\times + t/q \cdot c_1^{\times} \cdot s + t/q \cdot c_2^{\times} \cdot s^2 = \Delta \cdot [mm']_t + u_2

Hm.. these coefficients look like rational polynomials. Recall that the ciphertext has integer polynomials as elements. So we round each coefficient appearing in the left hand side to their nearest integers and then reduce the whole equation modulo qq:

[โŒŠt/qโ‹…c0ร—โŒ‰]q+[โŒŠt/qโ‹…c1ร—โŒ‰]qโ‹…s+[โŒŠt/qโ‹…c2ร—โŒ‰]qโ‹…s2=ฮ”โ‹…[mmโ€ฒ]t+u3ย ย (2)[\lfloor t/q \cdot c_0^{\times} \rceil]_q + [\lfloor t/q \cdot c_1^{\times}\rceil]_q \cdot s + [\lfloor t/q \cdot c_2^{\times}\rceil]_q \cdot s^2 = \Delta \cdot [mm']_t + u_3 \text{ } \ (2)

where u3u_3 is a "small" integer polynomial, that represents the "noise" after one multiplication.

๐Ÿ’ก The noise growth for multiplication grows a lot faster: โˆฅu3โˆฅโ‰ค2โ‹…nโ‹…tโ‹…Bโ‹…(2n+1)โ‹…(n+1)+8t2โ‹…n2,\|u_3\| \leq 2 \cdot n \cdot t \cdot B \cdot (2n+1)\cdot (n+1) + 8t^2 \cdot n^2, where BB is an upper bound for the "noise" of the ciphertexts that were multiplied. We refer the enthusiastic reader for more details to [[FV12] Lem. 2].

Phew, seems like we are done: we can consider as a ciphertext decrypting to [mmโ€ฒ]t[mm']_t the tuple of scaled and rounded coefficients mod qq from left hand side of (2)(2), denoted by (c0,c1,c2)(c_0, c_1, c_2). Of course, for a correct decryption, u3u_3 should have small enough coefficients. You can see below how these coefficients are computed in Python:

def multiplication_coeffs(ct1, ct2, q, t, poly_mod):
    """Multiply two ciphertexts.
        Args:
            ct1: first ciphertext.
            ct2: second ciphertext
            q: ciphertext modulus.
            t: plaintext modulus.
            poly_mod: polynomial modulus.
        Returns:
            Triplet (c0,c1,c2) encoding the multiplied ciphertexts.
        """

    c_0 = np.int64(np.round(polymul_wm(ct1[0], ct2[0], poly_mod) * t / q)) % q
    c_1 = np.int64(np.round(polyadd_wm(polymul_wm(ct1[0], ct2[1], poly_mod), polymul_wm(ct1[1], ct2[0], poly_mod), poly_mod) * t / q)) % q 
    c_2 = np.int64(np.round(polymul_wm(ct1[1], ct2[1], poly_mod) * t / q)) % q
    return c_0, c_1, c_2

But, as a popular movie character would say, "Houston, we have a problem". This tuple of coefficients has size 3, not 2 as the usual ciphertext. Moreover, the size of such tuple will grow linearly in the number of further multiplications performed on the ciphertexts. In order to restore the size of the ciphertext as 2, we will make use of the so called relinearization technique. ๐Ÿ’ฅ

Relinearization

๐Ÿ’ก The idea of Relinearization is to reduce the triplet (c0,c1,c2)(c_0,c_1,c_2) to a ciphertext pair (c0โ€ฒ,c1โ€ฒ)โˆˆRqร—Rq(c_0',c_1') \in R_q \times R_q that recovers [mmโ€ฒ]t[mm']_t when decrypted with the usual decryption algorithm. We would like to produce a pair (c0โ€ฒ,c1โ€ฒ)(c_0',c_1'), without using the secret ss, such that: [c0โ€ฒ+sโ‹…c1โ€ฒ]q=[c0+c1โ‹…s+c2โ‹…s2+r]q[c_0' + s\cdot c_1']_q = [c_0 + c_1\cdot s + c_2 \cdot s^2 + r ]_q, where rr is a "small" error. The correct decryption will be possible, as the "small" error rr will vanish because of the rounding in decryption.


As the name suggests it, we transform the degree 2 polynomial, c0+c1โ‹…s+c2โ‹…s2c_0+c_1\cdot s+c_2 \cdot s^2 into a linear polynomial, i.e. of degree 1. This involves giving extra info about s2s^2. Using a special public key, called relinearization key, we can linearize c2โ‹…s2c_2 \cdot s^2 (up to some small error) as

[c20+c21โ‹…s]q=[c2โ‹…s2+erelin]q.[c_{20}+c_{21} \cdot s]_q = [c_2\cdot s^2 + e_{relin}]_q.

Therefore, by Equation (2)(2), we can get a standard ciphertext pair as

cmul=(c0+c20,c1+c21),c_{mul} =(c_0+c_{20}, c_1+c_{21}),

that correctly decrypts to [mmโ€ฒ]t[mm']_t, as you can see below:

[c0+c20+sโ‹…(c1+c21)]q=[c0+c1โ‹…s+c2โ‹…s2+erelin]q=ฮ”โ‹…[mmโ€ฒ]t+vmult,\begin{aligned} [c_0 + c_{20} + s\cdot (c_1 + c_{21})]_q = [c_0 + c_1 \cdot s + c_2 \cdot s^2 + e_{relin}]_q = \Delta \cdot [mm']_t + v_{mult}, \end{aligned}

where vmult=u3+erelin.v_{mult} = u_3 + e_{relin}. Therefore, cmulc_{mul} decrypts correctly to [mmโ€ฒ]t[mm']_t if โˆฅvmultโˆฅ\|v_{mult}\| is less than ฮ”/2\Delta/2. So yay! we finally know how to get a ciphertext encoding multiplication!

Different versions of relinearization

To complete this discussion, we need to see how to construct the linear approximation of c2โ‹…s2c_2 \cdot s^2 and find out what the relinearization key is about. For this, we will go a bit deeper into (technical) details. Don't panic, we'll take you step by step. ๐Ÿ˜„

We are going to present two versions of linearizing c2โ‹…s2c_2\cdot s^2. First, keep in mind that s2s^2 should not be known, so we give the relinearization key as a masked version of s2s^2.

Let's think of the following situation: say we include in the public key the following relinearization key:

rlk=(rlk0,rlk1)=([โˆ’(aโ‹…s+e)+s2]q,a),rlk = (rlk_0, rlk_1) = ([-(a\cdot s+e)+s^2]_q,a),

for some uniform aa in RqR_q and a small error ee.

โ— Intuitively, the secret s2s^2 is hidden by something that looks like an RLWE sample. Notice that,

rlk0+rlk1โ‹…s=s2+e.rlk_0 + rlk_1 \cdot s = s^2 + e.

To obtain the approximation of c2โ‹…s2c_2\cdot s^2 we should multiply the above expression by c2c_2. By doing so, we end up with the rather large-norm term c2โ‹…ec_2\cdot e, due to the size of the coefficients of c2c_2. We cannot allow such a large "noise" as it will interfere with decryption. To avoid this "noise" blow-up we will employ two techniques described below.

๐Ÿ’ฅ Relinearization: Version 1

One strategy is to use base TT decomposition of the coefficients of c2c_2 to slice c2c_2 into components of small norm. To do this, we pick a base TT and write each coefficient of c2c_2 in this base. Recall that c2c_2 is a integer polynomial, modulo xn+1x^n+1, so of degree at most nโˆ’1n-1. If we write c2c_2 as a polynomial:

c2(x)=c2[0]+c2[1]โ‹…x+โ€ฆ+c2[nโˆ’1]โ‹…xnโˆ’1c_2(x) = c_2[0] + c_2[1]\cdot x+\ldots+c_2[n-1]\cdot x^{n-1}

then we can decompose each coefficient c2[i]c_2[i] in base TT. Notice that since c2c_2 has coefficients in [0,qโˆ’1][0,q-1], the maximum power appearing in these representations is Tโ„“T^{\ell}, where โ„“=โŒŠlogโกT(q)โŒ‹\ell = \lfloor \log_T(q)\rfloor. For base decomposition we use the function int2base\texttt{int2base}:

def int2base(n, b):
    """Generates the base decomposition of an integer n.
    Args:
        n: integer to be decomposed.
        b: base.
    Returns:
        array of coefficients from the base decomposition of n
        with the coeff[i] being the coeff of b ^ i.
    """
    if n < b:
        return [n]
    else:
        return [n % b] + int2base(n // b, b)  

The relinearization key, rlkrlk in this version, consists of masked variants of Tiโ‹…s2,T^i \cdot s^2, instead of s2s^2. More precisely, for 0โ‰คiโ‰คโ„“0 \leq i \leq \ell, this is defined as follows:

(rlk0[i],rlk1[i])=([โˆ’(aiโ‹…s+ei)+Tiโ‹…s2]q,ai)(rlk_0[i], rlk_1[i]) = ([-(a_i \cdot s + e_i) + T^i\cdot s^2]_{q}, a_i)

for aia_i chosen uniformly in RqR_q and eie_i chosen according to the distribution ฯ‡\chi over RR (yep, same error distribution as in the description of the scheme). Below you can find the implementation of the function that generates the evaluation (relinearization) key (rlk0[i],rlk1[i])0โ‰คiโ‰คโ„“(rlk_0[i],rlk_1[i])_{0\leq i\leq\ell}.

def evaluate_keygen_v1(sk, size, modulus, T, poly_mod, std2):
    """Generate a relinearization key using version 1.
        Args:
            sk: secret key.
            size: size of the polynomials.
            modulus: coefficient modulus.
            T: base.
            poly_mod: polynomial modulus.
            std2: standard deviation for the error distribution.
        Returns:
            rlk: relinearization key.

        """
    n = len(poly_mod) - 1
    l = np.int(np.log(modulus) / np.log(T))
    rlk0 = np.zeros((l + 1, n), dtype=np.int64)
    rlk1 = np.zeros((l + 1, n), dtype=np.int64)
    for i in range(l + 1):
        a = gen_uniform_poly(size, modulus)
        e = gen_normal_poly(size, 0, std2)
        secret_part = T ** i * poly.polymul(sk, sk)
        b = np.int64(polyadd(
        polymul_wm(-a, sk, poly_mod),
        polyadd_wm(-e, secret_part, poly_mod), modulus, poly_mod))

        b = np.int64(np.concatenate( (b, [0] * (n - len(b)) ) )) # pad b 
        a = np.int64(np.concatenate( (a, [0] * (n - len(a)) ) )) # pad a    

        rlk0[i] = b
        rlk1[i] = a
    return rlk0, rlk1

Now, given rlkrlk, let's look at how we compute the linear approximation of c2โ‹…s2c_2 \cdot s^2. Let the polynomials c2(i)c_2(i) be the base TT decomposition of c2c_2, such that:

c2=โˆ‘i=0โ„“c2(i)โ‹…Ti.c_2 = \displaystyle \sum_{i=0}^{\ell} c_2(i)\cdot T^i.

We can get the linear approximation given by (c20,c21)(c_{20}, c_{21}), where:

c20=โˆ‘i=0โ„“rlk0[i]โ‹…c2(i)ย andย c21=โˆ‘i=0โ„“rlk1[i]โ‹…c2(i).c_{20} = \sum_{i=0}^{\ell} rlk_0[i]\cdot c_2(i) \text{ and } c_{21} = \sum_{i=0}^{\ell} rlk_1[i]\cdot c_2(i).

Therefore, c20+c21โ‹…s=c2โ‹…s2+erelin_v1c_{20} + c_{21} \cdot s = c_2 \cdot s^2 + e_{\text{relin\_v1}} where erelin_v1e_{\text{relin\_v1}} is an error term from RqR_q.

Click here for more details.
c20+c21โ‹…s=โˆ‘i=0โ„“[โˆ’(aiโ‹…s+ei)+Tiโ‹…s2]c2(i)+โˆ‘i=0โ„“aiโ‹…sโ‹…c2(i)=โˆ’โˆ‘i=0โ„“eiโ‹…c2(i)+c2โ‹…s2.\begin{aligned} c_{20} + c_{21} \cdot s &= \sum_{i=0}^{\ell} [-(a_i\cdot s+e_i)+T^i\cdot s^2] c_2(i) + \sum_{i=0}^{\ell} a_i \cdot s \cdot c_2(i)\\ &= -\sum_{i=0}^{\ell} e_i \cdot c_2(i) + c_2 \cdot s^2. \end{aligned}

๐Ÿ’ก By doing base TT decomposition we get a "small" relinearization noise: โˆฅerelin_v1โˆฅโ‰ค(โ„“+1)โ‹…Bโ‹…Tโ‹…n/2\|e_{\text{relin\_v1}}\| \leq (\ell+1)\cdot B \cdot T \cdot n/2 where BB is an upper bound on the errors eie_i.


โ“ Now the question is how to compute the polynomials c2(i)c_2(i). The coefficients of these polynomials prove to be nothing but the columns of the matrix Reps\texttt{Reps}.

If you're curious to see why, click here: So far, we have stored the representations of each coefficient of $c_2$, $c_2[i]$, let's say as rows in an $n \times (\ell+1)$ matrix $\mathtt{Reps}$. Therefore

c2[i]=Reps[i][0]+Reps[i][1]โ‹…T+โ€ฆ+Reps[i][โ„“]โ‹…Tโ„“.c_2[i] = \mathtt{Reps}[i][0] + \mathtt{Reps}[i][1] \cdot T+ \ldots + \mathtt{Reps}[i][\ell] \cdot T^{\ell}.

If we multiply this by xix^i, we get:

c2[i]โ‹…xi=Reps[i][0]โ‹…xi+Reps[i][1]โ‹…xiโ‹…T+โ€ฆ+Reps[i][โ„“]โ‹…xiโ‹…Tโ„“.c_2[i] \cdot x^i = \mathtt{Reps}[i][0] \cdot x^i + \mathtt{Reps}[i][1] \cdot x^i \cdot T+ \ldots + \mathtt{Reps}[i][\ell] \cdot x^i \cdot T^{\ell}.

Summing up all these for each 0โ‰คiโ‰คnโˆ’10\leq i \leq n-1, on the left hand side we actually get c2c_2:

c2=โˆ‘i=0nโˆ’1Reps[i][0]โ‹…xi+(โˆ‘i=0nโˆ’1Reps[i][1]โ‹…xi)โ‹…T+โ€ฆ+(โˆ‘i=0nโˆ’1Reps[i][โ„“]โ‹…xi)โ‹…Tโ„“.c_2 = \displaystyle \sum_{i=0}^{n-1} \mathtt{Reps}[i][0] \cdot x^i + (\sum_{i=0}^{n-1}\mathtt{Reps}[i][1] \cdot x^i) \cdot T+ \ldots + (\sum_{i=0}^{n-1} \mathtt{Reps}[i][\ell] \cdot x^i) \cdot T^{\ell}.

So, if we denote the above sums by polynomials c2(j)c_2(j), we simply get

c2=โˆ‘j=0โ„“c2(j)โ‹…Tj.c_2 = \displaystyle \sum_{j=0}^{\ell} c_2(j)\cdot T^j.

Phew, what a relief! Notice that the coefficients for each polynomial c2(j)c_2(j) are given by the jj-th column of Reps\mathtt{Reps}.

So after all this math, we finally got the linear approximation of c2โ‹…s2c_2 \cdot s^2 and thus, we can derive the standard ciphertext encoding the multiplication of the plaintexts. Here comes the code:

def mul_cipher_v1(ct1, ct2, q, t, T, poly_mod, rlk0, rlk1):
    """Multiply two ciphertexts.
    Args:
        ct1: first ciphertext.
        ct2: second ciphertext
        q: ciphertext modulus.
        t: plaintext modulus.
        T: base
        poly_mod: polynomial modulus.
        rlk0, rlk1: output of the EvaluateKeygen_v1 function.
    Returns:
        Tuple representing a ciphertext.
    """
    n = len(poly_mod) - 1
    l = np.int64(np.log(q) / np.log(T))  #l = log_T(q)

    c_0, c_1, c_2 = multiplication_coeffs(ct1, ct2, q, t, poly_mod)
    c_2 = np.int64(np.concatenate( (c_2, [0] * (n - len(c_2))) )) #pad
   
    #Next, we decompose c_2 in base T: 
    #more precisely, each coefficient of c_2 is decomposed in base T such that c_2 = sum T**i * c_2(i)
    Reps = np.zeros((n, l + 1), dtype = np.int64)
    for i in range(n):
        rep = int2base(c_2[i], T)
        rep2 = rep + [0] * (l + 1 - len(rep)) #pad with 0
        Reps[i] = np.array(rep2, dtype=np.int64)
    # Each row Reps[i] is the base T representation of the i-th coefficient c_2[i].
    # The polynomials c_2(j) are given by the columns Reps[:,j].

    c_20 = np.zeros(shape=n)
    c_21 = np.zeros(shape=n)
    # Here we compute the sums: rlk[j][0] * c_2(j) and rlk[j][1] * c_2(j) 
    for j in range(l + 1):
        c_20 = polyadd_wm(c_20, polymul_wm(rlk0[j], Reps[:,j], poly_mod), poly_mod)
        c_21 = polyadd_wm(c_21, polymul_wm(rlk1[j], Reps[:,j], poly_mod), poly_mod)

    c_20 = np.int64(np.round(c_20)) % q
    c_21 = np.int64(np.round(c_21)) % q

    new_c0 = np.int64(polyadd_wm(c_0, c_20, poly_mod)) % q
    new_c1 = np.int64(polyadd_wm(c_1, c_21, poly_mod)) % q

    return (new_c0, new_c1)

๐Ÿ’ฅ Relinearization: Version 2

This version is much simpler and cleaner than the previous one (yay!) and uses the so-called modulus switching technique. Recall that if we try to naively mask s2s^2 in the reliniarization key, then there is a large blow-up in the noise because of the c2c_2 multiplication. We want to avoid this to get a correct decryption.

In this version we mask s2s^2 modulo a a different modulus, pโ‹…qp\cdot q, with a much larger pโ‰ซqp\gg q, as shown below.

rlk=(rlk0,rlk1)=([โˆ’(aโ€ฒโ‹…s+eโ€ฒ)+pโ‹…s2]pโ‹…q,aโ€ฒ),rlk = (rlk_0, rlk_1) = ([-(a' \cdot s + e') + p\cdot s^2]_{p\cdot q}, a'),

for a uniform aโ€ฒa' in Rpโ‹…qR_{p\cdot q} and eโ€ฒe' drawn according to an error distribution ฯ‡โ€ฒ\chi' over R.R. Remember that our goal is to produce an approximation of [c2โ‹…s2]q[c_2\cdot s^2]_q. The idea is that when we scale from pโ‹…qp\cdot q back to qq, the noise gets divided by the large integer pp, significantly reducing its size.

In a safe implementation the distribution ฯ‡โ€ฒ\chi' should be distinct from ฯ‡\chi and its parameters should be carefully chosen for security reasons. Since security is not our main goal in this blog post, you can check the paper for further details.

def evaluate_keygen_v2(sk, size, modulus, poly_mod, extra_modulus, std2):
    """Generate a relinearization key using version 2.
        Args:
            sk: secret key
            size: size of the polynomials.
            modulus: coefficient modulus.
            poly_mod: polynomial modulus.
            extra_modulus: the "p" modulus for modulus switching.
            st2: standard deviation for the error distribution.
        Returns:
            rlk0, rlk1: relinearization key.
        """
    new_modulus = modulus * extra_modulus
    a = gen_uniform_poly(size, new_modulus)
    e = gen_normal_poly(size, 0, std2)
    secret_part = extra_modulus * poly.polymul(sk, sk)

    b = np.int64(polyadd_wm(
        polymul_wm(-a, sk, poly_mod),
        polyadd_wm(-e, secret_part, poly_mod), poly_mod)) % new_modulus
    return b, a

The linear approximation of [c2โ‹…s2]q[c_2 \cdot s^2]_q can be computed from the pair:

(c20,c21)=([โŒŠc2โ‹…rlk0pโŒ‰]q,[โŒŠc2โ‹…rlk1pโŒ‰]q).(c_{20}, c_{21}) = \Big(\Big[\Big\lfloor\frac{c_2 \cdot rlk_0}{p}\Big\rceil\Big]_q, \Big[\Big\lfloor\frac{c_2 \cdot rlk_1}{p}\Big\rceil\Big]_q\Big).

Indeed, [c20+c21โ‹…s]q=[c2โ‹…s2]q+erelin_v2,[c_{20} + c_{21} \cdot s]_q = [c_2 \cdot s^2]_q + e_{\text{relin\_v2}}, for a "small" error erelin_v2e_{\text{relin\_v2}} in RqR_q.

For an intuition of why this happens, click here.Skipping some details, this holds true because of the following simple computation:
c2โ‹…rlk0p+c2โ‹…rlk1pโ‹…s=c2โ‹…[โˆ’(aโ€ฒโ‹…s+eโ€ฒ)+pโ‹…s2]pโ‹…q+c2โ‹…aโ€ฒโ‹…sp=c2โ‹…s2+โˆ’c2โ‹…eโ€ฒp+qโ‹…K,\begin{aligned} \frac{c_2 \cdot rlk_0}{p} + \frac{c_2 \cdot rlk_1}{p} \cdot s &= \frac{c_2\cdot [-(a'\cdot s+e') + p\cdot s^2]_{p\cdot q}+c_2 \cdot a' \cdot s}{p}\\ &= c_2\cdot s^2 +\frac{-c_2\cdot e'}{p} + q \cdot K, \end{aligned}

where KโˆˆRK \in R such that [โˆ’(aโ€ฒโ‹…s+eโ€ฒ)+pโ‹…s2]pq=โˆ’(aโ€ฒโ‹…s+eโ€ฒ)+pโ‹…s2+pqโ‹…K[-(a'\cdot s+e') + p\cdot s^2]_{pq} = -(a'\cdot s+e') + p\cdot s^2 + pq\cdot K.

We're cheating a bit here, since the pair involves the roundings of the terms to their nearest integers. Still, we're not far from the truth, since for any real number aa, โŒŠaโŒ‰\lfloor a \rceil differs from aa by a small quantity ฮตโˆˆ[โˆ’12,12]\varepsilon \in [-\frac{1}{2},\frac{1}{2}]. (we can also extend this coefficient-wise to polynomials).


๐Ÿ’ก We get a "small" relinearization noise, erelin_v2โ‰ˆ(c2โ‹…eโ€ฒ)/pe_{\text{relin\_v2}} \approx (c_2\cdot e')/p, for large p:p: โˆฅerelin_v2โˆฅโ‰คqโ‹…Bโ€ฒโ‹…np+n+12.\|e_{\text{relin\_v2}}\| \leq \frac{q \cdot B' \cdot n}{p}+\frac{n+1}{2}.


Next, we provide the easy code implementation for multiplying ciphertexts using this version.

def mul_cipher_v2(ct1, ct2, q, t, p, poly_mod, rlk0, rlk1):
    """Multiply two ciphertexts.
    Args:
        ct1: first ciphertext.
        ct2: second ciphertext.
        q: ciphertext modulus.
        t: plaintext modulus.
        p: modulus-swithcing modulus.
        poly_mod: polynomial modulus.
        rlk0, rlk1: output of the EvaluateKeygen_v2 function.
    Returns:
        Tuple representing a ciphertext.
    """
    c_0, c_1, c_2 = multiplication_coeffs(ct1, ct2, q, t, poly_mod)

    c_20 = np.int64(np.round(polymul_wm(c_2, rlk0, poly_mod) / p)) % q
    c_21 = np.int64(np.round(polymul_wm(c_2, rlk1, poly_mod) / p)) % q

    new_c0 = np.int64(polyadd_wm(c_0, c_20, poly_mod)) % q
    new_c1 = np.int64(polyadd_wm(c_1, c_21, poly_mod)) % q
    return (new_c0, new_c1)

Version 1 vs. Version 2 ๐ŸฅŠ

Recall that we can derive a fresh standard ciphertext that decrypts to [mmโ€ฒ]t[mm']_t as cmul=(c0+c20,c1+c21)c_{mul} =(c_0+c_{20}, c_1+c_{21}), since

c0+c20+sโ‹…(c1+c21)=ฮ”โ‹…[mmโ€ฒ]t+vmult,c_0 + c_{20} + s\cdot (c_1 + c_{21}) = \Delta \cdot [mm']_t + v_{mult},

where vmult=u3+erelinv_{mult} = u_3 + e_{\text{relin}} and erelinโˆˆ{erelin_v1,erelin_v2}e_{\text{relin}} \in \{e_{\text{relin\_v1}}, e_{\text{relin\_v2}}\}.

We need to make sure that cmulc_{mul} decrypts correctly to [mmโ€ฒ]t[mm']_t. For this, it suffices to choose the parameters such that โˆฅu3โˆฅ+โˆฅerelinโˆฅโ‰คฮ”/2.\|u_3\| + \|e_{\text{relin}}\| \leq \Delta/2.

Now that we have two versions of relinearization, which help us in deriving cmulc_{mul}, we may wonder which one we can use:

RelinearizationSize of rlkrlk (in bits)Bound of ereline_{\text{relin}}
Version 12(โ„“+1)โ‹…nโ‹…logโกq2(\ell+1)\cdot n \cdot \log q(โ„“+1)โ‹…Bโ‹…Tโ‹…n/2(\ell+1)\cdot B \cdot T \cdot n/2
Version 22nโ‹…logโกpq2n \cdot \log pqqโ‹…Bโ€ฒโ‹…np+n+12\frac{q \cdot B' \cdot n}{p}+\frac{n+1}{2}

โžก๏ธ size of rlkrlk: Version 1 gives a relinearization key as โ„“+1\ell+1 pairs of polynomials in Rq,R_q, whereas Version 2 gives just one such pair. Recall that โ„“=โŒŠlogโกTqโŒ‹\ell=\lfloor \log_T{q}\rfloor and see that this decreases as long as the base TT increases.

โžก๏ธ bound of ereline_{\text{relin}}: The upper bounds of ereline_{\text{relin}} for both versions are according to [FV12], Lem.3. We consider BB as a bound taken so that the error distribution ฯ‡\chi takes values in [โˆ’B,B][-B,B] with high probability. We similarly define Bโ€ฒB' for the case of the error distribution ฯ‡โ€ฒ\chi', used in Version 2. Notice that for Version 1, a larger TT leads to more noise (but smaller rlkrlk), whereas in Version 2, a larger pp leads to smaller noise (but larger rlkrlk). For choosing the parameters in safe implementation, we refer the reader to [FV12] Sec. Realinearisation Version 2.

Setting the parameters

We set the parameters for our toy implementation just to make sure it always correctly decrypts at least one ciphertext multiplication. For that, we made sure that โˆฅu3โˆฅ+โˆฅerelinโˆฅ<ฮ”/2\|u_3\| + \|e_{\text{relin}}\|< \Delta /2. In practice, for the current choice it seems that decryption always works for 33 ciphertext multiplications when using V2 relinearization, for example. This is due to the fact that the theoretical bounds are worst-case bounds. For the same parameters we can make hundreds of addition (in practice it seems that decryption works even for 10001000 additions ๐Ÿ˜ฎ). Ciphertext addition is almost for free in terms of noise growth! ๐Ÿ’ฅ We invite you to play around with the parameters and the bounds and try to see how many multiplications you can get (as they are the costly operations).

Let's play! ๐ŸŽ‰

Now we can multiply ciphertexts, as we have implemented two versions of this: mul_cipher_v1\texttt{mul\_cipher\_v1} and mul_cipher_v2\texttt{mul\_cipher\_v2}, using relinearization. We can further add ciphertexts by using add_cipher\texttt{add\_cipher}. So yay! we can finally perform computations on encrypted data.

Bonus: if you're curious, you can try computing more complex (polynomial) operations on encrypted data, such as multiplying three ciphertexts. Here we only wanted to show how to perform one multiplication. For more levels of multiplications, you should set the parameters as in [FV12, Thm1] so that you decrypt correctly ๐Ÿ˜‰.

Bonus2: you can also perform plain operations, such as adding or multiplying plaintexts to ciphertexts, by using add_plain\texttt{add\_plain} and mul_plain,\texttt{mul\_plain}, (with a reduced noise growth) both from here. For further details on how these work, you should check this. ๐Ÿค“

So what are you waiting for? Go check it out!

import rlwe_he_scheme_updated as rlwe_updated
import numpy as np

if __name__ == '__main__':
    # Scheme's parameters
    # polynomial modulus degree
    n = 2 ** 2
    # ciphertext modulus
    q = 2 ** 14
    # plaintext modulus
    t = 2
    # base for relin_v1
    T = int(np.sqrt(q)) 
    #modulusswitching modulus
    p = q ** 3

    # polynomial modulus
    poly_mod = np.array([1] + [0] * (n - 1) + [1])
    
    #standard deviation for the error in the encryption
    std1 = 1
    #standard deviation for the error in the evaluateKeyGen_v2
    std2 = 1

    # Keygen
    pk, sk = rlwe_updated.keygen(n, q, poly_mod, std1)

    #EvaluateKeygen_version1
    rlk0_v1, rlk1_v1 = rlwe_updated.evaluate_keygen_v1(sk, n, q, T, poly_mod, std1)

    #EvaluateKeygen_version2
    rlk0_v2, rlk1_v2 = rlwe_updated.evaluate_keygen_v2(sk, n, q, poly_mod, p, std2)
 
    # Encryption
    pt1, pt2 = [1, 0, 1, 1], [1, 1, 0, 1]
    cst1, cst2 = [0, 1, 1, 0], [0, 1, 0, 0]

    ct1 = rlwe_updated.encrypt(pk, n, q, t, poly_mod, pt1, std1)
    ct2 = rlwe_updated.encrypt(pk, n, q, t, poly_mod, pt2, std1)

    print("[+] Ciphertext ct1({}):".format(pt1))
    print("")
    print("\t ct1_0:", ct1[0])
    print("\t ct1_1:", ct1[1])
    print("")
    print("[+] Ciphertext ct2({}):".format(pt2))
    print("")
    print("\t ct1_0:", ct2[0])
    print("\t ct1_1:", ct2[1])
    print("")

    # Evaluation
    ct3 = rlwe_updated.add_plain(ct1, cst1, q, t, poly_mod)
    ct4 = rlwe_updated.mul_plain(ct2, cst2, q, t, poly_mod)
    #ct5 = (ct1 + cst1) + (cst2 * ct2)
    ct5 = rlwe_updated.add_cipher(ct3, ct4, q, poly_mod)
    # ct6 = ct1 * ct2
    ct6 = rlwe_updated.mul_cipher_v1(ct1, ct2, q, t, T, poly_mod, rlk0_v1, rlk1_v1)
    ct7 = rlwe_updated.mul_cipher_v2(ct1, ct2, q, t, p, poly_mod, rlk0_v2, rlk1_v2)
    # Decryption
    decrypted_ct3 = rlwe_updated.decrypt(sk, q, t, poly_mod, ct3)
    decrypted_ct4 = rlwe_updated.decrypt(sk, q, t, poly_mod, ct4)
    decrypted_ct5 = rlwe_updated.decrypt(sk, q, t, poly_mod, ct5)
    decrypted_ct6 = rlwe_updated.decrypt(sk, q, t, poly_mod, ct6)
    decrypted_ct7 = rlwe_updated.decrypt(sk, q, t, poly_mod, ct7)
    
    print("[+] Decrypted ct3=(ct1 + {}): {}".format(cst1, decrypted_ct3))
    print("[+] Decrypted ct4=(ct2 * {}): {}".format(cst2, decrypted_ct4))
    print("[+] Decrypted ct5=(ct1 + {} + {} * ct2): {}".format(cst1, cst2, decrypted_ct5))
    print("[+] pt1 + {} + {} * pt2): {}".format(cst1, cst2, rlwe_updated.polyadd(
                                                rlwe_updated.polyadd(pt1, cst1, t, poly_mod),
                                                rlwe_updated.polymul(cst2, pt2, t, poly_mod),
                                                t, poly_mod)))
    print("[+] Decrypted ct6=(ct1 * ct2): {}".format(decrypted_ct6))
    print("[+] Decrypted ct7=(ct1 * ct2): {}".format(decrypted_ct7))
    print("[+] pt1 * pt2: {}".format(rlwe_updated.polymul(pt1, pt2, t, poly_mod)))