Network Working Group                                            W. Ladd
Internet-Draft                                   Grad Student UC Berkley
Intended status: Informational                                   R. Salz
Expires: February 13, 2015                           Akamai Technologies
                                                               S. Turner
                                                              IECA, Inc.
                                                         August 12, 2014

                        The Curve25519 Function


   This document specifies the Curve25519 function, an ECDH (Elliptic-
   Curve Diffie-Hellman) key-agreement scheme for use in cryptographic
   applications.  It was designed with performance and security in mind.
   This document is based on information in the public domain.

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   This Internet-Draft will expire on February 13, 2015.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

1.  Introduction

   This document specifies the Curve25519 function, an ECDH (Elliptic-
   curve Diffie-Hellman) key-agreement scheme for use in cryptographic
   applications.  It was designed with performance and security in mind.
   This document is based on information in the public domain.

   This document provides a stable reference for the Curve25519 function
   [Curve25519] to which other specifications may refer when defining
   their use of Curve25519.  It specifies how to use Curve25519 for key
   exchange.  This document defines the algorithm, the "wire format"
   (how to serialize and parse bytes sent over a network, for example),
   and provides some implementation guidance to avoid known side-channel
   timing exposures.

   This document does not specify the use of Curve25519 in any other
   specific protocol, such as TLS (Transport Layer Security) or IPsec
   (Internet Protocol Security).  It does not specify how to use
   Curve25519 for digital signatures.

   Readers are assumed to be familiar with the concepts of elliptic
   curves, modular arithmetic, group operations, and finite fields
   [RFC6090] as well as rings [Curve25519].

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Notation and Definitions

   The following notation and definitions are used in this document
   (notation is to the left of the ":"):

   A: A value used in the elliptic-curve equation E.

   E: An elliptic-curve equation.

   p: A prime.

   GF(p): The field with p elements.

   _#: Subscript notation, where # is a number or letter.

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   =: Assignment.

   ^: Exponentiation.

   +, -, *, /: Addition, subtraction, multiplication, and division,

   Note that all operations are performed modulo p.

3.  The Curve25519 Function

   Let p = 2^255 - 19.  Let E be the elliptic curve with the equation
   y^2 = x^3 + 486662 * x^2 + x over GF(p).

   Each element x of GF(p) has a unique little-endian representation as
   32 bytes x[0] ... x[31], such that x[0] + 256 * x[1] + 256^2 * x[2] +
   ... + 256^31 * x[31] is congruent to x modulo p, and x[31] is
   minimal.  Implementations MUST only produce points in this form.  On
   receiving a point, implementations MUST mask the leftmost bit of byte
   31 to zero.  This is done to preserve compatibility with point
   formats which reserve the sign bit for use in other protocols and
   increase resistance to implementation fingerprinting.
   Implementations MUST reject numbers in the range [2^255-19, 2^255-1],

   Let X denote the projection map from a point (x,y) on E, to x,
   extended so that X of the point at infinity is zero.  X is surjective
   onto GF(p) if the y coordinate takes on values in GF(p) and in a
   quadratic extension of GF(p).

   Then Curve25519(s, X(Q)) = X(sQ) is a function defined for all
   integers s and elements X(Q) of GF(p).  Proper implementations use a
   restricted set of integers for s and only x-coordinates of points Q
   defined over GF(p).  The remainder of this document describes how to
   compute this function quickly and securely, and use it in a Diffie-
   Hellman scheme.

4.  Implementing the Curve25519 Function

   Let s be a 255 bits long integer, where
   s = sum s_i * 2^i with s_i in {0, 1}.

   Computing Curve25519(s, x) is done by the following procedure, taken
   from [Curve25519] based on formulas from [Mont].  All calculations
   are performed in GF(p), i.e., they are performed modulo p.  The
   parameter a24 is a24 = (486662 - 2) / 4 = 121665.

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       x_1 = x
       x_2 = 0
       z_2 = 1
       x_3 = x
       z_3 = 1
       For t = 254 down to 0:
           // Conditional swap; see text below.
       (x_2, x_3) = cswap (s_t, x_2, x_3)
       (z_2, z_3) = cswap (s_t, z_2, z_3)
           A = x_2 + z_2
           AA = A^2
           B = x_2 - z_2
           BB = B^2
           E = AA - BB
           C = x_3 + z_3
           D = x_3 - z_3
           DA = D * A
           CB = C * B
           x_3 = (DA + CB)^2
           z_3 = x_1 * (DA - CB)^2
           x_2 = AA * BB
           z_2 = E * (AA + a24 * E)
           // Conditional swap; see text below.
       (x_2, x_3) = cswap (s_t, x_2, x_3)
       (z_2, z_3) = cswap (s_t, z_2, z_3)
       Return x_2 * (z_2^(p - 1))

   In implementing this procedure, due to the existence of side-channels
   in commodity hardware, it is important that the pattern of memory
   accesses and jumps not depend on the values of any of the bits of s.
   It is also important that the arithmetic used not leak information
   about the integers modulo p (such as having b * c distinguishable
   from c * c).

   The cswap instruction SHOULD be implemented in constant time
   (independent of s_t) as follows:

   cswap(s_t, x_2, x_3) dummy = s_t * (x_2 - x_3) x_2 = x_2 - dummy x_3
   = x_3 + dummy Return (x_2, x_3)

   where s_t is 1 or 0.  Alternatively, an implementation MAY use the

     dummy = mask(s_t) AND (x_2 XOR x_3)
     x_2 = x_2 XOR dummy
     x_3 = x_3 XOR dummy

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   where mask(s_t) is the all-1 or all-0 word of the same length as x_2
   and x_3, computed, e.g., as mask(s_t) = 1 - s_t.  The latter version
   is often more efficient.

5.  Use of the Curve25519 function

   The Curve25519 function can be used in an ECDH protocol as follows:

   Alice generates 32 random bytes in f[0] to f[31].  She masks the
   three rightmost bits of f[0] and the leftmost bit of f[31] to zero
   and sets the second leftmost bit of f[31] to 1.  This means that f is
   of the form 2^254 + 8 * {0, 1, ..., 2^(251) - 1} as a little-endian

   Alice then transmits K_A = Curve25519(f, 9) to Bob, where 9 is the
   number 9.

   Bob similarly generates 32 random bytes in g[0] to g[31], applies the
   same masks, computes K_B = Curve25519(g, 9) and transmits it to

   Alice computes Curve25519(f, Curve25519(g, 9)); Bob computes
   Curve25519(g, Curve25519(f, 9)) using their generated values and the
   received input.

   Both of them now share K = Curve25519(f, Curve25519(g, 9)) =
   Curve25519(g, Curve25519(f, 9)) as a shared secret.  Alice and Bob
   can then use a key-derivation function, such as hashing K, to compute
   a key.

6.  Test Vectors

   The following test vectors are taken from [NaCl].  All numbers are
   shown as little-endian hexadecimal byte strings:

   Alice's private key, f:

    77 07 6d 0a 73 18 a5 7d 3c 16 c1 72 51 b2 66 45
    df 4c 2f 87 eb c0 99 2a b1 77 fb a5 1d b9 2c 2a

   Alice's public key, Curve25519(f, 9):

    85 20 f0 09 89 30 a7 54 74 8b 7d dc b4 3e f7 5a
    0d bf 3a 0d 26 38 1a f4 eb a4 a9 8e aa 9b 4e 6a

   Bob's private key, g:

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    5d ab 08 7e 62 4a 8a 4b 79 e1 7f 8b 83 80 0e e6
    6f 3b b1 29 26 18 b6 fd 1c 2f 8b 27 ff 88 e0 eb

   Bob's public key, Curve25519(g, 9):

    de 9e db 7d 7b 7d c1 b4 d3 5b 61 c2 ec e4 35 37
    3f 83 43 c8 5b 78 67 4d ad fc 7e 14 6f 88 2b 4f

   Their shared secret, K:

    4a 5d 9d 5b a4 ce 2d e1 72 8e 3b f4 80 35 0f 25
    e0 7e 21 c9 47 d1 9e 33 76 f0 9b 3c 1e 16 17 42

7.  Security Considerations

   Curve25519 meets all standard assumptions on DH and DLP difficulty.

   In addition, Curve25519 is twist secure: the co-factor of the curve
   is 8, that of the twist is 4.  Protocols that require contributory
   behavior must ban outputs K_A = 0, K_B = 0 or K = 0.

   Curve25519 is designed to enable very high performance software
   implementations, thus reducing the cost of highly secure cryptography
   to a point where it can be used more widely.

8.  IANA Considerations


9.  Acknowledgements

   We would like to thank Tanja Lange (Technische Universiteit
   Eindhoven) for her review and comments.

10.  References

10.1.  Normative References

              Bernstein, D., "Curve25519 - new Diffie-Hellman speed
              records", April 2006,

   [Mont]     Montgomery, P., "Speeding the Pollard and elliptic curve
              methods of factorization", 1983,

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC6090]  McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
              Curve Cryptography Algorithms", RFC 6090, February 2011.

10.2.  Informative References

   [NaCl]     Bernstein, D., "Cryptography in NaCl", 2013,

Authors' Addresses

   Watson Ladd
   Grad Student UC Berkley


   Rich Salz
   Akamai Technologies
   8 Cambridge Center
   Cambridge, MA 02142

   Phone: +1-617-714-6169

   Sean Turner
   IECA, Inc.
   Suite 106
   Fairfax, VA 22031

   Phone: +1-703-628-3180

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