Crypto Forum Research Group Rohit Khera
Internet-Draft Pivotal
Intended Category: Informational
Expires April 24 2018 October 24, 2017
Post-Quantum Key Exchange From Learning With Errors Over Rings
draft-khera-lpr-ring-lwe-kex-00
Abstract
This note describes a key exchange method based on the ring-LWE
(RLWE) assumption. It builds upon several results, including
Regev's landmark quantum reduction from certain worst case lattice
problems (approx. GapSVP and SIVP) to random instances of the
search variant of a particular learning problem (LWE). It also
builds on the follow on work of Lyubashevsky, Peikert and Regev on
the average case hardness of the RLWE search variant for
polynomially bounded numbers of RLWE samples, along with novel
applications of automorphism groups in number fields for a RLWE
search to decision reduction (thereby demonstrating
pseudorandomness of RLWE in these number fields). Subsequently,
these results were adopted for the construction of Diffie-Hellman
like key exchange methods by Peikert, and then by Lindner and
Peikert followed by Ding and then by Ding, Xie and Lin who proposed
efficient variants of such protocols. Subsequent work by Peikert
proposed another efficient variant, phrased as a key encapsulation
method, incorporating a low bandwidth "reconciliation" technique
allowing two parties to exactly agree on a uniformly distributed
secret value from noisy RLWE instances. This was followed by a
concrete instantiation with parameter sets by Bos, Costello,
Naehrig, Stebila, followed by another instantiation by Alkim,
Ducas, Poppelmann and Schwabe with the same ring polynomial but a
smaller modulus and a different reconciliation method. Unlike most
other public key cryptography based key exchange methods, it is
believed that RLWE based key exchange would remain secure in the
event that an adversary is able to build a quantum computer.
This document is a product of the Crypto Forum Research Group
(CFRG).
Status of this Memo
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provisions of BCP 78 and BCP 79. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
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Table of Contents
1. Introduction........................................3
2. Notation, Definitions and Operators.................4
2.1 Algebra and Number Theory
2.2 Set Notation
2.3 Random Variables and Distributions
2.4 Arithmetic
2.5 Miscellaneous
3. Error Distribution and Embeddings...................11
3.1 Sampling
4. Functions...........................................13
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4.1 Modular Rounding Function
4.2 Cross Rounding Function
4.3 Randomized Doubling Function
4.4 Reconciliation Function.
4.5 Extending to the rings OK_q and OK_2q
4.5.1 Extended Modular Rounding Function
4.5.2 Extended Cross Rounding Function
4.5.3 Extended Reconciliation Function
4.5.4 Extended Randomized Doubling Function
5. Flow Diagram........................................16
6. Efficient Polynomial Operations.....................16
7. Protocol............................................17
7.1 Server
7.1.1 Ring Element 'a'
7.1.2 Server Secret Key and Error
7.1.3 Server Public Key
7.1.4 Server Key and Parameter Exchange
7.1.5 Client Key and Cross Rounding Vector Receipt
7.1.6 Server Reconciliation and Key Derivation
7.2 Client
7.2.1 Client Secret Key and Error
7.2.2 Server Key and Parameter Receipt
7.2.3 Client Public Key
7.2.4 Client Doubling and Cross Rounding
7.2.5 Client Key and Cross Rounding Vector Exchange
7.2.6 Client Modular Rounding and Key Derivation
8. TLS Extensions....................................20
8.1 Fundamental RLWE Suites
8.2 Fundamental RLWE Key Exchange Algorithms
8.2.1 RLWE_RSA
8.2.2 RLWE_ECDSA
8.3 Fundamental TLS Extensions for RLWE
8.3.1 Fundamental RLWE Parameters Extension
8.3.2 Client Hello Extension
8.3.3 Server Hello Extension
8.3.4 Server Certificate
8.3.5 Server Key Exchange
8.3.6 Certificate Request
8.3.7 Client Certificate
8.3.8 Client Key Exchange
8.3.9 Certificate Verify
8.4 Hybrid TLS Extensions for RLWE
9. Authenticity.....................................28
10. Code Implementations and Licensing...............28
11. IANA Considerations..............................29
12. Security Considerations..........................29
12.1 Security Parameters
12.2 Ring Element 'a'
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12.3 Side Channels
13. References.......................................31
13.1 Normative References
13.2 Informative References
14. Acknowledgements................................35
15. Author's Address................................35
1. Introduction
Recent years have seen the development of key exchange protocols
from problems on lattices (discrete additive subgroups in vector
spaces). This note describes one such method based on the ring-LWE
problem. We start by describing some of the main results connecting
lattice problems with lattice based cryptosystems. Regev [Reg2005]
demonstrated a quantum reduction from certain worst case lattice
problems (approx. GapSVP and SIVP) to random instances of the
search variant of a particular learning problem (LWE). This means
that an oracle that returns the LWE secret vector from randomly
selected LWE instances implies an efficient quantum algorithm for
approximating lattice problems. Peikert demonstrated a classical
reduction from approx. GapSVP to decision LWE, partially de-
quantizing Regev's result [Pei2009a]. This reduction, however, does
not extend to approx. SIVP and requires a modulus exponential in
the lattice dimension. Lyubashevsky, Peikert and Regev [LPR2013a]
used Regev's quantum reduction along with aspects and tools from
algebraic number theory to provide a quantum reduction from approx.
SVP on ideal lattices (in the worst case) to random instances of
the search variant of RLWE. This was followed by a novel
application of automorphisms groups of cyclotomic fields for a
classical search to decision reduction, thereby proving the pseudo-
randomness of RLWE for sets of samples polynomially bounded in the
lattice dimension. In a companion publication [LPR2013b]
Lyubashevsky, Peikert and Regev provided a toolkit of algorithms
and techniques for applications.
The following is a description of work around the construction of
key exchange methods from the ring-LWE problem. Peikert in 2009
[Pei2009b] and Lidner and Peikert [LP2011] in 2011 proposed
cryptosystems and key exchange methods from both standard and RLWE
assumptions. Ding [DI2012] and Ding, Xie and Lin [DXL2012] proposed
optimized variants of such methods based on so called "robust
extractors" that allow two parties to recover the same value from
two closely separated ring elements. This was followed work by
Peikert [Pei2014] who presented a key encapsulation method
incorporating a low bandwidth "reconciliation" technique, which
once again, allowed two parties to exactly agree on a uniformly
distributed secret value from noisy RLWE instances. Subsequently
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Bos, Costello, Naehrig, Stebila [BCNS2014] published a concrete
instantiation with parameter sets, followed by another
instantiation by Alkim, Ducas, Poppelmann and Schwabe [ADPS2015]
with the same ring polynomial but a smaller modulus and a different
error distribution.
2. Notation, Definitions and Operators
2.1 Algebra and Number Theory
Q : The rational numbers.
Z : The integers.
Z/nZ : Integers modulo n.
Also written as Z_n.
R : The real numbers.
C : The complex numbers.
[K:F] : For a field K containing F,
the degree of
K/F as an extension of
fields. Equivalently the
dimension of K as a vector
space over F.
phi_m(x) : The m'th cyclotomic polynomial
This note uses the value
m = 2048
\zeta : An abstract root of phi_m(x).
\zeta_m : An m'th root of unity.
Q[X] : Polynomials with rational
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coefficients
Z[X] : Polynomials with integer coefficients
K = Q(\zeta) : A simple extension
generated over the rationals
by the algebraic
number \zeta, the root of an
irreducible polynomial over
the rationals. If zeta
is an abstract root of phi_m(x)
as defined above, then
Q(\zeta) is the cyclotomic
field of m'th roots of unity
and is \iso to the quotient
Q[X]/.
Power Basis : A basis for the field
Q(\zeta) when taken as a
vector space over Q,
consisting of the elements
{1, \zeta , \zeta^2 ,...,
\zeta^(n-1)} where
n = [Q(\zeta):Q] and
\zeta is the root of an
irreducible polynomial over Q.
In the case of the cyclotomic
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field K, the power basis is
also an integral basis for
the ring of algebraic integers
in K.
OK : The ring of algebraic integers
of the number field K. As
mentioned, the power basis of
K spans OK as a Z-module of
rank n and is an integral
basis for the full ring
of algebraic integers in K.
q : A large prime modulus such
that q = 1 mod 2n
where n = [Q(\zeta):Q].
This note uses the
value q = (2^31) - 1.
mod : The modular reduction operator
OK_q = OK/qOK : The quotient of OK mod the
ideal qOK.
OK^V : A fractional ideal, the
"trace product" dual of OK.
F^n : An n-dimensional vector space
over the field F. Alternatively,
if F is a ring, the F-module of
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rank n.
/x/^n in F^n : Denotes a vector in F^n. Written
in component form as
/x/^n = {x_1, x_2, ..., x_n}.
X_ : Cartesian product, A X_ B denotes
the Cartesian product of A and B.
M_:OK -> C^n : Minkowski embedding, also
called the canonical embedding,
comprising injective homomorphisms
from the ring OK to C^n where
n = [Q(\zeta):Q].
In this context, for
two positive integers d_1 and
d_2 such that n = d_1 + 2*d_2,
M_ is a map from OK to the space
R^d1 X_ C^(2*d_2) iso to C^n.
Coef-Embedding : Coefficient embedding. For an
element a_ in the ring OK, a
representation of a_ in a
polynomial basis of Q(\zeta).
2.2 Set Notation
{,} : Set brackets. {a,b,c}, for
example, denotes the set
consisting of the elements
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a,b and c.
[,) : right-open interval. As an
example, [a,b) for an integer
x denotes the set of integers
satisfying the inequalities
a <= x < b.
{0,1}^n : The set of n bit strings.
\union : The union of sets.
A \union B \union C,
for example, denotes the
union of sets A,B and C.
\intersect : The intersection of sets.
\in : Denotes set membership.
2.3 Random Variables and Distributions
x <- P_X() : Denotes the sampling of x in
the set X from the probability
distribution P_X() over X.
Normal_R(,s_) : The (centered) 1-D Gaussian
distribution over the reals
with parameter s_.
d-Normal_Z_q(,s_) : A discretization of
Normal_R(,s_) over
the ring Z_q.
Spherical_R^n(,s_) : The (centered) spherically
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symmetric n-dimensional
Gaussian distribution
with parameter s_ over
the vector space R^n.
d-Spherical_Z_q^n(,s_) : A discretization of
Spherical(,s_)_R^n over the
d-Spherical_OK_q(,s_) module (Z_q)^n.
This leads to the related
definition
d-Spherical_OK_q(,s_) over
the ring OK_q.
An element w in OK_q can
be written as a polynomial
in powers of X such that
w = w_0 + w_1*X + ...
+ w_(n-1)*X^(n-1) where
n = [Q(\zeta):Q].
To sample an element from
d-Spherical_OK_q(,s_), it
suffices to independently
sample the coefficients
w_0, ..., w_(n-1) from
d-Normal_Z_q(,s_).
Sampling an element w \in OK_q
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according to this method is
denoted as
w <- d-Spherical_OK_q(,s_).
d-Uniform_Z_q() : For x_ \in Z_q, the uniform
distribution over Z_q.
d-Uniform_Z_q^n() : The uniform
distribution over the module
d-Uniform_OK_q() (Z_q)^n.
This leads to the related
definition d-Uniform_OK_q()
over the ring OK_q.
An element w in OK_q can be
written as a polynomial in
powers of X such that
w = w_0 + w_1*X + ...
+ w_(n-1)*X^(n-1) where
n = [Q(\zeta):Q].
To sample an element from
d-Uniform_OK_q(), it
suffices to independently
sample the coefficients
w_0, ..., w_(n-1) from
d-Uniform_Z_q().
Sampling an element
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w \in OK_q according to this
method is denoted as
w <- Uniform_OK_q().
2.4 Arithmetic
* : a * b denotes the product of
a multiplied by b where a and b
are field or ring elements.
/ : a / b denotes the quotient
of a by b.
+ : a + b denotes the sum
of a and b.
- : a - b denotes the difference
of a and b.
2.5 Miscellaneous
floor(x) : Given a real number x, outputs
the nearest integer less than
or equal to x.
nint(x) : Given a real number x, outputs
floor(x + 1/2) with ties broken
upward.
3. Error Distribution and Embeddings
This section provides the rationale behind choice of the error
distribution, starting with Regev's reduction from approx. GapSVP
and SIVP to LWE [Reg2005], where the error is sampled from a
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discrete Gaussian. This is an approach that can be traced back to
Micciancio and Regev's [MR2004] utilization of Gaussian measures to
obtain tighter bounds on the approx. SVP to SIS reduction through
Gaussian sampling of offsets to random lattice points.
While the LWE error is a one dimensional Gaussian, RLWE requires
that error polynomials are sampled from n-dimensional Gaussians
over the Minkowski embedding of the ring OK. More precisely, the
literature ([LPR2013a]) defines an error distribution over the
space R^d1 X_ C^(2*d_2) (refer to the definition of the Minkowski
embedding in section 2) as a distribution over the tensor product
over Q of Q(\zeta) and the Reals (denoted K_R) . Further, in the
full description of the RLWE distribution, the work exploits the
fact that fractional ideals of Q(\zeta) canonically embed as
lattices. The RLWE secret is then drawn from a distribution over
the fractional ideal OK_V, defined as the dual of OK under a trace
product. Finally, the full RLWE instance is defined as a tuple over
(OK_q X_ K_R/qOK_V).
Certain RLWE applications, on the other hand, sample errors and
secret polynomials from distributions over the ring OK_q [BGV2011],
[GHS2011]. In the context of power of 2 cyclotomics, this leads to
a RLWE variant commonly known as polynomial LWE (PLWE Assumption -
Hermite Normal Form (HNF), [BV2011]).
In a similar vein, key exchange methods [Pei2014] employ the PLWE
variant by describing the ring-LWE instance as a tuple over (OK_q
X_ OK_q) where both the secret and the error are drawn from a
distribution over the ring OK_q and this is the variant considered
in this note.
When considering general RLWE hardness for search, the reductions
require a solution for any Gaussian error distribution. The
decision problem requires that Gaussian parameters are chosen at
random. Despite this fact, this note employs a discretized fixed
spherical Gaussian. Hardness can be established with such a
distribution if the adversary is constrained to have a bounded
number of samples [LPR2013a]. In practice, constructions including
the key exchange method specified here, comply with this
constraint.
3.1 Sampling
[BCNS2014] describe an adaptation of inverse transform sampling
[Dev1986] of errors from a Gaussian. [ADPS2015] on the other hand
sample errors from a centered binomial distribution partly owing
to its implementation simplicity. The binomial distribution is a
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suitable alternative for practical applications, however, this
note follows the [BCNS2014] implementation and specifies Gaussian
error. Though the [BCNS2014] implementation uses the inversion
method to transform uniform variates in [0,1] to Gaussian errors,
this note does not specify a sampling method and leaves it as
implementation choice.
4. Functions
4.1 Modular Rounding Function [Pei2014]
For the modulus q, an integer p that divides q, and for for x in
Z_q, define the modular rounding function modR_q_2(x) : Z_q ->
Z_2 as
modR_q_2(x) = nint((2/q) * x) mod 2
4.2 Cross Rounding Function [Pei2014]
For a modulus q, and for x in Z_q, define the cross rounding
function crossR_q_2(x) : Z_q -> Z_2 as
crossR_q_2(x) = floor((4/q) * x) mod 2
4.3 Randomized Doubling Function [Pei2014]
Sample e_ from the set {-1, 0, 1} such that
Pr(0) = 1/2 Pr(-1) = 1/4 Pr(1) = 1/4
Where Pr(x) is the probability of event x.
Define the function dbl(x) : Z_q -> Z_2q as
dbl(x) = 2x - e_
4.4 Reconciliation Function [Pei2014]
Define the following sets:
I_0 = {0,1,...,nint(q/2) -1 }
I_1 = {-floor(q/2),...,-1} mod q
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E = [-q/4,q/4) \intersect Z
For an element w_ in Z_2q and b \in {0,1}, define the function
rec_2q(w_,b) : Z_2q X_ Z_2 -> Z_2 as
{ 0 if w_ \in I_b + E mod 2q
rec_2q(w_,b) =
{ 1 otherwise
4.5 Extending to the rings OK_q and OK_2q
4.5.1 Extended Modular Rounding Function
The modular rounding function, previously defined as modR_q_2(x)
: Z_q -> Z_2 is extended to elements in OK_2q co-efficient wise
in a polynomial basis of OK_2q. That is for an element w in OK_2q
written as
w = w_0 + w_1*X + ... + w_(n-1)*X^(n-1)
where w_i \in Z_2q and n = [Q(\zeta):Q], the extended modular
rounding function is defined as
modR_OK_2q_2(w) = ( modR_2q_2(w_0), modR_2q_2(w_1), ... ,
modR_2q_2(w_(n-1)) ) : OK_2q -> {0,1}^n
4.5.2 Extended Cross Rounding Function
The cross rounding function, previously defined as crossR_q_2 :
Z_q -> Z_2 is extended to elements in OK_2q coefficient wise as
crossR_OK_2q_2(w) = ( crossR_2q_2(w_0), crossR_2q_2(w_1), ... ,
crossR_2q_2(w_(n-1)) ) : OK_2q -> {0,1}^n
For their arguments, both of these functions take an element in
OK_2q and output an n-bit vector.
4.5.3 Extended Reconciliation Function
The reconciliation function, previously defined as rec_2q(w_,b) :
Z_2q X_ Z_2 -> Z_2 is also extended to elements in OK_2q co-
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efficient wise in a polynomial basis of OK_2q. For an n-bit
vector b_vec containing coefficient wise cross rounding
information for an element in OK_2q:
b_vec = (b_0, b_1, ..., b_(n-1)),
the extended reconciliation function is defined as
rec_OK_2q(w,bvec) = ( rec_2q(w_0, b_0), rec_2q(w_1, b_1),..., \
rec_2q(w_(n-1), b_(n-1)) ) : (OK_2q X_ {0,1}^n) -> {0,1}^n
For its arguments, this function takes an element w in OK_2q and
the n-bit vector containing co-efficient wise cross rounding
information for w and returns an n-bit vector.
4.5.4 Extended Randomized Doubling Function
Finally, the randomized doubling function, previously defined as
dbl(x) : Z_q -> Z_2q is also extended to elements in OK_q co-
efficient wise in a polynomial basis of OK_q. For an element in
OK_q defined as
w = w_0 + w_1*X + ... + w_(n-1)*X^(n-1)
where w_i \in Z_2q and n = [Q(\zeta):Q], the doubling function
dbl_OK_q(w) is defined as
dbl_OK_q(w) = dbl(w_0) + dbl(w_1)*X ... + \
dbl(w_(n-1))*X^(n-1) : OK_q -> OK_2q
For its arguments, the extended doubling function takes an
element in OK_q and returns an element in OK_2q.
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5. Flow Diagram
Server Client
------ ------
a < d-Uniform_OK_q()
s_1, e_1 <- d-Spherical_OK_q(,s_)
s_2, e_2, e_3 <- d-Spherical_OK_q(,s_)
b = a*s_1 + e_1 -------> a,b
u = a*s_2 + e_2
v = b*s_2 + e_3
v_ = dbl(v)
v* = crossR_OK_2q_2(v_)
u,v* <--------
pms = rec_OK_2q(2*u*s_1,v*)
ms = KDF(pms)
pms = modR_OK_2q_2(v_)
ms = KDF(pms)
a,b, s_1, s_2, e_2, e_3, u and v are elements in OK_q, v_ is
an element in OK_2q, v* and pms (the pre-master secret) are both
1024 bit vectors, ms is the master secret. KDF() is a key
derivation function. In practice, it is expected that the key
derivation method specified in TLS 1.2 section 8.1 [RFC5426] will
be used (refer to subsequent section on TLS extensions). For an
alternate approach to generation of the parameter 'a' [ADPS2015]
refer the security section of this note.
Figure 1: Flow Diagram
6. Efficient Polynomial Operations
This note does not specify methods for efficient polynomial
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operations, this is a choice that is left to for implementations to
consider.The Number Theoretic Transform (NTT) has been used as a
technique for fast polynomial multiplication in rings of algebraic
integers ([PG2012], [PG2013], [ADPS2015]), and is therefore
particularly well suited for lattice based cryptography.
7. Protocol
7.1 Server
7.1.1 Ring Element 'a'
Server generates the ring element a by sampling from the the
uniform distribution over OK_q
a <- d-Uniform_OK_q()
This can be done by sampling each coefficient of a independently
from the uniform distribution over Z_q
7.1.2 Server Secret Key and Error
Server generates its ephemeral secret key s_1 and the error e_1
sampling the ring elements from the discrete spherical Gaussian
distribution over OK_q with parameter s_.
s_1 <- d-Spherical_OK_q(,s_)
e_1 <- d-Spherical_OK_q(,s_)
This can be done by sampling each coefficient of s_1 and e_1
independently from the discrete Gaussian distribution over Z_q
centered at 0 with parameter s_.
7.1.3 Server Public Key
Server computes its ephemeral public key b by multiplying the
element a and its ephemeral secret key s_1 and then adding the
error e_1
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b = a*b + e_1
7.1.4 Server Key and Parameter Exchange
Server returns the ring element a and the its ephemeral public
key b to the client
7.1.5 Client Key and Cross Rounding Vector Receipt
Server receives the client public key u along with cross rounding
information v* from the client
7.1.5 Server Reconciliation and Key Derivation
The server then computes h = 2*u*s_1 by multiplying the ephemeral
client public key with its ephemeral secret key and doubling the
result. The server then computes the premaster secret (pms) as:
pms = rec_OK_2q(h,v*)
An approved key derivation function can then be used for deriving
the master secret key. In practice, its expected that the key
derivation method specified in TLS 1.2 section 8.1 [RFC5426] will
be used.
7.2 Client
7.2.1 Client Secret Key and Error
The client generates its ephemeral secret key s_2 and the error
terms e_2 and e_3 by sampling the ring elements from the discrete
spherical Gaussian distribution over OK_q with parameter s_.
s_2 <- d-Spherical_OK_q(,s_)
e_2 <- d-Spherical_OK_q(,s_)
e_2 <- d-Spherical_OK_q(,s_)
This can be done by sampling each coefficient of s_2 and e_2 and
e_3 independently from the discrete Gaussian distribution over
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Z_q centered at 0 with parameter s_.
7.2.2 Server Key and Parameter Receipt
The client receives the parameter a from the server and the
ephemeral server public key b.
7.2.3 Client Public Key
The client computes its ephemeral public key u by multiplying
elements a and its ephemeral secret key s_2 and then adding the
error e_2.
u = a*s_2 + e_2
7.2.4 Client Doubling and Cross Rounding
Client computes the element v by multiplying the ephemeral server
public key b with its secret key s_2 and adding the error term
e_3.
v = b*s_2 + e_3
The client then applies the extended randomized doubling function
to v and obtains the element v_ in OK_2q:
v_ = dbl_OK_q(v)
The client then computes the cross rounding of v_ by applying the
extended cross rounding function to v_ and obtains the n-bit
vector v*
v* = CrossRound_OK_2q(v_)
7.2.5 Client Key and Cross Rounding Vector Exchange
The client returns its ephemeral public key u and the cross
rounding vector v* to the server.
7.2.6 Client Modular Rounding and Key Derivation
The client then applies the extended modular rounding function to
v_ and computes the premaster secret (pms) as:
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pms = modR_OK_2q_2(v_)
An approved key derivation function can then be used for deriving
the master secret key. In practice, its expected that the key
derivation method specified in TLS 1.2 section 8.1 [RFC5426] will
be used.
8. TLS Extensions
This section addresses authentication as it relates to key exchange
along with guidance for integration of this key exchange method
with the TLS protocol.
Authenticated key exchange (AKE) protocols allow two parties to
mutually establish ephemeral authenticated session keys for two way
communications. There is a body of literature in the area of
formalizing models for AKE, including the significant work of
Canetti and Krawczyk [CN2002) on demonstrating security of so
called "post-specified peer" SIGMA ("SIGn-and-MAc") protocols in
the simulation paradigm. Whereas protocols such as IKE achieve some
of these goals, the passive lattice based key exchange protocol
described here does not. The approach to address this deficiency is
to integrate it within the TLS handshake protocol. There are two
ways to do this, the first as a "fundamental" replacement for FFC /
ECC discrete log based classical key exchange methods [NIST-SP-800-
56A] which will be described below, and the second being a "hybrid"
approach that incorporates this lattice based method with discrete
log based key exchange (which is clearly the more conservative
approach that should be considered by implementations).
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Client Server
------ ------
ClientHello -------->
ServerHello
Certificate*
ServerKeyExchange*
CertificateRequest*+
<-------- ServerHelloDone
Certificate*+
ClientKeyExchange
CertificateVerify*+
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<-------- Finished
Application Data <-------> Application Data
* message is not sent under some conditions
+ message is not sent unless client authentication is desired
Figure 2: TLS handshake
8.1 Fundamental RLWE Suites
This note therefore follows after the guidance in [BCNS14] and
its associated implementation for TLS versions supporting AEAD
modes by specifying the following cipher suites. Also, this note
follows the blueprint for specifying TLS extensions set forth in
[RFC4492] ( which defines the TLS extensions for Elliptic Curve
cipher suites):
+-----------------------------------------+
| |
| TLS_RLWE_RSA_AES128_GCM_SHA256 |
| |
| TLS_RLWE_ECDSA_AES128_GCM_SHA256 |
| |
+-----------------------------------------+
Figure 3: Fundamental RLWE Cipher Suites
8.2 Fundamental RLWE Key Exchange Algorithms
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The following server key exchange algorithms are specified
8.2.1 RLWE_RSA
For this method the server's certificate must contain a RSA
capable signing key and be signed with RSA. The server sends its
ephemeral RLWE public key, the ring element 'a' and a
specification of the lattice parameters in the ServerKeyExchange
message. These parameters MUST be signed with RSA using the
private key corresponding to the public key in the server's
Certificate. The client generates an ephemeral RLWE public and
secret keys based on the server's lattice parameters and ring
element 'a' its ephemeral public key and the cross rounding
vector in the ClientKeyExchange message. The client then performs
the modular rounding operation whereas the server performs the
reconciliation operation (as described in the protocol section
above). Client and server then use the resultant shared secret as
the premaster secret.
8.2.2 RLWE_ECDSA
This key exchange algorithm is the same as ECDH_RSA except that
the server's certificate MUST be signed with ECDSA rather than
RSA.
8.3 Fundamental TLS extensions for RLWE
A single new TLS extension is specified in this section, called
the Fundamental RLWE Parameter Set extension (which specifies
the ring polynomial, the modulus q, the distribution type,
standard deviation and expected value, see section immediately
below for more details).
Though a single set of parameters is described here, in the
future these extensions should allow for negotiating other
lattice parameters. The client should enumerate the lattice
parameters it supports in its ClientHello message. The server
should in a similar way enumerate the lattice parameters it
supports in its ServerHello message. A TLS client that proposes
RLWE cipher suites in its ClientHello message SHOULD include this
extension. Servers implementing RLWE cipher suites MUST support
this extension, and when a client uses this extension, servers
MUST NOT negotiate the use of a RLWE cipher suite unless they can
complete the handshake under the choice of RLWE parameters
supported by the client. The client MUST NOT include this
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extension in the ClientHello message if it does not propose any
RLWE cipher suites.
8.3.1 Fundamental RLWE Parameters Extension
enum {
sec_cyclotomic_2048 (1), reserved (..)
} RingPolynomial;
enum {
sec_mod_2_32_1 (1), reserved (..)
} Modulus;
enum {
sec_gaussian (1), reserved (..)
} Distribution;
enum {
sec_std_8_Sqrt_2_by_Pi (1), reserved (..)
} StandardDeviation
enum {
sec_ev_0 (1), reserved (..)
} ExpectedValue;
struct {
RingPolynomial poly;
Modulus mod;
Distribution dist;
StandardDeviation std;
ExpectedValue ev;
} FundamentalRLWEParamSet;
struct { FundamentalRLWEParamSet paramSetList<1..2^8-1>;
} FundamentalRLWEParamSetList;
In this version of the note, the only valid instantiation of the
struct FundamentalRLWEParamSet is the following:
struct FundamentalRLWEParamSet params =
{ sec_cyclotomic_2048, sec_mod_2_32_1, sec_gaussian, \
sec__std_8_Sqrt_2_by_Pi, sec_ev_0 };
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Implementations MUST ensure that passed parameters agree with the
above. The above namespace will need to be maintained by IANA.
8.3.2 Client Hello Extension
This section specifies the TLS extension that can be included
with the ClientHello message for RLWE based key exchange. This
extension is the Fundamental RLWE Parameter Set extension.
When this extension is sent:
The extension SHOULD be sent along with any ClientHello message
that proposes RLWE cipher suites. The extension allows a client
to enumerate the RLWE parameter sets that it supports. The
general structure of TLS extensions is described in [RFC4366],
and this note specification adds the following to ExtensionType.
enum { FundamentalRLWE(..) } ExtensionType;
FundamentalRLWE (Fundamental RLWE Parameter Set extension):
Indicates the Fundamental RLWE parameters supported by the
client. For this extension, the opaque extension_data field
contains the FundamentalRLWEParamSetList. See the preceding
section for details.
Actions of the sender:
A client that proposes RLWE cipher suites in its ClientHello
message appends this extension, In the current version of this
note, clients clients SHOULD only send the single parameter set
outlined in the preceding section.
Actions of the receiver:
A server that receives a ClientHello containing this extension
MUST use the client's enumerated capabilities to guide its
selection of an appropriate cipher suite. One of the proposed
RLWE cipher suites must be negotiated only if the server can
successfully complete the handshake while using the Fundamental
RLWE parameter set supported by the client.
8.3.3 Server Hello Extension
This section specifies a TLS extension that can be included with
the ServerHello message for RLWE based key exchange (the
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Fundamental RLWE Parameter Set Extension).
When this extension is sent:
The Fundamental RLWE Parameter Set Extension is included in a
ServerHello message in response to a ClientHello message
containing the Fundamental RLWE Parameter Set Extension when
negotiating an RLWE cipher suite.
Meaning of this extension:
This extension allows a server to enumerate RLWE parameters that
it supports for the parameter and key material exchanges in its
ServerKeyExchange message.
Structure of this extension:
The server's Fundamental RLWE Parameter Set Extension has the
same structure as the client's Fundamental RLWE Parameter Set
Extension (see preceding section).
Actions of the sender:
A server that selects a RLWE cipher suite in response to a
ClientHello message (that includes a Fundamental RLWE Parameter
Set) and appends this extension to its ServerHello message.
Actions of the receiver:
A client that receives a ServerHello message containing a
Fundamental RLWE Parameter Set Extension MUST respect the
server's choice of RLWE parameters.
8.3.4 Server Certificate
This message is sent for all non-anonymous Fundamental RLWE key
exchange algorithms. No additional or modified processing is
required for this message for the fundamental RLWE key exchange
methods described here.
8.3.5 Server Key Exchange
When this message is sent:
This message is sent when using the RLWE_ECDSA and RLWE_RSA key
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exchange algorithms. This message is used to convey the (i) the
fundamental RLWE parameter set (ii) The public ring element 'a'
(refer to preceding sections on protocol flow and description),
along with the server's ephemeral public key b.
struct {
FundamentalRLWEParamSet params;
opaque a <2^12>;
opaque b <2^12>
} ServerFundamentalRLWEParams;
The ServerKeyExchange message is extended as follows:
enum { fundamental_RLWE_kex } KeyExchangeAlgorithm;
This indicates the ServerKeyExchange message contains the ring
element 'a' and the ephemeral server RLWE public key.
select (KeyExchangeAlgorithm) {
case fundamental_RLWE_kex:
ServerFundamentalRLWEParams params;
Signature signed_params;
} ServerKeyExchange;
params: Specifies the server ephemeral RLWE public key, the
ring element 'a' and associated fundamental RLWE parameters
signed_params: Consists of a hash of the params, along with the
appropriate signature corresponding to the key exchange method
(i.e. RLWE_RSA or RLWE_ECDSA). The private key corresponding to
the certified public key in the server's Certificate message is
used for signing.
Actions of the sender:
The server selects the Fundamental RLWE parameters (see prior
section for a description of valid parameters), ephemeral public
key and the ring element 'a' and conveys this information to the
client in the ServerKeyExchange message using the format defined
above.
Actions of the receiver:
The client verifies the signature and retrieves the server's RLWE
parameters, ephemeral public key and the ring element 'a' from
the ServerKeyExchange message.
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8.3.6 Certificate Request
This message sent when requesting client authentication. No
additional or modified processing is required for this message
for the Fundamental RLWE key exchange methods described here.
8.3.7 Client Certificate
This message is sent in response to a CertificateRequest. No
additional or modified processing is required for this message
for the fundamental RLWE key exchange methods described here.
8.3.8 Client Key Exchange
This message contains the client's ephemeral RLWE public key and
cross rounding information.
Meaning of the message: This message is used to convey ephemeral
data relating to the key exchange belonging to the client.
Structure of this message: The TLS ClientKeyExchange message is
extended as follows.
struct {
FundamentalRLWEParamSet params;
opaque u <2^12>;
opaque cr <2^7>;
} ClientFundamentalRLWEParams;
This message contains the client's ephemeral RLWE public key and
cross rounding information.
struct {
select (KeyExchangeAlgorithm) {
case fundamental_RLWE_kex:
ClientFundamentalRLWEParams;
} exchange_keys;
} ClientKeyExchange;
Actions of the sender:
The client generates its ephemeral public key upon receipt of the
parameter 'a' from the server and sends this along with the cross
rounding vector to the server.
Actions of the receiver:
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The server retrieves the client's ephemeral RLWE public key from
the ClientKeyExchange message and ensures that it is consistent
with the agreed upon Fundamental RLWE parameters.
8.3.9 Certificate Verify
Pending
8.4 Hybrid RLWE Suites
This section, which is pending, will specify TLS extensions for
the use of RLWE key exchange in conjunction with FFC/ECC discrete
log based key exchange.
The hybrid method will be the preferred key exchange method
specified in this note.
+----------------------------------------------------+
| |
| TLS_RLWE_DHE_RSA_AES128_GCM_SHA256 |
| |
| TLS_RLWE_ECDHE_RSA_AES128_GCM_SHA256 |
| |
| TLS_RLWE_DHE_ECDSA_AES128_GCM_SHA256 |
| |
| TLS_RLWE_ECDHE_ECDSA_AES128_GCM_SHA256 |
| |
+----------------------------------------------------+
Figure 4: Hybrid RLWE Cipher Suites
9. Authenticity
As discussed in the previous section on TLS integration, the
proposed approach relies on classical signature schemes RSA and
ECDSA [FIPS186-4] for authenticity. For compatibility with 128 bit
security, implementations should use 3072 RSA and ECDSA on the nist
curve p256.
10. Code Implementations and Licensing
The following code is the closest code implementation of the method
described in this note and is attributed to [BCNS14]:
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https://github.com/dstebila/rlwekex The licensing terms are from:
http://unlicense.org
An OpenSSL fork incorporating RLWE key exchange is available here
under the terms of the OpenSSL license:
https://github.com/dstebila/openssl-rlwekex/tree/OpenSSL_1_0_1-
stable
The availability of [ADPS2015] implementation is also noted. This
implementation utilizes a smaller modulus and a different
reconciliation method, and is available under a public license
from: https://github.com/tpoeppelmann/newhope and from:
https://cryptojedi.org/crypto/#newhope
11. IANA Considerations
IANA is required to maintain the namespaces for the following TLS
extensions:
1) FundamentalRLWEParamSet
2) ServerFundamentalRLWEParams
3) ClientFundamentalRLWEParams
4) Pending - namespaces for hybrid key exchange
12. Security Considerations
The security of the key exchange method described here stems from a
quantum reduction from approx. SVP on ideal lattices in the worst
case to random instances of the search RLWE problem. The
pseudorandomness of RLWE has been proven through a classical search
to decision reduction for Galois number fields. The reader is
referred to the introduction of this document as well as the
section on error distributions for more details and for citations
to the relevant literature.
The security of the RLWE instantiations depend on the choice of
error distribution. In particular, security with a fixed spherical
error has been established for bounded numbers of RLWE samples, a
constraint that is achieved in the key exchange method described
here. The reader is referred to the introduction and the section on
error distributions for more details.
Incorrect choice of parameters and errors can lead to vulnerable
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RLWE instantiations. Rather than state the nature and cryptanalysis
of such instantiations, the reader is referred to the literature on
this subject such as [Pei2016a].
This key exchange method relies on the public ring element a_
(refer to description of the high level flow). Rather than specify
this to be a global, fixed parameter, this note takes the approach
of sampling this element uniformly from the ring OK/qOK for every
key exchange.
12.1 Security Parameters
In order to fully describe the RLWE distribution used in this key
exchange method, the ring polynomial, modulus "q" and the 1-D
Gaussian standard deviation and expected value need to be
defined. In order to do so, this note adopts the following
parameter sets from [BCNS2014]:
+---------------------------------------------+
| |
| Ring polynomial : 2048th cyclotomic |
| |
| Modulus q : (2^31) -1 |
| |
| Error distribution : Gaussian |
| |
| Guassian parameter (s_): 8 / Sqrt(2*Pi) |
| |
| Expected value : 0 |
| |
+---------------------------------------------+
Figure 5: Parameter Sets
These parameters provide a security of 128 bits against classical
attacks as described in the literature ([LP2011],[BCNS2014]).
12.2 Ring Element 'a'
This key exchange method relies on the public ring element a_
(refer to description of the high level flow). Rather than
specify this to be a global, fixed parameter, in this note, the
server samples this element uniformly from the ring OK/qOK for
every key exchange and passes this element to the client (in the
TLS setting this is done in the Server Key Exchange message). It
is noted that [ADPS2015] on the other hand use the approach where
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one of the parties samples a seed and generates a by application
of the SHAKE-128 function. This is an approach that may be
considered in a future version of this draft.
12.3 Side Channels
This note treats certain computational aspects such as efficient
polynomial multiplication as an implementation choice. In a
similar vein, it is expected that implementations make their own
determination around countermeasures against side channel
attacks.
13. References
13.1 Normative References
[FIPS186-4] FIPS pub. 186-4, "Federal Information Processing
Standards Publication, Digital Signature Standard (DSS)"
[NIST-SP-800-56A] NIST Special Publication 800-56A Revision 2,
"Recommendation for Pair-Wise Key Establishment Schemes Using
Discrete Logarithm Cryptography"
[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and
B. Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites for
Transport Layer Security (TLS)", RFC 4492, May 2006.
[RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen,
J., and T. Wright, "Transport Layer Security (TLS) Extensions",
RFC4366, April 2006.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434, October
1998.
[RFC4506] Eisler, M., "XDR: External Data Representation
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Standard", RFC 4506, May 2006.
[RFC5246] T. Dierks and E. Rescorla. The Transport Layer Security
(TLS) Protocol Version 1.2. RFC 5246 (Proposed Standard)
13.2 Informative References
[ADPS2015] Erdem Alkim and Leo Ducas and Thomas Poppelmann and
Peter Schwabe, " Post-quantum key exchange - a new hope", IACR
Cryptology ePrint Archive report 2015/1092, 2015
[BCNS2014] Joppe W. Bos and Craig Costello and Michael Naehrig and
Douglas Stebila, "Post-quantum key exchange for the TLS protocol
from the ring learning with errors problem", Cryptology ePrint
Archive, Report 2014/599, 2014.
[BGV2011] Zvika Brakerski and Craig Gentry and Vinod
Vaikuntanathan, "Fully Homomorphic Encryption without
Bootstrapping", Cryptology ePrint Archive, Report 2011/277
[BV2011] Brakerski, Zvika and Vaikuntanathan, Vinod, "Fully
Homomorphic Encryption from Ring-LWE and Security for Key Dependent
Messages", Advances in Cryptology -- CRYPTO 2011 Ed. Rogaway,
Phillip, Springer Berlin Heidelberg pages 505 - 524
[Can2000] Ran Canetti, "Universally Composable Security: A New
Paradigm for Cryptographic Protocols", Cryptology ePrint Archive,
Report 2000/067, 2000, http://eprint.iacr.org/2000/067
[CN2002] Ran Canetti and Hugo Krawczyk, "Security Analysis of IKE's
Signature-based Key-Exchange Protocol", Cryptology ePrint Archive,
Report 2002/120, 2002, http://eprint.iacr.org/2002/120
[Dev1986] L. Devroye, "Non-Uniform Random Variate Generation",
Springer-Verlag, New York (1986) Available from:
http://www.nrbook.com/devroye/
[DI2012] Jintai Ding, "A simple provably secure key exchange scheme
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based on the learning with errors problem". IACR Cryptology ePrint
Archive report 2012/688.
[DXL2012] Jintai Ding, Xiang Xie, Xiaodong Lin, "A Simple Provably
Secure Key Exchange Scheme Based on the Learning with Errors
Problem", Cryptology ePrint Archive Report 2012/688, 2012.
[GHS2011] Craig Gentry and Shai Halevi and Nigel P. Smart, "Fully
Homomorphic Encryption with Polylog Overhead", Cryptology ePrint
Archive, Report 2011/566
[LP2011] "Lindner, Richard and Peikert, Chris", "Better Key Sizes
(and Attacks) for LWE-Based Encryption", In proceedings "Topics in
Cryptology -- CT-RSA 2011: The Cryptographers' Track" at the RSA
Conference 2011, San Francisco, CA, USA, February 14-18, 2011.
Proceedings", 2011, Ed. Kiayias, Aggelos, Springer Berlin
Heidelberg, pgs 319--339
[LPR2013a] Lyubashevsky, Vadim and Peikert, Chris and Regev, Oded,
"On Ideal Lattices and Learning with Errors over Rings". J. ACM,
November 2013 Vol.60/6, 2013.
[LPR2013b] Vadim Lyubashevsky and Chris Peikert and Oded Regev, "A
Toolkit for Ring-LWE Cryptography". Cryptology ePrint Archive,
Report 2013/293}, 2013.
[MR2004] Daniele Micciancio, Oded Regev, "Worst-Case to Average-
Case Reductions Based on Gaussian Measures", vol. 00, pp. 372-381,
2004, FOCS.2004.72
[Pei2009a] Chris Peikert, "Public-key cryptosystems from the worst-
case shortest vector problem". In STOC, pages 333-342. 2009
[Pei2009b] Chris Peikert, "Public-key cryptosystems from the worst-
case shortest vector problem, 2009".
https://web.eecs.umich.edu/~cpeikert/pubs/svpcrypto.pdf
[Pei2014] Chris Peikert} "Lattice Cryptography for the Internet",
Cryptology ePrint Archive Report 2014/070, 2014
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[Pei2016a] Chris Peikert, Chris,,"How (Not) to Instantiate Ring-
LWE", from "Security and Cryptography for Networks: 10th
International Conference, SCN 2016, Proceedings", Ed. Zikas,
Vassilis and De Prisco, Roberto, Springer International Publishing
pages 411 - 430
[Pei2016b] Chris Peikert, "A Decade of Lattice Cryptography",
Cryptology ePrint Archive, Report 2015/939, 2015
[PG2012] Poppelmann, Thomas and Guneysu, Tim, "Towards Efficient
Arithmetic for Lattice-Based Cryptography on Reconfigurable
Hardware", Ed. Hevia, Alejandro and Neven, Gregory", 2012, Springer
Berlin Heidelberg
[PG2013] Poppelmann, Thomas and Guneysu, Tim, "Towards Practical
Lattice-Based Public-Key Encryption on Reconfigurable Hardware", in
Selected Areas in Cryptography -- SAC 2013: 20th International
Conference, Burnaby, BC, Canada, August 14-16, 2013, Revised
Selected Papers, Ed. Lange, Tanja, and Lauter, Kristin and
Lisonvek, Petr, 2014, Springer Berlin Heidelberg
[Reg2005] Regev, O., "On lattices, learning with errors, random
linear codes , and cryptography". In Proc. 37th ACM Symp. on Theory
of Computing (STOC), pages 84-93 (2005).
14. Acknowledgements
The author would like to thank Pivotal for support in putting
together this draft. The author would also like to thank Eric Malm
for insightful discussions and perspectives into the relevant
aspects of algebra and algebraic number theory.
15. Author's Address
Rohit Khera
Pivotal
875 Howard St.
San Francisco
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CA 94103
EMail: rkhera@pivotal.io
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