Lattices, Cryptography and Computing with Encrypted Data Vinod Vaikuntanathan M.I.T.

Lattices, Cryptography and Computing with Encrypted Data

Vinod VaikuntanathanM.I.T

As e+

“small” error

Combinatorially nice: Optimal rate etc.

Can we decode efficiently (even in the unique decoding regime)?

Seems very hard!

Decoding Random Linear CodesDecoding Lattices

TODAY: Lattice-based Cryptography

As e+

“small” error

Decoding Lattices

(search) LWEn,q,B [Regev’05]: For random secret s Zqn

Learning With Errors (LWE)

( a1 , b1 = a1 , s + e1 )

O s

( a2 , b2 = a2 , s + e2 ) …

( am , bm =am , s + em )

“noisy” random linear equation

Uniformly random in Zq

n

“Small” error |e1| < B

Find s

s +a1 a2 am…e

(decisional) LWEn,q,B : For random secret s Zqn

Learning With Errors (LWE)

¡~a= (a[1]; : : : ;a[n]);b= h~a;~si +e

¢¼¡~a;u

¢

( a1 , b1 = a1 , s + e1 )

O sO rand

( a1 , u1 )

( a2 , b2 = a2 , s + e2 ) …

( am , bm =am , s + em )

( a2 , u2 ) … ( am , um)

random in Zq

Theorem [Reg05,Pei09]: Decisional LWE as hard as Search

LWE/Lattice-based Cryptography

Robust

─ No sub-exponential or quantum attacks

Based on worst-case hardness

Amazingly Versatile─ Advanced Crypto: Homomorphic Encryption,

Functional Encryption, Software Obfuscation,…

─ Only known constructions use lattices

─ Solve LWE on average Solve in worst-case Approx. shortest vectors on worst-case lattices[Regev05, Peikert09, BLPRS13] THIS TALK

Warmup: Secret-key Encryption

• Decryption: Decs(a,b) = ( b - a, s ) (mod 2).

– Correctness: b - a, s = b - ∑a[ i ]∙s[ i ] = m + 2e (over Zq).

decryption succeeds if e < q/4.

Message M

secret key sksecret key sk

eavesdropper

C = Enc(sk,M)

Semantic Security [GM’82]: Encryption of any M0 and M1 are “computationally indistinguishable”

M = Dec(sk,C)

Secret-key Encryption from LWE




• KeyGen:– Sample random “short” vector t Zq

n and set sk = t






n and set sk = t

• Bit Encryption Encsk(m):

– Sample uniformly random a Zqn, “short” noise e Zq

– The ciphertext CT = (a, b = a, t + 2e + m) Zq

n X Zq

Semantic Security from LWE






n and set sk = t

• Bit Encryption Encsk(m):

– Sample uniformly random a Zqn, “short” noise e Zq

– The ciphertext CT = (a, b = a, t + 2e + m) Zq

n X Zq

• Decryption Decsk(CT): Output (b − a, t mod q) mod 2.

– Correctness: b − a, t mod q = 2e + m mod q = 2e + m

(as long as |2e+m| < q/2)

All-or-nothingHave Secret Key, Can Decrypt

No Secret Key, No Go

M

Message M

Encryption

Fully Homomorphic Encryption

Compute arbitrary functions on encrypted data?

[Rivest, Adleman and Dertouzos’78]

Enc(Data)

Enc(F(Data))

Encryption

Powerful server / cloud


Compute arbitrary functions on encrypted data?

[Rivest, Adleman and Dertouzos’78]

Enc(data), F → Enc(F(data))

[Gentry’09, BV’11, LTV’12]: Fully homomorphic (FHE)

(all known constructions based on lattices)

[Goldwasser-Micali’82,…]: Additively homomorphic

[El Gamal’85,…]: Multiplicatively homomorphic

The Big PictureSTEP 1 “Somewhat Homomorphic” (SwHE) Encryption

Evaluate arithmetic circuits of depth d = ε log n *

[Gen09,DGHV10,SV10,BV11a,BV11b,BGV12,LTV12,GHS’12]

* (0 < ε < 1 is a constant, and n is the security parameter)

d =

ε lo

g n

C

EVAL

The Big Picture

“Bootstrapping” Theorem [Gen09] (Qualitative)

“Homomorphic enough” Encryption * FHE

Homomorphic enough = Can evaluate its own Dec Circuit (plus some)

Dec

CT sk

msg

Decryption Circuit

C

EVAL

STEP 2

The Big Picture

“Somewhat Homomorphic” (SwHE) Encryption

Evaluate arithmetic circuits of depth d = ε log n


Depth Boosting / Modulus Reduction [BV11b]

Boost the SwHE to depth d = nε

“Bootstrapping” Method



STEP 1

STEP 2

STEP 3

CT = (a ,b)

Additive Homomorphism

CT’ = (a’, b’)

Look at Ciphertexts through the Decryption Lens

b − a, t = 2e + m b’ − a’, t = 2e’ + m’

CT = (a ,b)


CT’ = (a’, b’)

b − a, t = 2e + m b’ − a’, t = 2e’ + m’

Let c = (a ,b) and s = (-t, 1) Let c’ = (a’ ,b’) and s = (-t, 1)

c, s = 2e + m c’, s = 2e’ + m’

CT = c


CT’ = c’

Claim: cadd = c+c’

c, s = 2e + m c’, s = 2e’ + m’

c, s = 2e + m

c’, s = 2e’ + m’

c+c’, s = 2(e+e’) + (m+m’)

Decs(cadd) = 2E + (m+m’) (mod 2) = (m+m’) (mod 2)

+

E

Proof:

Cadd

Multiplicative Homomorphism

CT = c CT’ = c’

c, s = 2e + m c’, s = 2e’ + m’

Claim: cmult = ?

c, s = 2e + m

c’, s = 2e’ + m’

c, s ∙ c’, s = (2e+m) ∙ (2e’+m’)

X


CT = c CT’ = c’

c, s = 2e + m c’, s = 2e’ + m’

Claim: cmult = ?

c, s = 2e + m

c’, s = 2e’ + m’

c, s ∙ c’, s = mm’ + 2(em’+e’m+2ee’)

X

Quadratic equation in the variables s[i]

E


CT = c CT’ = c’

c, s = 2e + m c’, s = 2e’ + m’

Claim: cmult = ?

c, s = 2e + m

c’, s = 2e’ + m’

c c’, s s = mm’ + 2(em’+e’m+2ee’)

X

E

Tensor Product:

• c c’ = (c[1]∙c’[1], …, c[i]∙c’[j],…, c[n+1]∙c’[n+1])

• c, c’ live in (n+1) dim → c c’ lives in (n+1)2-dim

• KEY FACT: c, s ∙ c’, s = c c’, s s


CT = c CT’ = c’

c, s = 2e + m c’, s = 2e’ + m’

Claim: cmult = c c’

c, s = 2e + m

c’, s = 2e’ + m’

c c’, s s = mm’ + 2(em’+e’m+2ee’)

X

Dec(s s, cmult) = 2E + mm’ (mod 2) = mm’ (mod 2)

E

Problem: Ciphertext size blows up!

(Zqn+1 → Zq

(n+1)^2)


Key Idea [BV’11]: RelinearizationFind linear functions of s that represents these quadratic func.

or, of new secret s’

cmult, s s = 2E + mm’

Multiplicative Homomorphismcmult, s s = 2E + mm’

Key Idea [BV’11]: RelinearizationFind linear functions of s’ that represent these quadratic func.

New KeyGen:

• Sample t,t’Zqn and set sk = (t,t’).

• Evaluation key evk :i,j. Enct’ ( s[ i ]s[ j ] )



New KeyGen:


• Evaluation key evk : sample Ai,j , Ei,j

i,j. (Ai,j , Bi,j = Ai,j , t’ + 2Ei,j + s[ i ]s[ j ])

LWE Security still

holds.



New KeyGen:


• Evaluation key evk : sample Ai,j , Ei,j

i,j. Bi,j − Ai,j , t’ = 2Ei,j + s[ i ]s[ j ]



New KeyGen:


• Evaluation key evk :

i,j. Ci,j , s’ ≈ s[ i ]s[ j ]

(denoting s’ = (-t’, 1) and Ci,j = (Ai,j, Bi,j) as before)



New KeyGen:


• Evaluation key evk :

i,j. Ci,j , s’ ≈ s[ i ]s[ j ]

Linear fn(in s’)

Quadratic fn(in s)

Plug back into quadratic equation:

cmult[i,j] ∙ Ci,j , s’ ≈ 2*Error + mm’

Linear in s’.

Cheating Alert


Plug back into quadratic equation:

cmult[i,j] ∙ Ci,j , s’ ≈ mm’+2*Error

Linear in s’.

Homomorphic Mult:

1.First compute cmult = c c’

2.Compute and output cmult[i,j] ∙ Ci,j

(where Ci,j are from the evaluation key)

The Reservoir Analogy

noise=0

noise=q/2Additive Homomorphism: ξ → 2 ξ

initial noise= ξ

Mult. Homomorphism: ξ → ξ2 + n2B log q

2ξ

~ ξ2

AFTER d LEVELS:

noise B → (worst case)

Correctness Security

(How homomorphic is this?)

The Reservoir Analogy

noise=0

noise=q/2Additive Homomorphism: ξ → 2 ξ

initial noise= ξ

Mult. Homomorphism: ξ → ξ2 + n2B log q

~ ξ2

AFTER d LEVELS:

noise B → (worst case)

(How homomorphic is this?)

The Big Picture









STEP 1

STEP 2

STEP 3

Bootstrapping

Bootstrapping Theorem [Gen09]

– If you can homomorphically evaluate depth d circuits (you have a d-HE) and

– the depth of your decryption circuit < d

* FHE

Bootstrapping

“Homomorphic enough” Encryption FHE

Bootstrapping Theorem [Gen09]

d-HE with decryption depth < d * FHE

Bootstrapping = “Valve” at a fixed height

noise=0

noise=q/2

(that depends on decryption depth)

noise=Bdec

Say n(Bdec)2 < q/2

Bootstrapping: How

“Best Possible” Noise Reduction = Decryption!

Dec

CT SK

m

Decryption Circuit

“Very Noisy” ciphertext

“Noiseless ciphertext”

But the evaluatordoes not have SK!

Bootstrapping, Concretely

Next Best = Homomorphic Decryption!

EncPK(m)

Dec

CT EncPK(SK)

Assume Enc(SK) is public.

(OK assuming the scheme is “circular secure”)

*

Noise = Binput

Noise = Bdec

Bdec Independent of Binput

The Big Picture









STEP 1

STEP 2

STEP 3

Boosting Depth from log n to nε

(in one slide)

• The Culprit: Multiplication– Increases error from B to about B2

• Let us pause for a moment: Is B2 > B?– Not if B < 1!

• Why not scale ciphertexts by q and work over [0,1)?– Quite amazingly, this works out and gives us an error

growth of B → nB– Error grows singly exponentially with circuit depth

The Big Picture









STEP 1

STEP 2

STEP 3

Lattices are awesome!

BASIC CRYPTO [Ajtai’96,Ajtai-Dwork’97, Goldreich-Goldwasser-Halevi’97, Micciancio-Regev’04, Regev’05]

One-way functions, hash functions, public-key encryption

[Ajtai’99,Gentry-Peikert-V’08, Peikert-V-Waters’08]

Trapdoor functions, Identity-based Encryption, secure computation

[Gentry’09, Brakerski-V’11, Brakerski-Gentry-V’12]


[Gorbunov-V-Wee’13, Goldwasser-KP-V-Z’13]

Attribute-based and Functional Encryption

THIS TALK

[Garg-GHRSW’13] Program Obfuscation

ADVANCED CRYPTO

Merci Beaucoup!

Lattices, Cryptography and Computing with Encrypted Data Vinod Vaikuntanathan M.I.T.

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