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Network Working Group                                            W. Ladd
Internet-Draft                                               UC Berkeley
Intended status: Standards Track                                P. Longa
Expires: September 28, 2017                           Microsoft Research
                                                               R. Barnes
                                                          March 27, 2017



   This document specifies Curve4Q, a twisted Edwards curve proposed in
   [Curve4Q] that takes advantage of arithmetic over the field
   GF(2^127-1) and two endomorphisms to achieve the speediest Diffie-
   Hellman key agreements over a group of order approximately 2^246,
   which provides around 128 bits of security.  Curve4Q implementations
   are more than two times faster than those of Curve25519 and, when not
   using endomorphisms, are between 1.2 and 1.6 times faster.

Status of This Memo

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

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   This Internet-Draft will expire on September 28, 2017.

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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents

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   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.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Mathematical Prerequisites  . . . . . . . . . . . . . . . . .   3
   3.  Representation of Curve Points  . . . . . . . . . . . . . . .   4
   4.  Scalar multiplication . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Alternative Point Representations and Addition Laws . . .   6
     4.2.  Multiplication without Endomorphisms  . . . . . . . . . .   8
     4.3.  Multiplication with Endomorphisms . . . . . . . . . . . .   9
       4.3.1.  Endomorphisms . . . . . . . . . . . . . . . . . . . .   9
       4.3.2.  Scalar Decomposition and Recoding . . . . . . . . . .  11
       4.3.3.  Final Computation . . . . . . . . . . . . . . . . . .  12
   5.  Diffie-Hellman Key Agreement  . . . . . . . . . . . . . . . .  13
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   8.  Informative References  . . . . . . . . . . . . . . . . . . .  15
   Appendix A.  Constants  . . . . . . . . . . . . . . . . . . . . .  16
   Appendix B.  Point Decompression  . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   Public key cryptography continues to be computationally expensive,
   particularly on less powerful devices.  While recent advances in
   efficient formulas for addition and doubling have substantially
   reduced the cost of elliptic curve operations in terms of field
   operations, the number of group operations involved in scalar
   multiplication has not been reduced in the curves considered for IETF
   use.  Using curves with efficiently computable endomorphisms can
   reduce the number of group operations by turning one long scalar
   multiplication into the sum of several multiplications by smaller
   scalars, which can be evaluated more efficiently.

   For curves over quadratic extension fieldss, there are more
   endomorphism families to choose from, and the field operations are
   often more efficient compared to prime fields of the same size.  The
   ideal case is given by curves equipped with two distinct
   endomorphisms, so that it becomes possible to divide scalars into
   four parts.  We focus on curves defined over the field GF(p^2) for
   the Mersenne prime p = 2^127 - 1, which offers extremely efficient
   arithmetic.  Together, these improvements substantially reduce
   computation time compared to other proposed Diffie-Hellman key

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   exchange and digital signature schemes.  However, the combined
   availability of these features severely restricts the curves that can
   be used for cryptographic applications.

   As described in [Curve4Q], the elliptic curve "Curve4Q" defined in
   this document is a special instance of the recent endomorphism-based
   constructions from [Qcurve] and [GLV4], and is the only known
   elliptic curve that (1) permits a four dimensional decomposition
   (using two endomorphisms) over GF(p^2) and (2) has a large prime
   order subgroup.  The order of this subgroup is approximately 2^246,
   which provides around 128 bits of security.  No other known elliptic
   curve with such a decomposition has a larger prime order subgroup
   over this field.  This "uniqueness" allays concerns about selecting
   curves vulnerable to undisclosed attacks.

   Curve4Q can be used to implement Diffie-Hellman key exchange, as
   described below.  It is also possible to use Curve4Q as the basis for
   digital signature scheme (e.g., [SchnorrQ]).

2.  Mathematical Prerequisites

   Curve4Q is defined over the finite field GF(p^2), where p is the
   Mersenne prime 2^127 - 1.  Elements of this finite field have the
   form (a + b * i), where a and b are elements of the finite field
   GF(p) (i.e., integers mod p) and i^2 = -1.

   Let A = a0 + a1*i and B = b0 + b1*i be two elements of GF(p^2).
   Below we present formulas for computing addition, subtraction,
   multiplication, squaring, conjugation and inversion.

   A + B = (a0 + b0) + (a1 + b1)*i

   A - B = (a0 - b0) + (a1 - b1)*i

   A * B = (a0*b0 - a1*b1) + ((a0 + a1)*(b0 + b1)-(a0*b0 + a1*b1))*i
         = (a0*b0 - a1*b1) + (a0*b1 + a1*b0)*i

   A * A = (a0 + a1)*(a0 - a1) + 2*a0*a1*i

   conj(A) = a0 - a1*i

   1/A = conj(A) / (a0^2 + a1^2)

   The GF(p) division in the formula for 1/A can be computed using an
   exponentiation via Fermat's little theorem: 1/a = a^(p - 2) =
   a^(2^127 - 3) for any element a of GF(p).  One can use a fixed
   addition chain to compute a^(2^127 - 3) (e.g., see [FourQlib]).

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   Curve4Q is the twisted Edwards curve E over GF(p^2) defined by the
   following curve equation:

   E: -x^2 + y^2 = 1 + d * x^2 * y^2, with

   d = 0x00000000000000e40000000000000142 +
       0x5e472f846657e0fcb3821488f1fc0c8d * i

   Let E(GF(p^2)) be the set of pairs (x, y) of elements of GF(p^2)
   satisfying this equation.  This set forms a group with the addition
   operation (x1, y1) + (x2, y2) = (x3, y3), where:

             x1 * y2 + y1 * x2                y1 * y2 + x1 * x2
   x3 = ---------------------------, y3 = ---------------------------
         1 + d * x1 * y1 * x2 * y2         1 - d * x1 * y1 * x2 * y2

   As d is not a square in GF(p^2), and -1 is, this formula never
   involves a division by zero when applied to points on the curve.
   That is, the formula is complete and works without exceptions for any
   input in E(GF(p^2)).  The identity element is (0, 1), and the inverse
   of (x, y) is (-x, y).  The order of this group is #E = 2^3 . 7^2 . N,
   where N is the following 246-bit prime:

   N = 0x29cbc14e5e0a72f05397829cbc14e5dfbd004dfe0f79992fb2540ec7768ce7

   Points P on E such that [N]*P = (0, 1) are N-torsion points.  Given a
   point P and Q which are both N-torsion points, it is difficult to
   find m such that Q = [m]*P.  This is the elliptic curve discrete
   logarithm problem, which is closely related to the security of
   Diffie-Hellman key exchanges as the best known attacks on the Diffie-
   Hellman problem involve solving the discrete logarithm problem.  The
   best known algorithms take approximately 2^123 group operations.

   This group has two different efficiently computable endomorphisms, as
   described in [Curve4Q].  As discussed in [GLV] and [GLS], these
   endomorphisms allow a multiplication by a large scalar to be computed
   using multiple multiplications by smaller scalars, which can be
   evaluated in much less time overall.

3.  Representation of Curve Points

   Elements a in GF(p) are represented as 16 byte little endian integers
   which are the numbers in the range [0, p).  The 16 bytes b[0], b[1],
   ... b[15] represent b[0] + 256*b[1] + 256^2*b[2] + ... +
   256^15*b[15].  Since we are representing numbers in the range [0,
   2^127-1), the top bit of b[15] is always zero.

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   An element x0 + x1*i of GF(p^2) is represented on the wire by the
   concatenation of the encodings for x0 and x1.  A point (x, y) on
   Curve4Q is serialized in a compressed form as the representation of y
   with a modified top bit.  This top bit is used to disambiguate
   between x and -x during decoding.

   To carry out this disambiguation we use the lexicographic order of
   elements in GF(p^2): define two elements a = a0 + a1*i and b = b0 +
   b1*i with all their coordinates in [0, p); a is greater than b if a0
   is greater than b0.  If a0 and b0 are equal, a is greater than b if
   a1 is greater than b1.

   Set the coordinate value x and its negative -x.  The top bit of a
   compressed point is 0 if x is smaller than -x.  Otherwise, the top
   bit is 1.

   |--------------- y ---------------|
   |       y0     |0|       y1     |s|

   To decode an encoded point from a 32-byte sequence B:

   o  Parse out the encoded values y = y0 + y1 * i and s

   o  Check that y0 and y1 are both less than p

   o  Solve x^2 = (y^2 - 1) * (d * y^2 + 1) for x

   o  If s is 0, return the smaller of x and -x (in the lexicographic

   o  If s is 1, return the larger of x and -x

   o  Check that (x,y) is a valid point on the curve

   The appendix Appendix B details an algorithm for decoding a point
   following the steps above.

   We call the operation of compressing a point P into 32 bytes
   Compress(P), and decompression Expand(S).  Expand(Compress(P))=P for
   all the points P on the curve, and Compress(Expand(S))=S if and only
   if S is a valid representation of a point.

   Not all 32 byte strings represent valid points.  Implementations MUST
   reject invalid strings and check that decompression is successful.
   Strings are invalid if they are not possible outputs of the
   compression operator.  In particular the values of y0 and y1 MUST be
   less then p.

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4.  Scalar multiplication

   Below, we present two algorithms for scalar multiplication on
   Curve4Q.  Each algorithm takes as input a 256-bit unsigned integer m
   and an N-torsion point P and computes the product [m]*P.

   The first algorithm uses a simple fixed-window exponentiation without
   exploiting endomorphisms.  The second algorithm uses endomorphisms to
   accelerate computation.  The execution of operations in both
   algorithms has a regular pattern in order to enable constant-time
   implementations and protect against timing and simple side channel
   attacks.  Both algorithms use the same addition and doubling

   First, we discuss explicit formulas and efficient projective
   coordinate representations.

4.1.  Alternative Point Representations and Addition Laws

   We use coordinates based on extended twisted Edwards coordinates
   introduced in [TwistedRevisited]: the tuple (X, Y, Z, T) with Z
   nonzero and Z * T = X * Y corresponds to a point (x, y) satisfying x
   = X/Z and y = Y/Z.  The neutral point in this representation is (0,
   1, 1, 0).  The following slight variants are used in the optimized
   scalar multiplication algorithm in order to save computations: point
   representation R1 is given by (X, Y, Z, Ta, Tb), where T=Ta * Tb;
   representation R2 is (N, D, E, F) = (X + Y, Y- X, 2Z, 2dT);
   representation R3 is (N, D, Z, T) = (X + Y, Y - X, Z, T); and
   representation R4 is (X, Y, Z).  Similar "caching" techniques were
   discussed in [TwistedRevisited] to accelerate repeated additions of
   the same point.  Converting between these representations is

   o  R1: (X, Y, Z, Ta, Tb), Ta * Tb = T, Z * T = X * Y

   o  R2: (N, D, E, F) = (X + Y, Y - X, 2 * Z, 2 * d * T)

   o  R3: (N, D, Z, T) = (X + Y, Y - X, Z, T)

   o  R4: (X, Y, Z)

   A point doubling (DBL) takes an R4 point and produces an R1 point.
   For addition, we first define an operation ADD_core that takes an R2
   and an R3 point and produces an R1 point.  This can be used to
   implement an operation ADD which takes an R1 and an R2 point as
   inputs (and produces an R1 point) by first converting the R1 point to
   R3, and then executing ADD_core.  Exposing these operations and the
   multiple representations helps save time by avoiding redundant

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   computations: the conversion of the first argument to ADD can be done
   once if the argument will be used in multiple additions.

   Below, we list the explicit formulas for the required point
   operations.  These formulas, which are adapted from [Twisted] and
   [TwistedRevisited], are complete: they have no exceptional cases, and
   therefore can be used in any algorithm for computing scalar multiples
   without worrying about exceptional procedure attacks [Exceptional].
   Note that we do not explicitly note the point format every time an
   addition or doubling is used, and assume that conversions are done
   when required.

   DBL and ADD_core are computed as follows:

   DBL(X1, Y1, Z1):
     A = X1^2
     B = Y1^2
     C = 2 * Z1^2
     D = A + B
     E = (X1 + Y1)^2 - D
     F = B - A
     G = C - F
     X3 = E * G
     Y3 = D * F
     Z3 = F * G
     Ta3 = E
     Tb3 = D
   return(X3, Y3, Z3, Ta3, Tb3)

   ADD\_core(N1, D1, E1, F1, N2, D2, Z2, T2):
      A = D1 * D2
      B = N1 * N2
      C = T2 * F1
      D = Z2 * E1
      E = B - A
      F = D - C
      G = D + C
      H = B + A
      X3 = E * F
      Y3 = G * H
      Z3 = F * G
      Ta3 = E
      Tb3 = H
   return (X3, Y3, Z3, Ta3, Tb3)

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4.2.  Multiplication without Endomorphisms

   We begin by taking our input point P, and computing a table of points
   containing T[0] = [1]P, T[1] = [3]P, ... , T[7] = [15]P as follows:

   Q = DBL(P)
   Convert Q to R2 form
   T[0] = P
   Convert T[0] to R2 form
   for i=1 to 7:
       T[i] = ADD_core(Q, T[i-1])
       Convert T[i] to R2 form

   Next, take m and reduce it modulo N.  Then, add N if necessary to
   ensure that m is odd.  At this point, we recode m into a signed digit
   representation consisting of 63 signed, odd digits d[i] in base 16.
   The following algorithm accomplishes this task.

   for i=0 to 61:
       d[i] = (m mod 32) - 16
       m = (m - d[i]) / 16
   d[62] = m

   Finally, the computation of the multiplication is as follows.

   Let ind = (abs(d[62]) - 1) / 2
   Let sign = sgn(d[62])
   Q = sign * T[ind]
   Convert Q into R4 form
   for i from 61 to 0:
       Q = DBL(Q)
       Q = DBL(Q)
       Q = DBL(Q)
       Q = DBL(Q)
       ind = (abs(d[i]) - 1) / 2
       sign = sgn(d[i])
       S = sign * T[ind]
       Q = ADD(Q, S)
   return Q

   As sign is either -1 or 1, the multiplication sign * T[ind] is simply
   a conditional negation.  To negate a point (N, D, E, F) in R2 form
   one computes (D, N, E, -F).  The table lookups and conditional
   negations must be carefully implemented as described in ``Security
   Considerations'' to avoid side-channel attacks.  This algorithm MUST
   NOT be applied to points which are not N-torsion points; it will
   produce the wrong answer.

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4.3.  Multiplication with Endomorphisms

   This algorithm makes use of the identity [m]*P = [a_1]*P +
   [a_2]*phi(P) + [a_3]*psi(P) + [a_4]*psi(phi(P)), where a_1, a_2, a_3,
   and a_4 are 64-bit scalars that depend on m.  The overall product can
   then can be computed using a small table of 8 precomputed points and
   64 doublings and additions.  This is considerably fewer operations
   than the number of operations required by the algorithm above, at the
   cost of a more complicated implementation.

   We describe each phase of the computation separately: the computation
   of the endomorphisms, the scalar decomposition and recoding, the
   creation of the table of precomputed points and, lastly, the
   computation of the final results.  Each section refers to constants
   listed in an appendix in order of appearance.

4.3.1.  Endomorphisms

   The two endomorphisms phi and psi used to accelerate multiplication
   are computed as phi(Q) = tau_dual(upsilon(tau(Q)) and psi(Q) =
   tau_dual(chi(tau(Q))).  Below, we present procedures for tau,
   tau_dual, upsilon and chi, adapted from [FourQlib].  Tau_dual
   produces an R1 point, while the other procedures produce R4 points.

   Note: Tau produces points on a different curve, while upsilon and chi
   are endomorphisms of that different curve.  Tau and tau_dual are the
   isogenies mentioned in the mathematical background above.  As a
   result the intermediate results do not satisfy the equations of the
   curve E.  Implementers who wish to check the correctness of these
   intermediate results are referred to [Curve4Q].

   tau(X1, Y1, Z1):
      A = X1^2
      B = Y1^2
      C = A + B
      D = A - B
      X2 = ctau * X1 * Y1 * D
      Y2 = -(2 * Z1^2 + D) * C
      Z2 = C * D
   return(X2, Y2, Z2)

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   tau_dual(X1, Y1, Z1):
     A = X1^2
     B = Y1^2
     C = A + B
     Ta2 = B - A
     D = 2 * Z1^2 - Ta2
     Tb2 = ctaudual * X1 * Y1
     X2 = C * Tb2
     Y2 = D * Ta2
     Z2 = C * D
   return(X2, Y2, Z2, Ta2, Tb2)

   upsilon(X1, Y1, Z1):
      A = cphi0 * X1 * Y1
      B = Y1 * Z1
      C = Y1^2
      D = Z1^2
      F = D^2
      G = B^2
      H = C^2
      I = cphi1 * B
      J = C + cphi2 * D
      K = cphi8 * G + H + cphi9 * F
      X2 = conj(A * K * (I + J) * (I - J))
      L = C + cphi4 * D
      M = cphi3 * B
      N = (L + M) * (L - M)
      Y2 = conj(cphi5 * D * N * (H + cphi6 * G + cphi7 * F))
      Z2 = conj(B * K * N)
   return(X2, Y2, Z2)

   chi(X1, Y1, Z1):
      A = conj(X1)
      B = conj(Y1)
      C = conj(Z1)^2
      D = A^2
      F = B^2
      G = B * (D + cpsi2 * C)
      H = -(D + cpsi4 * C)
      X2 = cpsi1 * A * C * H
      Y2 = G * (D + cpsi3 * C)
      Z2 = G * H
   return(X2, Y2, Z2)

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4.3.2.  Scalar Decomposition and Recoding

   This stage has two parts.  The first one consists in decomposing the
   scalar into four 64-bit integers, and the second one consists in
   recoding these integers into a form that can be used to efficiently
   and securely compute the scalar multiplication.

   The decomposition step uses four fixed vectors called b1, b2, b3, b4,
   with four 64 bit entries each.  In addition, we have integer
   constants L1, L2, L3, L4, which are used to implement rounding.  All
   these values are listed in Appendix A.  In addition, we use two
   constant vectors derived from these inputs:

   o  c = 5 * b2 - 3 * b3 + 2 * b4

   o  c' = 5 * b2 - 3 * b3 + 3 * b4 = c + b4

   Given m, first compute t[i] = floor(L[i] * m / 2^256) for i between 1
   and 4.  Then compute the vector sum a = (m, 0, 0, 0) - t1 b1 - t2 b2
   - t3 b3 - t4 b4.  Precisely one of a + c and a + c' has an odd first
   coordinate: this is the vector v that is fed into the scalar recoding
   step.  Note that the entries of this vector are 64 bits, so
   intermediate values in the calculation above can be truncated to this

   The recoding step takes the vector v=(v1, v2, v3, v4) from the
   previous step and outputs two arrays m[0]..m[64] and d[0]..d[64].
   Each entry of d is between 0 and 7, and each entry in m is -1 or 0.
   The recoding algorithm is detailed below.  bit(x, n) denotes the nth
   bit of x, counting from least significant to most, starting with 0.

   m[64] = -1
   for i = 0 to 63 do:
       b1 = bit(v1, i+1)
       d[i] = 0
       m[i] = b1

       for j = 2 to 4 do:
           bj = bit(vj, 0)
           d[i] = d[i] + bj * 2^(j-2)
           c = (b1 or bj) xor b1
           vj = vj / 2 + c
   d[64] = v2 + 2 * v3 + 4 * v4

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4.3.3.  Final Computation

   We now describe the last step in the endomorphism based algorithm for
   computing scalar multiplication.  On inputs m and P, the algorithm
   first precomputes a table of images of P under the endomorphisms,
   then recodes m, then uses these intermediate artifacts to compute the
   scalar product.

   First, compute a table T of 8 points in representation R2 as shown
   below.  Computations Q = psi(P), R = phi(P) and S = psi(phi(P)) are
   carried out using formulas from Section 4.3.1.

   Q is phi(P) in R3
   R is psi(P) in R3
   S is psi(Q) in R2
   T[0] is P in R2
   T[1] is ADD_core(Q, T[0])  # (P + Q)
   Convert T[1] to R2
   T[2] is ADD_Core(R, T[0])  # (P + R)
   Convert T[2] to R2
   T[3] is ADD_Core(R, T[1])  # (P + Q + R)
   Convert T[3] to R2
   T[4] is ADD_Core(S, T[0])  # (P + S)
   Convert T[4] to R2
   T[5] is ADD_Core(S, T[1])  # (P + Q + S)
   Convert T[5] to R2
   T[6] is ADD_Core(S, T[2])  # (P + R + S)
   Convert T[6] to R2
   T[7] is ADD_Core(S, T[3])  # (P + Q + R + S)
   Convert T[7] to R2

   Second, apply the scalar decomposition and recoding algorithm from
   Section 4.3.2 to m, to produce the two arrays m[0]..m[64] and

   Define s[i] to be 1 if m[i] is 1 and -1 if m[i] is 0.  Then the
   multiplication is completed as follows:

   Q = s[64] * T[d[64]]
   Convert Q to R4
   for i=63 to 0 do:
       Q = DBL(Q)
       Q = ADD(Q, s[i] * T[di])
   return Q = (X/Z, Y/Z)

   Multiplication by s[i] is simply a conditional negation.  To negate
   an R2 point (N, D, E, F) one computes (D, N, E , -F).  It is
   important to do this (as well as the table lookup) in constant time,

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   i.e., the execution of branches and memory accesses MUST NOT depend
   on secret values (see ``Security Considerations'' for more details).

   The optimized multiplication algorithm above only works properly for
   N-torsion points.  Implementations MUST NOT use this algorithm on
   anything that is not known to be an N-torsion point.  Otherwise, it
   will produce the wrong answer, with extremely negative consequences
   for security.

5.  Diffie-Hellman Key Agreement

   The above scalar multiplication algorithms can be used to implement
   Diffie-Hellman with cofactor.

   DH(m, P):
         Ensure P on curve and if not return FAILURE

         P1 = DBL(P)                 # [2]P
         P2 = ADD(P1, P)             # [3]P
         P3 = DBL(DBL(DBL(DBL(P2)))) # [48]P
         Q = ADD(P3, P)              # [49]P
         Q = DBL(DBL(DBL(Q))         # [392]P

         Compute [m]*Q

         If Q is the neutral point, return FAILURE
   Return [m]\*Q in affine coordinates

   The role of the multiplication by 392 is to ensure that Q is an
   N-torsion point so that the scalar multiplication [m]*P in the DH
   function above may be used safely to produce correct results.  In
   other words, as the cofactor is greater than one, Diffie-Hellman
   computations using Curve4Q MUST always clear the cofactor (i.e.,
   multiply by 392, as explained above).

   The base point G for Diffie-Hellman operations has the following
   affine coordinates:

   Gx = 0x1A3472237C2FB305286592AD7B3833AA +
   Gy = 0x0E3FEE9BA120785AB924A2462BCBB287 +
   G = (X, Y)

   The tables used in multiplications of this generator (small multiples
   of G for the multiplication without endomorphisms, or endomorphism
   images for the optimized multiplication with endomorphisms) can be
   pre-generated to speed up the first, fixed-point DH computation.

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   Two users, Alice and Bob, can carry out the following steps to derive
   a shared key: each picks a random string of 32 bytes, mA and mB,
   respectively.  Alice computes the public key A = Compress([mA]*G),
   and Bob computes the public key B = Compress([mB]*G).  They exchange
   A and B, and then Alice computes KAB = DH(mA, Expand(B)) while Bob
   computes KBA = DH(mB, Expand(A)), which produces the shared point K =
   KAB = KBA.  The y coordinate of K, represented as a 32 byte string as
   detailed in Section 3 is the shared secret.

   Before decompressing A and B using the function Expand(), each user
   SHOULD verify that the 128th bit of the received public key is zero
   (i.e., the non-imaginary part of the corresponding y-coordinate
   should be < 2^127).

   If the received strings are not valid points, the DH function has
   failed to compute an answer.  Implementations SHOULD return a random
   32 byte string as well as return an error, to prevent bugs when
   applications ignore return codes.  They MUST signal an error when
   decompression fails.

   Implementations MAY use any method to carry out these calculations,
   provided that it agrees with the above function on all inputs and
   failure cases, and does not leak information about secret keys.  For
   example, refer to the constant-time fixed-base scalar multiplication
   algorithm implemented in [FourQlib] to accelerate the computation of
   multiplications by the generator G.

6.  IANA Considerations

   [RFC Editor: please remove this section prior to publication] This
   document has no IANA actions.

7.  Security Considerations

   The best known algorithms for the computation of discrete logarithms
   on Curve4Q are parallel versions of the Pollard rho algorithm in
   [Distinguished].  On Curve4Q these attacks take on the order of 2^123
   group operations to compute a single discrete logarithm.  The
   additional endomorphisms have large order, and so cannot be used to
   accelerate generic attacks.  Quadratic fields are not affected by any
   of the index calculus attacks used over larger extension fields.

   Implementations MUST check that input points properly decompress to
   points on the curve.  Removing such checks may result in extremely
   effective attacks.  The curve is not twist-secure: implementations
   using single coordinate ladders MUST validate points before operating
   on them.  In the case of protocols that require contributory
   behavior, when the identity is the output of the DH primitive it MUST

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   be rejected and failure signaled to higher levels.  Notoriously
   [RFC5246] without [RFC7627] is such a protocol.

   Implementations MUST ensure that execution of branches and memory
   addresses accessed do not depend on secret data.  The time
   variability introduced by secret-dependent operations have been
   exploited in the past via timing and cache attacks to break
   implementations.  Side-channel analysis is a constantly moving field,
   and implementers must be extremely careful to ensure that operations
   do not leak any secret information.  Using ephemeral private scalars
   for each operation (ideally, limiting the use of each private scalar
   to one single operation) can reduce the impact of side-channel
   attacks.  However, this might not be possible for many applications
   of Diffie-Hellman key agreement.

   In the future quantum computers may render the discrete logarithm
   problem easy on all abelian groups through Shor's algorithm.  Data
   intended to remain confidential for significantly extended periods of
   time SHOULD NOT be protected with any primitive based on the hardness
   of factoring or the discrete log problem (elliptic curve or finite

8.  Informative References

   [Curve4Q]  Costello, C. and P. Longa, "FourQ: four-dimensional
              decompositions on a Q-curve over the Mersenne prime",
              2016, <https://eprint.iacr.org/2015/565.pdf>.

              van Oorschot, P. and M. Wiener, "Parallel Collision Search
              with Cryptanalytic Applications", 1996,

              Izu, T. and T. Takagi, "Exceptional procedure attack on
              elliptic curve cryptosystems", 2003,

              Costello, C. and P. Longa, "FourQlib", 2016,

   [GLS]      Galbraith, S., Lin, X., and M. Scott, "Endomorphisms for
              Faster Elliptic Curve Cryptography on a Large Class of
              Curves", 2009, <https://www.iacr.org/archive/

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   [GLV]      Gallant, R., Lambert, R., and S. Vanstone, "Faster Point
              Multiplication on Elliptic Curves with Efficient
              Endomorphisms", 2001, <https://www.iacr.org/archive/

   [GLV4]     Guillevic, A. and S. Ionica, "Four-dimensional GLV via the
              Weil restriction", 2013,

   [Invsqr]   Hamburg, M., "Fast and compact elliptic-curve
              cryptography", 2012,

   [Qcurve]   Smith, B., "The Q-curve Construction for Endomorphism-
              Accelerated Elliptic Curves", 2016,

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [RFC7627]  Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
              Langley, A., and M. Ray, "Transport Layer Security (TLS)
              Session Hash and Extended Master Secret Extension",
              RFC 7627, DOI 10.17487/RFC7627, September 2015,

              Costello, C. and P. Longa, "SchnorrQ: Schnorr Signatures
              on FourQ", 2016, <https://www.microsoft.com/en-

   [Twisted]  Bernstein, D., Birkner, P., Joye, M., Lange, T., and C.
              Peters, "Twisted Edwards Curves", 2008,

              Hisil, H., Wong, K-H., Carter, G., and E. Dawson, "Twisted
              Edwards Curves Revisited", 2008, <http://iacr.org/archive/

Appendix A.  Constants

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   ctau = 0x1964de2c3afad20c74dcd57cebce74c3 +
          0x000000000000000c0000000000000012 * i

   ctaudual = 0x4aa740eb230586529ecaa6d9decdf034 +
              0x7ffffffffffffff40000000000000011 * i

   cphi0 = 0x0000000000000005fffffffffffffff7 +
           0x2553a0759182c3294f65536cef66f81a * i
   cphi1 = 0x00000000000000050000000000000007 +
           0x62c8caa0c50c62cf334d90e9e28296f9 * i
   cphi2 = 0x000000000000000f0000000000000015 +
           0x78df262b6c9b5c982c2cb7154f1df391 * i
   cphi3 = 0x00000000000000020000000000000003 +
           0x5084c6491d76342a92440457a7962ea4 * i
   cphi4 = 0x00000000000000030000000000000003 +
           0x12440457a7962ea4a1098c923aec6855 * i
   cphi5 = 0x000000000000000a000000000000000f +
           0x459195418a18c59e669b21d3c5052df3 * i
   cphi6 = 0x00000000000000120000000000000018 +
           0x0b232a8314318b3ccd3643a78a0a5be7 * i
   cphi7 = 0x00000000000000180000000000000023 +
           0x3963bc1c99e2ea1a66c183035f48781a * i
   cphi8 = 0x00000000000000aa00000000000000f0 +
           0x1f529f860316cbe544e251582b5d0ef0 * i
   cphi9 = 0x00000000000008700000000000000bef +
           0x0fd52e9cfe00375b014d3e48976e2505 * i

   cpsi1 = 0x2af99e9a83d54a02edf07f4767e346ef +
           0x00000000000000de000000000000013a * i
   cpsi2 = 0x00000000000000e40000000000000143 +
           0x21b8d07b99a81f034c7deb770e03f372 * i
   cpsi3 = 0x00000000000000060000000000000009 +
           0x4cb26f161d7d69063a6e6abe75e73a61 * i
   cpsi4 = 0x7ffffffffffffff9fffffffffffffff6 +
           0x334d90e9e28296f9c59195418a18c59e * i

   L1 = 0x7fc5bb5c5ea2be5dff75682ace6a6bd66259686e09d1a7d4f
   L2 = 0x38fd4b04caa6c0f8a2bd235580f468d8dd1ba1d84dd627afb
   L3 = 0x0d038bf8d0bffbaf6c42bd6c965dca9029b291a33678c203c
   L4 = 0x31b073877a22d841081cbdc3714983d8212e5666b77e7fdc0

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   b1 = [ 0x0906ff27e0a0a196, -0x1363e862c22a2da0,
          0x07426031ecc8030f, -0x084f739986b9e651]
   b2 = [ 0x1d495bea84fcc2d4, -0x0000000000000001,
          0x0000000000000001,  0x25dbc5bc8dd167d0]
   b3 = [ 0x17abad1d231f0302,  0x02c4211ae388da51,
         -0x2e4d21c98927c49f,  0x0a9e6f44c02ecd97]
   b4 = [ 0x136e340a9108c83f,  0x3122df2dc3e0ff32,
         -0x068a49f02aa8a9b5, -0x18d5087896de0aea]

Appendix B.  Point Decompression

   The following algorithm is an adaptation of the decompression
   algorithm from [SchnorrQ].  It decodes a 32-byte string B which is
   formatted as detailed in Section 3.  The result is a valid point P =
   (x, y) that satisfies the curve equation, or a message of FAILED if
   the decoding had a failure.

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   Sign(x0 + x1*i):
       s0 = X[0] >> 126
       s1 = X[1] >> 126
       if X[0] != 0:
           return s0
           return s1

   Compress(X, Y):
       B = Y encoded following {{representation-of-curve-points}}
       Set the top bit to Sign(X)
       return B

   Expand(B = [y, s]):
       Parse out the encoded values y = y0 + y1 * i and s
       if y0 or y1 >= p:
           return FAILED

       u = y^2 - 1             # Set u = u0 + u1 * i
       v = d*y^2 + 1           # Set v = v0 + v1 * i

       t0 = u0*v0 + u1*v1;
       t1 = u1*v0 - u0*v1;
       t2 = v0^2 + v1^2
       t3 = (t0^2 + t1^2)^(2^125)

       t = 2*(t0 + t3)
       if t = 0:
           t = 2*(t0 - t3)

       a = (t * t2^3)^(2^125-1)
       b = (a * t2) * t
       x0 = b/2
       x1 = (a * t2) * t1
       if t2 * b^2 != t:
           Swap x0 and x1

       x = x0 + x1 * i
       if Sign(x) != s:
         x = -x

       if -x^2+y^2 != 1+d*x^2*y^2:  # Check curve equation with x
           x = conj(x)
       if -x^2+y^2 != 1+d*x^2*y^2:  # ... or its conjugate
           return FAILED
       return P = (x,y)

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Authors' Addresses

   Watson Ladd
   UC Berkeley

   Email: watsonbladd@gmail.com

   Patrick Longa
   Microsoft Research

   Email: plonga@microsoft.com

   Richard Barnes

   Email: rlb@ipv.sx

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