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lwip                                                           R. Struik
Internet-Draft                               Struik Security Consultancy
Intended status: Informational                          October 30, 2017
Expires: May 3, 2018


               Alternative Elliptic Curve Representations
               draft-struik-lwip-curve-representations-00

Abstract

   This document specifies how to represent Montgomery curves and
   (twisted) Edwards curves as curves in short-Weierstrass form and
   illustrates how this can be used to implement elliptic curve
   computations using existing implementations that already implement,
   e.g., ECDSA and ECDH using NIST prime curves.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in RFC
   2119 [RFC2119].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 3, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents



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   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Fostering Code Reuse with New Elliptic Curves . . . . . . . .   2
   2.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   3
   4.  Normative References  . . . . . . . . . . . . . . . . . . . .   3
   Appendix A.  Some (non-Binary) Elliptic Curves  . . . . . . . . .   4
     A.1.  Curves in short-Weierstrass Form  . . . . . . . . . . . .   4
     A.2.  Montgomery Curves . . . . . . . . . . . . . . . . . . . .   4
     A.3.  Twisted Edwards Curves  . . . . . . . . . . . . . . . . .   4
   Appendix B.  Elliptic Curve Group Operations  . . . . . . . . . .   5
     B.1.  Group Law for Weierstrass Curves  . . . . . . . . . . . .   5
     B.2.  Group Law for Montgomery Curves . . . . . . . . . . . . .   5
     B.3.  Group Law for Twisted Edwards Curves  . . . . . . . . . .   6
   Appendix C.  Relationship Between Curve Models  . . . . . . . . .   6
     C.1.  Mapping between twisted Edwards Curves and Montgomery
           Curves  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     C.2.  Mapping between Montgomery Curves and Weierstrass Curves    7
     C.3.  Mapping between twisted Edwards Curves and Weierstrass
           Curves  . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Appendix D.  Curve25519 and Cousins . . . . . . . . . . . . . . .   8
     D.1.  Curve Definition and Alternative Representations  . . . .   8
     D.2.  Switching between Alternative Representations . . . . . .   8
     D.3.  Domain Parameters . . . . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Fostering Code Reuse with New Elliptic Curves

   It is well-known that elliptic curves can be represented using
   different curve models.  Recently, IETF has standardized elliptic
   curves that are claimed to have better performance and improved
   robustness against "real world" attacks than curves represented in
   the traditional "short" Weierstrass model.  This draft specifies an
   alternative representation of points on Curve25519, as specified in
   RFC 7748, and of points on Ed25519, as specified in RFC 8032, as
   points on a specific "short" Weierstrass curve, called Wei25519.

   Use of Wei25519 allows easy definition of signature schemes and key
   agreement schemes already specified for traditional NIST prime
   curves, thereby allowing easy integration with existing



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   specifications, such as NIST SP 800-56a and FIPS Pub 186-4, and ANSI
   X9.63-2005, and fostering code reuse on platforms that already
   implement some of these schemes.

   For details, we refer to the appendices.

2.  Security Considerations

   The different representations of elliptic curve points discussed in
   this draft are all obtained using a publicly known transformation.
   Since this transformation is an isomorphism, this transformation maps
   elliptic curve points to equivalent mathematical objects.

3.  IANA Considerations

   There is no IANA action required for this document.

4.  Normative References

   [ANSI-X9.62]
              ANSI X9.62-2005, "Public Key Cryptography for the
              Financial Services Industry: The Elliptic Curve Digital
              Signature Algorithm (ECDSA)", American National Standard
              for Financial Services, Accredited Standards Committee X9,
              Inc Anapolis, MD, 2005.

   [FIPS-186-4]
              FIPS 186-4, "Digital Signature Standard (DSS), Federal
              Information Processing Standards Publication 186-4", US
              Department of Commerce/National Institute of Standards and
              Technology Gaithersburg, MD, July 2013.

   [I-D.ietf-6lo-ap-nd]
              Sarikaya, B., Thubert, P., and M. Sethi, "Address
              Protected Neighbor Discovery for Low-power and Lossy
              Networks", draft-ietf-6lo-ap-nd-03 (work in progress),
              September 2017.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC7748]  Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
              for Security", RFC 7748, DOI 10.17487/RFC7748, January
              2016, <https://www.rfc-editor.org/info/rfc7748>.





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   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <https://www.rfc-editor.org/info/rfc8032>.

   [SP-800-56a]
              NIST SP 800-56a, "Recommendation for Pair-Wise Key
              Establishment Schemes Using Discrete Log Cryptography,
              Revision 2", US Department of Commerce/National Institute
              of Standards and Technology Gaithersburg, MD, June 2013.

Appendix A.  Some (non-Binary) Elliptic Curves

A.1.  Curves in short-Weierstrass Form

   Let GF(q) denote the finite field with q elements, where q is an odd
   prime power and where q is not divisible by three.  Let W_{a,b} be
   the Weierstrass curve with defining equation y^2 = x^3 + a*x + b,
   where a and b are elements of GF(q) and where 4*a^3 + 27*b^2 is
   nonzero.  The points of W_{a,b} are the ordered pairs (x, y) whose
   coordinates are elements of GF(q) and which satisfy the defining
   equation (the so-called affine points), together with the special
   point O (the so-called "point at infinity").This set forms a group
   under addition, via the so-called "chord-and-tangent" rule, where the
   point at infinity serves as the identity element.  See Appendix B.1
   for details of the group operation.

A.2.  Montgomery Curves

   Let GF(q) denote the finite field with q elements, where q is an odd
   prime power.  Let M_{A,B} be the Montgomery curve with defining
   equation B*v^2 = u^3 + A*u^2 + u, where A and B are elements of GF(q)
   with A unequal to (+/-)2 and with B nonzero.  The points of M_{A,B}
   are the ordered pairs (u, v) whose coordinates are elements of GF(q)
   and which satisfy the defining equation (the so-called affine
   points), together with the special point O (the so-called "point at
   infinity").This set forms a group under addition, via the so-called
   "chord-and-tangent" rule, where the point at infinity serves as the
   identity element.  See Appendix B.2 for details of the group
   operation.

A.3.  Twisted Edwards Curves

   Let GF(q) denote the finite field with q elements, where q is an odd
   prime power.  Let E_{a,d} be the twisted Edwards curve with defining
   equation a*x^2 + y^2 = 1+ d*x2*y2, where a and d are distinct nonzero
   elements of GF(q).  The points of E_{a,d} are the ordered pairs (x,
   y) whose coordinates are elements of GF(q) and which satisfy the



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   defining equation (the so-called affine points).  It can be shown
   that this set forms a group under addition if a is a square in GF(q),
   whereas d is not, where the point (0, 1) serves as the identity
   element.  (Note that the identity element satisfies the defining
   equation.)  See Appendix B.3 for details of the group operation.  An
   Edwards curve is a twisted Edwards curve with a=1.

Appendix B.  Elliptic Curve Group Operations

B.1.  Group Law for Weierstrass Curves

   For each point P on the Weierstrass curve W_{a,b}, the point at
   infinity O serves as identity element, i.e., P + O = O + P = P.

   For each point P:=(x, y) on the Weierstrass curve W_{a,b}, the point
   -P is the point (x, -y) and one has P + (-P) = O.

   Let P1:=(x1, y1) and P2:=(x2, y2) be distinct points on the
   Weierstrass curve W{a,b} and let Q:=P1 + P2, where Q is not the
   identity element.  Then Q:=(x, y), where

       x + x1 + x2 = lambda^2 and y + y1 = lambda*(x1 - x), where lambda
       = (y2 - y1)/(x2 - x1).

   Let P:= (x1, y1) be a point on the Weierstrass curve W{a,b} and let
   Q:=2P, where Q is not the identity element.  Then Q:= (x, y), where

       x + 2*x_1 = lambda^2 and y + y1 = lambda*(x1 - x), where
       lambda=(3*x1^2 + a)/(2*y1).

B.2.  Group Law for Montgomery Curves

   For each point P on the Montgomery curve M_{A,B}, the point at
   infinity O serves as identity element, i.e., P + O = O + P = P.

   For each point P:=(x, y) on the Montgomery curve M_{A,B}, the point
   -P is the point (x, -y) and one has P + (-P) = O.

   Let P1:=(x1, y1) and P2:=(x2, y2) be distinct points on the
   Montgomery curve M_{A,B} and let Q:=P1 + P2, where Q is not the
   identity element.  Then Q:=(x, y), where

       x + x1 + x2 = B*lambda^2 - A and y + y1 = lambda*(x1 - x), where
       lambda=(y2 - y1)/(x2 - x1).

   Let P:= (x1, y1) be a point on the Montgomery curve M_{A,B} and let
   Q:=2P, where Q is not the identity element.  Then Q:= (x, y), where




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       x + 2*x_1 = B*lambda^2 - A and y + y1 = lambda*(x1 - x), where
       lambda=(3*x1^2 + 2*A*x_1+1)/(2*y1).

B.3.  Group Law for Twisted Edwards Curves

   Note: The group laws below hold for twisted Edwards curves E_{a,d}
   where a is a square in GF(q), whereas d is not.  In this case, the
   addition formula below are defined for each pair of points, without
   exceptions.  Generalizations of this group law to other twisted
   Edwards curves are out of scope.

   For each point P on the twisted Edwards curve E_{a,d}, the point
   O=(0,1) serves as identity element, i.e., P + O = O + P = P.

   For each point P:=(x, y) on the twisted Edwards curve E_{a,d}, the
   point -P is the point (-x, y) and one has P + (-P) = O.

   Let P1:=(x1, y1) and P2:=(x2, y2) be points on the twisted Edwards
   curve E_{a,d} and let Q:=P1 + P2.  Then Q:=(x, y), where

       x = (x1*y2 + x2*y1)/(1 + d*x1*x2*y1*y2) and y = (y1*y2 -
       a*x1*x2)/(1 - d*x1*x2*y1*y2).

   Let P:=(x1, y1) be a point on the twisted Edwards curve E_{a,d} and
   let Q:=2P.  Then Q:=(x, y), where

       x = (2*x1*y1)/(1 + d*x1^2*y1^2) and y = (y1^2 - a*x1^2)/(1 -
       d*x1^2*y1^2).

Appendix C.  Relationship Between Curve Models

   The non-binary curves specified in Appendix A are expressed in
   different curve models, viz. as curves in short-Weierstrass form, as
   Montgomery curves, or as twisted Edwards curves.  These curve models
   are related, as follows.

C.1.  Mapping between twisted Edwards Curves and Montgomery Curves

   One can map points of the Montgomery curve M_{A,B} to points of the
   twisted Edwards curve E_{a,d}, where a:=(A+2)/B and d:=(A-2)/B and,
   conversely, map points of the twisted Edwards curve E_{a,d} to points
   of the Montgomery curve M_{A,B}, where A:=2(a+d)/(a-d) and where
   B:=4/(a-d).  For twisted Edwards curves we consider (i.e., those
   where a is a square in GF(q), whereas d is not), this defines a one-
   to-one correspondence, which - in fact - is an isomorphism between
   M_{A,B} and E_{a,d}, thereby showing that, e.g., the discrete
   logarithm problem in either curve model is equally hard.




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   For the Montgomery curves and twisted Edwards curves we consider, the
   mapping from M_{A,B} to E_{a,d} is defined by mapping the point at
   infinity O and the point (0, 0) of order two on M_{A,B} to,
   respectively, the point (0, 1) and the point (0, -1) of order two of
   E_{a,d}, while mapping each other point (u, v) on M_{A,B} to the
   point (x, y):=(u/v, (u-1)/(u+1)) on E_{a,d}. The inverse mapping from
   E_{a,d} to M_{A,B} is defined by mapping the point (0, 1) and the
   point (0, -1) of order two of E_{a,d} to, respectively, the point at
   infinity O and the point (0, 0) of order two on M_{A,B}, while each
   other point (x, y) of E_{a,d} is mapped to the point (u,
   v):=((1+y)/(1-y), (1+y)/((1-y)x)) of M_{A,B}.

   Implementations may take advantage of this mapping to carry out
   elliptic curve group operations originally defined for a twisted
   Edwards curve on the corresponding Montgomery curve, or vice-versa,
   and translating the result back to the original curve, thereby
   potentially allowing code reuse.

C.2.  Mapping between Montgomery Curves and Weierstrass Curves

   One can map points on the Montgomery curve M_{A,B} to points on the
   Weierstrass curve W_{a,b}, where a:=(3-A^2)/(3*B^2) and
   b:=(2*A^3-9*A)/(27*B^3).  This defines a one-to-one correspondence,
   which - in fact - is an isomorphism between M_{A,B} and W_{a,b},
   thereby showing that, e.g., the discrete logarithm problem in either
   curve model is equally hard.

   The mapping from M_{A,B} to W_{a,b} is defined by mapping the point
   at infinity O on M_{A,B} to the point at infinity O on W_{a,b}, while
   mapping each other point (u, v) of M_{A,B} to the point (x, y):=(u/
   B+A/3B, v/B) of W_{a,b}. Note that not all Weierstrass curves can be
   injectively mapped to Montgomery curves, since the latter have a
   point of order two and the former may not.  In particular, if a
   Weierstrass curve has prime order, such as is the case with the so-
   called "NIST curves", this inverse mapping is not defined.

   This mapping can be used to implement elliptic curve group operations
   originally defined for a twisted Edwards curve or for a Montgomery
   curve using group operations on the corresponding elliptic curve in
   short-Weierstrass form and translating the result back to the
   original curve, thereby potentially allowing code reuse.  Note that
   implementations for elliptic curves with short-Weierstrass form that
   hard-code the domain parameter a to a= -3 (which value is known to
   allow more efficient implementations) cannot always be used this way,
   since the curve W_{a,b} may not always be expressed in terms of a
   Weierstrass curve with a=-3 via a coordinate transformation.





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C.3.  Mapping between twisted Edwards Curves and Weierstrass Curves

   One can map points of the twisted Edwards curve E_{a,d} to points of
   the Weierstrass curve W_{a,b}, via function composition, where one
   uses the isomorphic mapping between twisted Edwards curve and
   Montgomery curves of Appendix C.1 and the one between Montgomery and
   Weierstrass curves of Appendix C.2.  Obviously, one can use function
   composition (now using the respective inverses) to realize the
   inverse of this mapping.

Appendix D.  Curve25519 and Cousins

D.1.  Curve Definition and Alternative Representations

   The elliptic curve Curve25519 is the Montgomery curve M_{A,B} defined
   over the prime field GF(p), with p:=2^{255}-19, where A:=486662 and
   B:=1.  This curve has order h*n, where h=8 and where n is a prime
   number.  For this curve, A^2-4 is not a square in GF(p), whereas A+2
   is.  The quadratic twist of this curve has order h1*n1, where h1=4
   and where n1 is a prime number.  For this curve, the base point is
   defined to be the ordered pair (Gu,Gv) of elements of GF(p), where
   Gu=9 and where Gv is an odd integer in the interval [0, p-1].

   This curve has the same group structure as (is "isomorphic" to) the
   twisted Edwards curve E_{a,d} defined over GF(p), with as base point
   the ordered pair (Gx,Gy) of elements of GF(p), where parameters are
   as specified in Appendix D.3.  This curve is denoted as Ed25519.  For
   this curve, the parameter a is a square in GF(p), whereas d is not,
   so the group laws of Appendix B.3 apply.

   The curve is also isomorphic to the elliptic curve W_{a,b} in short-
   Weierstrass form defined over GF(p), with as base point the ordered
   pair (Gx',Gy') of elements, where parameters are as specified in
   Appendix D.3.  This curve is denoted as Wei25519.

D.2.  Switching between Alternative Representations

   Each point (u,v) of Curve25519 corresponds to the point (x,y):=(u +
   A/3,y) of Wei25519, while the point at infinity of Curve25519
   corresponds to the point at infinity of Wei25519.  (Here, we used the
   mapping of Appendix C.2.)  Under this mapping, the base point (Gu,Gv)
   of Curve25519 corresponds to the base point (Gx',Gy') of Wei25519.
   The inverse mapping maps the point (x,y) on Wei25519 to (u,v):=(x -
   A/3,y) on Curve25519, while mapping the point at infinity of Wei25519
   to the point at infinity on Curve25519.  Note that this mapping
   involves a simple shift of the first coordinate and can be
   implemented via integer-only arithmetic as a shift of (p+A)/3 for the




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   isomorphic mapping and a shift of -(p+A)/3 for its inverse, where
   delta=(p+A)/3 is the element of GF(p) defined by

   delta  19298681539552699237261830834781317975544997444273427339909597
       334652188435537

       (=0x2aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaad2
       451)

   The curve Ed25519 is isomorphic to the curve Curve25519, where the
   base point (Gu,Gv) of Curve25519 corresponds to the base point
   (Gx,Gy) of Ed25519 and where the point at infinity and the point
   (0,0) of order two of Curve25519 correspond to, respectively, the
   point (0, 1) and the point (0, -1) of order two of Ed25519 and where
   each other point (u, v) of Curve25519 corresponds to the point (c*u/
   v, (u-1)/(u+1)) of Ed25519, where c is the element of GF(p) defined
   by

   c   sqrt(-(A+2))

       51042569399160536130206135233146329284152202253034631822681833788
       666877215207

       (=0x70d9120b 9f5ff944 2d84f723 fc03b081 3a5e2c2e b482e57d
       3391fb55 00ba81e7)

   (Here, we used the mapping of Appendix C.1.)  The inverse mapping
   from Ed25519 to Curve25519 is defined by mapping the point (0, 1) and
   the point (0, -1) of order two of Ed25519 to, respectively, the point
   at infinity and the point (0,0) of order two of Curve25519 and having
   each other point (x, y) of Ed25519 correspond to the point ((1 +
   y)/(1 - y), c(1 + y)/((1-y)x)).

   The curve Ed25519 is isomorphic to the Weierstrass curve Wei25519,
   where the base point (Gx,Gy) of Ed25519 corresponds to the base point
   (Gx',Gy') of Wei25519 and where the identity element (0,1) and the
   point (0,-1) of order two of Ed25519 correspond to, respectively, the
   point at infinity O and the point (A/3, 0) of order two of Wei25519
   and where each other point (x, y) of Ed25519 corresponds to the point
   (x', y'):=((1+y)/(1-y)+A/3, c(1+y)/((1-y)*x)) of Wei25519, where c
   was defined before.  (Here, we used the mapping of Appendix C.3.)
   The inverse mapping from Wei25519 to Ed25519 is defined by mapping
   the point at infinity O and the point (A/3, 0) of order two of
   Wei25519 to, respectively, the identity element (0,1) and the point
   (0,-1)) of order two of Ed25519 and having each other point (x, y) of
   Wei25519 correspond to the point (c*(3*x+A)/(3*y), (x+A-3)/(x+A+3)).





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   Note that these mappings can be easily realized in projective
   coordinates, using a few field multiplications only, thus allowing
   switching between alternative representations with negligible
   relative incremental cost.

D.3.  Domain Parameters

   The parameters of the Montgomery curve and the corresponding
   isomorphic curves in twisted Edwards curve and short-Weierstrass form
   are as indicated below.  Here, the domain parameters of Curve25519
   and Ed25519 are as specified in RFC 7748; the domain parameters of
   Wei25519 are "new".

   General parameters:

   p   2^{255}-19

       (=0x7fffffff ffffffff ffffffff ffffffff ffffffff ffffffff
       ffffffff ffffffed)

   h   8

   n   72370055773322622139731865630429942408571163593799076060019509382
       85454250989

       (=2^{252} + 0x14def9de a2f79cd6 5812631a 5cf5d3ed)

   h1  4

   n1  14474011154664524427946373126085988481603263447650325797860494125
       407373907997

       (=2^{253} - 0x29bdf3bd 45ef39ac b024c634 b9eba7e3)

   Montgomery curve-specific parameters:

   A   486662

   B   1

   Gu  9 (=0x9)

   Gv  14781619447589544791020593568409986887264606134616475288964881837
       755586237401

       (=0x20ae19a1 b8a086b4 e01edd2c 7748d14c 923d4d7e 6d7c61b2
       29e9c5a2 7eced3d9)




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   Twisted Edwards curve-specific parameters:

   a   -1 (-0x01)

   d   -121665/121666

       (=370957059346694393431380835087545651895421138798432190163887855
       33085940283555)

       (=0x52036cee 2b6ffe73 8cc74079 7779e898 00700a4d 4141d8ab
       75eb4dca 135978a3)

   Gx  15112221349535400772501151409588531511454012693041857206046113283
       949847762202

       (=0x216936d3 cd6e53fe c0a4e231 fdd6dc5c 692cc760 9525a7b2
       c9562d60 8f25d51a)

   Gy  4/5

       (=463168356949264781694283940034751631413079938662562256157830336
       03165251855960)

       (=0x66666666 66666666 66666666 66666666 66666666 66666666
       66666666 66666658)

   Weierstrass curve-specific parameters:

   a   19298681539552699237261830834781317975544997444273427339909597334
       573241639236

       (=0x2aaaaaaa aaaaaaaa aaaaaaaa aaaaaaaa aaaaaaaa aaaaaaaa
       aaaaaa98 4914a144)

   b   55751746669818908907645289078257140818241103727901012315294400837
       956729358436

       (=0x7b425ed0 97b425ed 097b425e d097b425 ed097b42 5ed097b4
       260b5e9c 7710c864)

   Gx' 19298681539552699237261830834781317975544997444273427339909597334
       652188435546

       (=0x2aaaaaaa aaaaaaaa aaaaaaaa aaaaaaaa aaaaaaaa aaaaaaaa
       aaaaaaaa aaad245a)

   Gy' 14781619447589544791020593568409986887264606134616475288964881837
       755586237401



Struik                     Expires May 3, 2018                 [Page 11]


Internet-Draft         lwip-curve-representations           October 2017


       (=0x20ae19a1 b8a086b4 e01edd2c 7748d14c 923d4d7e 6d7c61b2
       29e9c5a2 7eced3d9)

Author's Address

   Rene Struik
   Struik Security Consultancy

   Email: rstruik.ext@gmail.com










































Struik                     Expires May 3, 2018                 [Page 12]


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