Internet-Draft                                                D. Brown
Intended status: Experimental                               BlackBerry
Expires: 2019-10-06                                         2019-04-04

          Elliptic curve 2y^2=x^3+x over field size 8^91+5


  This document recommends using a special elliptic curve alongside
  dissimilar curves, such as NIST P-256, Curve25519, sect283k1,
  Brainpool, and random curves, as a cryptographic defense against an
  unlikely, undisclosed attack against mainstream curves.  Features of
  this curve 2y^2=x^3+x/GF(8^91+5) are: isomorphism to Miller curves
  from 1985; Montgomery form mappable to Edwards; simple field
  powering for inversion, Legendre symbol, and square roots; efficient
  endomorphism to speed up Diffie--Hellman with Bernstein's 2-D
  ladder; 34-byte keys; similarity to Bitcoin curve; hashing-to-point;
  low Kolmogorov complexity (low risk of backdoor).

Status of This Memo

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  provisions of BCP 78 and BCP 79.  Internet-Drafts are working
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  This document may not be modified, and derivative works of it may
  not be created, except to format it for publication as an RFC or to
  translate it into languages other than English.

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Table of Contents

    1.  Introduction
    1.1.  Background
    1.1.1. Notation
    1.2.  Motivation
    2.  Requirements Language (RFC 2119)
    3.  Encoding a point into 34 bytes
    3.1.  Encoding a point into bytes
    3.2.  Decoding bytes into a point
    4.  Point validation
    4.1.  When a public key MAY, SHOULD or MUST be validated
    4.1.1.  Precautionary mandatory validation
    4.1.2.  Simplified validation
    4.1.3.  Relatively safe cases of non-validation
    4.1.4.  Minimal validation
    4.2.  How to validate a point (given only x)
    5.  OPTIONAL encodings
    5.1.  Encoding scalar multipliers as 34 bytes
    5.2.  Encoding 34 bytes into a point (sketch)
    6.  IANA Considerations
    7.  Security considerations
    7.1.  Field choice
    7.2.  Curve choice
    7.3.  Encoding choices
    7.4.  General subversion concerns
    7.5.  Concerns about 'aegis'
    8.  References
    8.1.  Normative References
    8.2.  Informative References
    Appendix A.  Test vectors
    Appendix B.  Motivation: minimizing the room for backdoors
    Appendix C.  Pseudocode
    C.1.  Byte encoding
    C.2.  Byte decoding
    C.3.  Fermat inversion
    C.4.  Branchless Legendre symbol computation
    C.5.  Field multiplication and squaring
    C.6.  Field element partial reduction
    C.7.  Field element final reduction
    C.8.  Scalar point multiplication
    C.9.  Diffie--Hellman pseudocode
    C.10.  Elligator i
    D.  Primality proofs and certificates
    D.1.  Pratt certificate for the field size 8^91+5
    D.2.  Pratt certificate for subgroup order

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1.  Introduction

  This document relates to elliptic curve cryptography (ECC).  It
  specifies methods for using the elliptic curve 2y^2=x^3+x over the
  field of size 8^91+5.  It recommends using this curve in combination
  with a diverse set of curves, as a strongest-link multi-layer
  defense-in-depth against undisclosed attacks against some subset of

1.1.  Background

  This document presumes that its reader already has familiarity with
  elliptic curve cryptography (ECC).

1.1.1. Notation

  The symbol '^', as used in '2y^2=x^3+x' and '8^91+5' means
  exponentiation, also known as powering.  For example, y^3=yyy, or
  y*y*y, if * is used for multiplication, and 8^91 = 8*8*...*8, with
  91 eights in the product on the right.

    Note: This document does not use '^' the way that C (and similar
    programming languages) use it as bit-wise exclusive-or.

  In hexadecimal (base 16, big-endian) notation, the number 8^91+5 is


  with with 67 zeros between 2 and 5.

1.2.  Motivation

  The main motivation is that the description of the curve is very
  short (for an otherwise secure elliptic curve), thereby reducing the
  room for a secretly embedded trapdoor, as in [Teske].

  The best countermeasure against a secretly embedded trapdoor in an
  elliptic curve is to use a diverse combination of elliptic curves.
  So, this curve is only recommended for use in such a combination.

  The detailed motivations for curve 2y^2=x^3+x over field 8^91+5 are
  discussed in Appendix B (and in [B1]).

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2.  Requirements Language (RFC 2119)

  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
  document are to be interpreted as described in RFC 2119 [BCP14].

3.  Encoding a point into 34 bytes

  Elliptic curve cryptography uses points for public keys and raw
  shared secrets.

  Abstractly, points are mathematical objects.  For curve 2y^2=x^3+x,
  a point is either a pair (x,y), where x and y are elements of
  mathematical field, or a special point O, both of whose coordinates
  may be deemed as infinity.

  For 2y^2=x^3+x, the coordinates are x and y are field elements for
  this curve are integers modulo 8^91+5.

    Note: for practicality, an implementation will often represented
    the x-coordinate as a ratio [X:Z] of field elements.  Each field
    element has multiple representations, but [x:1] can viewed as
    normal representation of x.  (Infinity can be then represented by
    [1:0], though one must be careful.)

  To interoperably communicate, points must be encoded as byte

  This draft specifies an encoding of finite points (x,y) as strings
  of 34 bytes, as described in the following sections.

    Note: The 34-byte encoding is not injective. Each point is
    generally among a group of four points that share the same byte

    Note: The 34-byte encoding is not surjective.  Approximately half
    of 34-byte strings do not encode a finite point (x,y).

    Note: In many typical ECC schemes, the 34-byte encoding works
    well, despite being neither injective nor surjective.

3.1.  Encoding a point into bytes

  In short: a finite point (x,y) is encoded by the little-endian byte
  representation of x or -x, whichever fits into 34 bytes.

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  In detail: a point (x,y) is encoded into 34 bytes, as follows.

  First, ensure that x is fully reduced mod p=8^91+5, so that

   0 <= x < 8^91+5.

  Second, further reduce x by a flipping its sign.  Let

   x' =: min(x,p-x) mod 2^272.

  Third, set the byte string b to be the little-endian encoding of the
  reduced integer x', by finding the unique integers b[i] such that
  0<=b[i]<256 and

   (x' mod 2^272) = sum (0<=i<=33, b[i]*256^i).

  Pseudocode can be found in Appendix C.

    Note: the loss of information that happens upon replacing x by -x
    represents applying a complex multiplication by i on the curve,
    since [i](x,y) = (-x,iy) = (u,v) is also a point on the curve,
    because 2u^2 = 2(iy)^2 = -2y^2 = -(x^3+x) = (-x)^3 + (-x) = v^3 +
    v.  In many application, particularly Diffie--Hellman key
    agreement, this loss of information is carried through the final
    shared secret, which means that Alice and Bob can agree on the
    same secret 34 bytes.

  In elliptic curve algorithms where the original x coordinate and the
  decoded x coordinate need to match exactly, then the 34-byte
  encoding is probably not usable unless the following pre-encoding
  procedure is practical:

  Given a point x where x is larger than min(x,p-x), first replace x
  by x'=p-x, on the encoder's side, using the new value x' (instead of
  x) for any further step in the algorithm.  In other words, replace
  the point (x,y) by the point (x',y')=(-x,iy).  Most algorithms will
  also require a discrete logarithm d of (x,y), meaning (x,y) = [d] G
  for some point G.  Since (x',y') = [i](x,y), we can replace by d'
  such that [d']=[i][d].  Usually, [i] can be represented by an
  integer, say j, and we can compute d' = jd (mod ord(G)).

3.2.  Decoding bytes into a point

  In short: the bytes are little-endian decoded into an integer which
  becomes the x-coordinate.  Public-key validation done if needed.  If
  needed, the y-coordinate is recovered.

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  In greater detail: if byte i is b[i], with an integer value
  between 0 and 255 inclusive, then

   x = sum( 0<=i<=33, b[i]*256^i)

    Note: a value of -x (mod p) will also be suitable, and results in
    a point (-x,y') which might be different from the originally
    encoded point.  However, it will be one of the points [i](x,y) or
    -[i](x,y) where [i] means complex multiplication by [i].

  In many cases, such as Diffie--Hellman key agreement using the
  Montgomery ladder, neither the original value of x or -x nor
  coordinate y of the point is needed.  In these cases, the decoding
  steps can be considered completed.

    |                                                       |
    |        \  W  / /A\  |R) |N | I |N | /G   !            |
    |         \/ \/ /   \ |^\ | \| | | \| \_7  0            |
    |                                                       |
    |                                                       |
    |  WARNING: Some byte strings b decode to an invalid    |
    |  point (x,y) that does not belong to the curve        |
    |  2y^2=x^3+x.  In some situations, such invalid b can  |
    |  lead to a severe attack.  In these situations, the   |
    |  decoded point (x,y) MUST be validated, as described  |
    |  below in Section 4.                                  |
    |                                                       |

  In cases, where a value for at least of y, -y, iy, or -iy is needed
  such as Diffie--Hellman key agreement using some other coordinate
  system (such as one might need when converting to Edwards
  coordinates), the candidate value can be obtained by computing a
  square root:

    y = ((x^3+x)/2)^(1/2).

  In some cases, it is important for the decoded value of x to match
  the original value of x exactly.  In that case, the encoder should
  use the procedure that replace x by p-x, and adjusts the discrete
  logarithm appropriately.  These steps can be done by the encoder,
  with the decoder doing nothing.

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4.  Point validation

  In elliptic curve cryptography, scalar multiplying an invalid public
  key by a private key risks leaking information about the private

    Note: For curve 2y^2=x^3+x over 8^91+5, the underlying attacks are
    a little milder than the average a typical elliptic curve.

4.1.  When a public key MAY, SHOULD or MUST be validated

4.1.1.  Precautionary mandatory validation

  Every public key (and point) MAY be validated, as an extra
  precaution (i.e., defense in depth.

4.1.2.  Simplified validation

  If small but general implementation aims for high speed, then the
  implementation might not be able to the cost mandatory public key

  It SHOULD follow at least the rule that an distrusted public key is
  validated before combination with a static secret key.

    |   STATIC                                                      |
    |   SECRET                                                      |
    |    KEY      ------\                     _  ___                |
    |     +              )   PUBLIC |\/| | | (_`  |                 |
    |  UNPROVEN   ------/    KEY    |  | \_/ ._)  |  BE VALIDATED.  |
    |   PUBLIC                                                      |
    |    KEY                                                        |

4.1.3.  Relatively safe cases of non-validation

  In some application an implementation of ECC seem not to suffer an
  invalid curve attack.  This section lists these situations.

     Note: The main reason to omit public key validation is save

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  We classify these situations at two levels: safe, if it seems that
  no harm is possible, and relatively safe, if it there the attack is
  both no worse than another attack and requires something else to be

  To be verified.

  If the secret key is ephemeral, the public key is trusted (signed)
  and a Montgomery ladder is used, then omitting validation of the
  public key seems relatively safe.  This is mostly because if the
  holder of the public key is the attacker, then the trust system has
  been broken.  Furthermore, the attacker, as the intended recipient
  in a typical communication, should already be able to receive any
  data hidden by the secret key.

  To be extended.

4.1.4.  Minimal validation

  To maximize efficiency, an implementation may wish to minimize the
   amount of validation done down to the point of only resisting a

  To be completed.

    Note: a given point need only be validated once, if the
    implementation can track validation state.

  The curve 2y^2=x^3+x is not twist-secure: using the Montgomery
  ladder for scalar multiplication is not enough to thwart invalid
  public key attacks.

    Note: the twist of 2y^2=x^3+x/GF(8^91+5) curve has order:

    2^2 * 5 * 1526119141 * 788069478421 * 182758084524062861993 *

4.2.  How to validate a point (given only x)

  Upon decoding the 34 bytes into x, the next step is to compute
  z=2(x^3+x). Then one checks if z has a nonzero square root (in the
  field of size 8^91+5).  If z has a nonzero square root, then the
  represented point is valid, otherwise it is not valid.

  Equivalently, one can check that x^3 + x has no square root (that
  is, x^3+x is a quadratic non-residue).

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  To check z for a square root, one can compute the Legendre symbol
  (z/p) and check that is 1.  (Equivalently, one can check that

  The Legendre symbol can be computed using Gauss' quadratic
  reciprocity law, but this requires implementing modular integer
  arithmetic for moduli smaller than 8^91+5.

  More slowly, but perhaps more simply, one compute the Legendre
  symbol using powering in the field: (z/p) = z^((p-1)/2) =
  z^(2^272+2).  This will have value 0,1 or p-1 (which is equivalent
  to -1).

  More generally, in signature applications, where the y-coordinate is
  also needed, the computation of y, which involves computing a square
  root will generally include a check that x is valid.

  OPTIONAL: In some rare situations, it is also necessary to ensure
  that the point has large order, not just that it is on the curve.

  For points on this curve, each point has large order, unless it has
  torsion by 12.  In other words, if [12]P != O, then the point P has
  large order.

  OPTIONAL: In even rarer situations, it may be necessary to ensure
  that a point P also has a prime order n = ord(G).  The costly method
  to check this is checking that [n]P = O.  An alternative method is
  to try to solve for Q in the equation [12]Q=P, which involves
  methods such a division polynomials.

  To be completed.

5.  OPTIONAL encodings

  The following two encodings are not usually required to obtain
  interoperability in the typical ECC applications, but can sometimes
  be useful.

5.1.  Encoding scalar multipliers as 34 bytes

  To be completed.

  Basically, little-endian byte encoding of integers is recommended.

  The main application is to signatures.

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  Another application is for test vectors (to be completed).

5.2.  Encoding 34 bytes into a point (sketch)

   In niche applications, it may be desired to encode arbitrary bytes

    Note: This type of encoding is sometimes called hashing to a

    Note: Diffie--Hellman key exchange or digital signatures do not
    require encoding of arbitrary byte strings.

  Example reasons are anonymity, or hiding the presence of a key

    Note: the point encoding described earlier does a different job.
    It encodes every point as a byte string.  The task here is the
    opposite: to encode every byte string as a point.

    Note: Encoding a byte string as a point yields biased elliptic
    curve points, but has the advantage that the byte-strings are

  The encoding is called Elligator i, (see also [B1]), and is just a
  minor variation of the Elligator 2 construction [Elligator].
  Elligator 2 fails for curves with j-invariant 1728, which includes
  2y^2=x^3+x, so a minor tweak is made, obtain Elligator i.

  Fix a square root i of -1 in the field.

  Given any random field element r, compute

    x=i- 3i/(1-ir^2)

  If there is no y solving 2y^2=x^3+x for this x, then replace x by
  x+i and try to solve for y once again.

  If the first x fails, then the second x succeeds.

  So, now r determines a unique x.  To determine y, solve it per the
  equation, getting two roots.  Label the 2 roots y0 and y1 according
  to a deterministic rule.  Then choose y0 if the first x works, else
  choose y2.  This ensures that the map from r^2 to (x,y) is

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  Finally, to encode a byte string b, just let it represent a field
  element r.  Note that -r will be require more than 34 bytes.  So the
  map from b to (x,y) is now injective.

  The Elligator i map is reversible.

  To be completed.

6.  IANA Considerations

  This document requires no actions by IANA, yet.

7.  Security considerations

  No cryptographic algorithms is without risks.  Consequently, risks
  are comparative.  This section will not fully list the risks of all
  other forms of elliptic curve cryptography.  Instead it will list
  the most plausible risks of this curve, and only to a limited degree
  contrast these to a few other standardized curves.

7.1.  Field choice

  The field 8^91+5 has the following risks.

  - 8^91+5 is a special prime.  As such, it is perhaps vulnerable to
    some kind of attack.  For example, for some curve shapes, the
    supersingularity depends on the prime, and the curve size is
    related in a simple way to the field size, causing a potential
    correlation between the field size and the effectiveness of an
    attack, such as the Pohlig--Hellman attack.

    Many other standard curves, such as the NIST P-256 and
    Curve25519, also use special prime field sizes, so have a similar
    risk.  Yet other standard curves, such as the Brainpool, use
    pseudorandom field sizes, so have less risk to this threat.

  - 8^91+5, while implementable in five 64-bit words, has some risk of
    overflowing, or of not fully reducing properly.  Perhaps a smaller
    field, such as that used in Curve25519, has a simpler reduction
    and overflow-avoidance properties.

  - 8^91+5, by virtue of being well-above 256 bits in size, risks its
    user doing extra, and perhaps unnecessary, computation to protect
    their 128-bit keys, whereas smaller curves might be faster (as
    expected) yet still provide enough security.  In other words, the
    extra cost is wasteful, and partially a form of denial of service.

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  - 8^91+5 is smaller than some other six-symbol primes: 8^95-9,
    9^99+4 and 9^87+4.  Arguably, 8^91+5 fails to maximize field size,
    and thus potential Pollard rho resistance of the ECDLP, among
    six-symbol primes.  The primes 9^99+4 and 9^87+4 are not close to
    a power of two, so probably suffer from much slower implementation
    than 8^91+5, which is a significant cost.  The prime 8^95-9 is
    just a little below a power of two, so should have comparable
    efficiency for basic field arithmetic.  The field 8^95-9 is a
    little larger, but can still be implemented using five 64-bit
    words.  Being larger, 8^85-9, it has a slightly greater risk than
    8^91+5 of leading to an arithmetic overflow implementation fault
    in field arithmetic.  Also, field size 8^91+5 has very simple
    powering algorithms for computing field inverses, Legendre
    symbols, and square roots, all because it is just slightly above a
    power of two.  For field size 8^85-9, these powering algorithms
    require more complicated algorithms.

  - 8^91+5 is smaller than 2^283 (the field size for curve sect283k1
    [SEC2], [Zigbee]), and many other five-symbol and four-symbol
    prime powers (such as 9^97).  It provides less resistance to
    Pollard rho than such larger prime powers.  Recent progress in the
    elliptic curve discrete logarithm problem, [HPST] and [Nagao], is
    the main reason to prefer prime fields instead of power of prime
    fields.  A second reason to prefer a prime field (including the
    field of size 8^91+5) over small characteristic fields is the
    generally better software speed of large characteristic field.
    (Better software speed is mainly due to general-purpose hardware
    often having dedicated fast multiplication circuits:
    special-purpose hardware should make small characteristic field

  See [B1] for further discussion.

7.2.  Curve choice

  A first risk of using 2y^2=x^3+x is the fact that it is a special
  curve, with complex multiplication leading to an efficient
  endomorphism.  Many other standard curves, NIST P-256 [NIST-P-256],
  Curve25519, Brainpool [Brainpool], do not have any efficient
  endomorphisms.  Yet some standard curves do, NIST K-283 and
  secp256k1 (see [SEC2] and [BitCoin]).  Furthermore, it is not
  implausible [KKM] that special curves, including those efficient
  endomorphisms, may survive an attack on random curves.

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  A second risk of 2y^2=x^3+x over 8^91+5 is the fact that it is not
  twist-secure.  What may happen is that an implementer may use the
  Montgomery ladder in Diffie--Hellman and re-use private keys.  They
  may think, despite the (ample?) warnings in this document, that
  public key validation in unnecessary, modeling their implementation
  after Curve25519 or some other twist-secure curve.  This implementer
  is at risk of an invalid public key attack.  Moreover, the
  implementer has an incentive to skip public-key validation, for
  better performance.  Finally, even if the implementer uses
  public-key validation, then the cost of public-key validation is

  A third risk is a biased ephemeral private key generation in a
  digital signature scheme.  Most standard curves lack this risk
  because the field size is close to a power of two, and the cofactor
  is a power of two.  Curve 2y^2=x^3+x over 8^91+5 has a base point
  order which is approximately a power of two divided by nine (because
  its cofactor is 72=8*9.)  As such, it is more vulnerable than
  typical curves to biased ephemeral keys in a signature scheme.

  A fourth risk is a Cheon-type attack.  Few standard curves address
  this risk, and 2y^2=x^3+x over 8^91+5 is not much different.

  A fifth risk is a small-subgroup confinement attack, which can also
  leak a few bits of the private key.   Curve 2y^2=x^3+x over 8^91+5
  has 72 elements whose order divides 12.

7.3.  Encoding choices

  To be completed.

7.4.  General subversion concerns

  Although the main motivation of curve 2y^2=x^3+x over 8^91+5 is to
  minimize the risk of subversion via a backdoor ([Gordon], [YY],
  [Teske]), it is only fair to point out that its appearance in this
  very document can be viewed with suspicion as an possible effort at
  subversion (via a front-door).  (See [BCCHLV] for some further

  Any other standardized curve can be view with a similar suspicion
  (except, perhaps, by the honest authors of those standards for whom
  such suspicion seems absurd and unfair).  A skeptic can then examine
  both (a) the reputation of the (alleged) author of the standard,
  making an ad hominem argument, and (b) the curve's intrinsic merits.

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  By the very definition of this document, the reader is encouraged to
  take an especially skeptical viewpoint of curve 2y^2=x^3+x over
  8^91+5.  So, it is expected that skeptical users of the curve will

  - use the curve for its other merits (other than its backdoor
    mitigations), such as efficient endomorphism, field inversion,
    high Pollard rho resistance within five 64-bit words, meanwhile
    holding to the evidence-supported belief ECC that is now so mature
    that worries about subverted curves are just far-fetched nonsense,

  - as an additional of layer of security in addition to other
    algorithms (ECC or otherwise), as an extra cost to address the
    non-zero probability of other curves being subverted.

  To paraphrase, consider users seriously worried about subverted
  curves (or other cryptographic algorithms), either because they
  estimate as high either the probability of subversion or the value
  of the data needing protection.  These users have good reason to
  like 2y^2=x^3+x over 8^91+5 for its compact description.
  Nevertheless, the best way to resist subversion of cryptographic
  algorithms seems to be combine multiple dissimilar cryptographic
  algorithms, in a strongest-link manner.  Diversity hedges against
  subversion, and should the first defense against it.

7.5.  Concerns about 'aegis'

  The exact curve 2y^2=x^3+x over 8^91+5 was (seemingly) first
  described to the public in 2017 [AB].  So, it has a very low age.

  Furthermore, it has not been submitted for a publication with peer
  review to any cryptographic forum such as the IACR conferences like
  Crypto and Eurocrypt.  So, it has only been reviewed by very few
  eyes, most of which had little incentive to study it critically.

  Under the metric of aegis, as in age * eyes, it scores low.
  Counting myself (but not quantifying incentive) it gets an aegis
  score of 0.1 (using a rating 0.1 of my eyes factor in the aegis
  score: I have not discovered any major ECC attacks of my own.)  This
  is far smaller than some more well-studied curves.

  However, in its defense, the curve 2y^2=x^3+x over 8^91+5 has
  similarities to some of the better-studied curves with much higher

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  - Curve25519: has field size 8^85-19, which a little similar to
    8^91+5; has equation of the form by^2=x^3+ax+x, with b and a
    small, which is similar to 2y^2=x^3+x.  Curve25519 has been around
    for over 10 years, has (presumably) many eyes looking at it, and
    has been deployed thereby creating an incentive to study.  An
    estimated aegis for Curve25519 is 10000.

  - P-256: has a special field size, and maybe an estimated aegis of
    200000.  (It is a high-incentive target.  Also, it has received
    much criticism, showing some intent of cryptanalysis.  Indeed,
    there has been incremental progress in finding minor weakness
    (implementation security flaws), suggestive of actual
    cryptanalytic effort.)  The similarity to 2y^2=x^3+x over 8^91+5
    is very minor, so very little of the P-256 aegis would be relevant
    to this document.

  - secp256k1: has a special field size, though not quite as special
    as 8^91+5, and has special field equation with an efficient
    endomorphism by a low-norm complex algebraic integer, quite
    similar to 2y^2=x^3+x.  It is about 17 years old, and though not
    studied much in academic work, its deployment in Bitcoin has at
    least created an incentive to attack it.  An estimated aegis for
    secp256k1 is 10000.

  - Miller's curve: Miller's 1985 paper introducing ECC suggested,
    among other choices, a curve equation y^2=x^3-ax, where a is a
    quadratic non-residue.  Curve 2y^2=x^3+x is isomorphic to
    y^2=x^3-x, essentially one of Miller's curves, except that a=1 is
    a quadratic residue.  Miller's curve may not have been studied
    intensely as other curves, but its age matches that ECC itself.
    Miller also hinted that it was not prudent to use a special curve
    y^2=x^3-ax: such a comment may have encouraged some cryptanalysts,
    but discouraged cryptographers, perhaps balancing out the effect
    on the eyes factor the aegis.  An estimated aegis for Miller's
    curves is 300.

  Obvious cautions to the reader:

  - Small changes in a cryptographic algorithm sometimes cause large
    differences in security.  So security arguments based on
    similarity in cryptographic schemes should be given low priority.

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  - Security flaws have sometimes remained undiscovered for years,
    despite both incentives and peer reviews (and lack of hard
    evidence of conspiracy).  So, the eyes-part of the aegis score is
    very subjective, and perhaps vulnerable false positives by a herd
    effect.  Despite this caveat, it is not recommended to ignore the
    eyes factor in the aegis score: don't just flip through old books
    (of say, fiction), looking for cryptographic algorithms that might
    never have been studied.

8.  References

8.1.  Normative References

  [BCP14] Bradner, S., "Key words for use in RFCs to Indicate
          Requirement Levels", BCP 14, RFC 2119, March 1997,

8.2.  Informative References

  To be completed.

  [AB] A. Allen and D. Brown.  ECC mod 8^91+5, presentation to CFRG,

  [AMPS] Martin R. Albrecht, Jake Massimo, Kenneth G. Paterson, and
     Juraj Somorovsky.  Prime and Prejudice: Primality Testing Under
     Adversarial Conditions, IACR ePrint,
     2018. <>

  [B1] D. Brown.  ECC mod 8^91+5, IACR ePrint, 2018.

  [B2] D. Brown.  RKHD ElGamal signing and 1-way sums, IACR ePrint,
     2018. <>

  [KKM] A. Koblitz, N. Koblitz and A. Menezes.  Elliptic Curve
     Cryptography: The Serpentine Course of a Paradigm Shift, IACR
     ePrint, 2008.  <>

  [BCCHLV] D. Bernstein, T. Chou, C. Chuengsatiansup, A. Hulsing,
     T. Lange, R. Niederhagen and C. van Vredendaal.  How to
     manipulate curve standards: a white paper for the black hat, IACR
     ePrint, 2014. <>

  [Elligator] (((To do:))) fill in this reference.

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  [NIST-P-256] (((To do:))) NIST recommended 15 elliptic curves for
     cryptography, the most popular of which is P-256.

   [Zigbee] (((To do:))) Zigbee allows the use of a
     special curve, which was also recommended by NIST, called K-283,
     and also known as sect283k1.  These types of curves were
     introduced by Koblitz.  These types of curves were not
     recommended by NSA in Suite B.

  [Brainpool] (((To do:))) the Brainpool consortium (???) recommended
     some elliptic curves in which both the field size and the curve
     equation were derived pseudorandomly from a nothing-up-my-sleeve

  [SEC2] Standards for Efficient Cryptography.  SEC 2: Recommended
     Elliptic Curve Domain Parameters, version 2.0, 2010.

  [IT] T. Izu and T. Takagi.  Exceptional procedure attack on elliptic
     curve cryptosystems, Public key cryptography -- PKC 2003, Lecture
     Notes in Computer Science, Springer, pp. 224--239, 2003.

  [PSM] (((To do:))) Pointcheval, Smart, Malone-Lee.  Projective
     coordinates leak.

  [BitCoin] (((To do:))) BitCoin uses curve secp256k1, which has an
     efficient endomorphism.

  [Bleichenbacher] To do: Bleichenbacher showed how to attack DSA
      using a bias in the per-message secrets.

  [Gordon] (((To do:))) Gordon showed how to embed a trapdoor in DSA

  [HPST] Y. Huang, C. Petit, N. Shinohara and T. Takagi.  On
     Generalized First Fall Degree Assumptions, IACR ePrint 2015.

  [Nagao] K. Nagao.  Equations System coming from Weil descent and
     subexponential attack for algebraic curve cryptosystem, IACR
     ePrint, 2015.  <>

  [Teske] E. Teske.  An Elliptic Curve Trapdoor System, IACR ePrint,
     2003.  <>

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   [YY] (((To do:))) Yung and Young, generalized Gordon's ideas
  [Gordon] into
     Secretly-embedded trapdoor ... also known as a backdoor.

Appendix A.  Test vectors

  To be completed.

Appendix B.  Motivation: minimizing the room for backdoors

  To be completed.

  See [AB] and [B1] for some details.

  The field and curve are described with very few symbols, while
  retaining many basic security and speed features.

  A prime field was chosen due to recent asymptotic advances on
  discrete logarithms in low-characteristic fields [HPST] and
  [Nagao].  According to [Teske], some characteristic-two elliptic
  curves could be equipped with a secretly embedded backdoor.

    Note: this curve is isomorphic to the non-Montgomery curve
    y^2=x^3-x, which requires just 9 symbols in its description, 1
    fewer than required by 2y^2=x^3+x.

Appendix C.  Pseudocode

  This section uses a C-like pseudocode to describe some of the
  algorithms useful for implementing this curve.

  Real-world implementations adapting this pseudocode had better
  harden this pseudocode against real-world implementation issues.
  Better yet, real-world code could start from scratch, using the
  pseudocode only for comparison.

    Note: the pseudocode relies on some C idioms (hacks?), which might
    make the pseudocode unclear to those unfamiliar with these idioms.

    Note: this pseudocode was adapted from a few different
    experimental prototypes of the author, (which might not be
    consistent).  The pseudocode has not yet received any independent

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    Note: this pseudocode uses a terse non-conventional coding style,
    partly as an exercise in arbitrary source code compression (code
    golf), but also in the mathematics tradition of using many
    single-letter variable names, which enables seeing an entire
    formula in a single view and emphasizes the essential mathematical
    operations rather than the variable's purpose.

    Note: the pseudocode does not use the C operator ^ for bitwise XOR
    of integers, which (luckily) avoid possible confusion with the use
    of ^ as exponentiation operator in the rest of this document.

C.1.  Byte encoding

  Pseudocode for byte representation encoding process is

  bite(c b,f x) {
   i j=34,k=5; f t;
   for(;j--;) b[j]=x[j/7]>>((8*j)%55);
   for(;--k;) b[7*k-1]+=x[k]<<(8-k);

  The input variable is x and the output variable is b.  The declared
  types and functions are as follows:

  - type c: curve representative, length-34 array of non-negative
    8-bit integers ("characters"),

  - type f: field element, a length-5 array of 64-bit integers
    (negatives allowed), representing a field element as an integer in
    base 2^55,

  - type i: 64-bit integers (e.g. entries of f),

  - function mal: multiply a field element by a small integer (result
    stored in 1st argument),

  - function fix: fully reduce an integer modulo 8^91+5,

  - function cmp: compare two field element (after fixing), returning
    -1, 0 or 1.

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    Note: The two for-loops in the pseudocode are just radix
    conversion, from base 2^55 to base 2^8.  Because both bases are
    powers of two, this amount to moving bits around.  The entries of
    array b are compute modulo 256.  The second loop copies the bits
    that the first loop misses (the bottom bits of each entry of f).

    Note: Encoding is lossy, several different (x,y) may encode to the
    same byte string b.  Usually, if (x,y) generated as a part of
    Diffie--Hellman key exchange, this lossiness has no effect.

    Note: Encoding should not be confused with encryption.  Encoding
    is merely a conversion or representation process, whose inverse is
    called decoding.

C.2.  Byte decoding

  Pseudocode for decoding is:

  feed(f x,c b) {
   i j=34;
   for(;j--;) x[j/7]+=((i)b[j])<<((8*j)%55);

  with similar conventions as used in the pseudocode function bite
  (defined in the section on encoding), and some extra conventions:

  - the expression (i)b[j] means that 8-bit integer b[j] is converted
    to a 64-bit integer (so is no longer treated modulo 256).  (In C,
    this is operation is called casting.)

    Note: the decode function 'feed' only has 1 for-loop, which is the
    approximate inverse of the first of the 2 for-loops in the encode
    function 'bite'.  The reason the 'bite' needs the 2nd for-loop is
    due to the lossy conversion from integers to bytes, whereas in the
    other direction the conversion is not lossy.  The second loop
    recovers the lost information.

C.3.  Fermat inversion

  Projective coordinates help avoid costly inversion steps during
  scalar multiplication.

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  Projective coordinates are not suitable as the final representation
  of an elliptic curve point, for two reasons.

  - Projective coordinates for a point are generally not unique: each
    point can be represented in projective coordinates in multiple
    different ways.  So, projective coordinates are unsuitable for
    finalizing a shared secret, because the two parties computing the
    shared secret point may end up with different projective

  - Projective coordinates have been shown to leak information about
    the scalar multiplier [PSM], which could be the private
    key.  It would be unacceptable for a public key to leak
    information about the private key.  In digital signatures, even a
    few leaked bits can be fatal, over a few signatures

  Therefore, the final computation of an elliptic curve point, after
  scalar multiplication, should translate the point to a unique
  representation, such as the affine coordinates described in this

  For example, when using a Montgomery ladder, scalar multiplication
  yields a representation (X:Z) of the point in projective
  coordinates.  Its x-coordinate is then x=X/Z, which can be computed
  by computing the 1/Z and then multiplying by X.

  The safest, most prudent way to compute 1/Z is to use a side-channel
  resistant method, in particular at least, a constant-time method.
  This reduces the risk of leaking information about Z, which might in
  turn leak information about X or the scalar multiplier.  Fermat
  inversion, computation of Z^(p-2) mod p, is one method to compute
  the inverse in constant time (if the inverse exists).

  Pseudocode for Fermat inversion is:

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  i inv(f y,f x) {
    i j=272;f z;
    for(;j--;) squ(z,z);
    return !!cmp(y,(f){});

  Other inversion techniques, such as the binary extended GCD, may be
  faster, but generally run in variable-time.

  When field elements are sometimes secret keys, using a variable-time
  algorithm risk leaking these secrets, and defeating security.

C.4.  Branchless Legendre symbol computation

  Pseudocode for branchlessly computing if a field element x has a
  square root:

  i has_root(f x) {
    i j=270;f y,z;
    return 0==cmp(y,(f){1});

    Note: Legendre symbol is usually most appropriately applied to
    public keys, which mostly obviates the need for side-channel
    resistance.  In this case, the implementer can use quadratic
    reciprocity for greater speed.

C.5.  Field multiplication and squaring

  To be completed.

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    Note (on security): Field multiplication can be achieved most
    quickly by using hardware integer multiplication circuits.  It is
    critical that those circuits have no bugs or backdoors.
    Furthermore, those circuits typically can only multiple integers
    smaller than the field elements.  Larger inputs to the circuits
    will cause overflows.  It is critical to avoid these overflows,
    not just to avoid interoperability failures, but also to avoid
    attacks where the attackers supplies inputs likely induce
    overflows [bug attacks], [IT].  The following pseudocode
    should therefore be considered only for illustrative purposes.
    The implementer is responsible for ensuring that inputs cannot
    cause overflows or bugs.

  The pseudocode below for multiplying and squaring: uses unrolled
  loops for efficiency, uses refactoring for source code compression,
  relies on a compiler optimizer to detect common sub-expressions (in

  #define TRI(m,_)\
  #define CYC(M) ff zz; TRI(+M,-20*M); mod(z,zz);
  #define MUL(j,k) x[j]*(ii)y[k]
  #define SQR(j,k) x[j]*(ii)x[k]
  #define SQU(j,k) SQR(j>k?j:k,j<k?j:k)
  mul(f z,f x,f y) {CYC(MUL);}
  squ(f a,f x) {CYC{SQU};}

  This pseudocode makes uses of some extra C-like pseudocode features:

  - #define is used to create macros, which expand within the source
    code (as in C pre-processing).

  - type ii is 128-bit integer

  - multiplying a type i by a type ii variable yields a type ii
    variable.  If both inputs can fit into a type i variable, then
    the result has no overflow or reduction: it is exact as a product
    of integers.

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  - type ff is array of five type ii values.  It is used to represent
    a field in a radix expansion, except the limbs (digits) can be
    128-bits instead of 64-bits.  The variable zz has type ff and is
    used to intermediately store the product of two field element
    variables x and y (of type f).

  - function mod takes an ff variable and produce f variable
    representing the same field element.  A pseudocode example may be
    defined further below.

  TO DO: Add some notes (answer these questions):

  - How small the limbs of the inputs to function mul and squ must be
    to ensure no overflow occurs?

  - How small are the limbs of the output of functions mul and squ?

C.6.  Field element partial reduction

  To be completed.

  The function mod used by pseudocode function mul and squ above is
  defined below.

  #define QUO(x)(x>>55)
  #define MOD(x)(x&((((i)1)<<5)-1))
  #define Q(j) QUO(QUO(zz[j]))
  #define P(j) MOD(QUO(zz[j]))
  #define R(j)     MOD(zz[j])
  mod(f z,ff zz){

  TO DO: add notes answering these questions:

  - How small must be the input limbs to avoid overflow?

  - How small are the output limbs (to know how to safely use of
    output in further calculations).

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C.7.  Field element final reduction

  To be completed.

  The partial reduction technique is sometimes known as lazy
  reduction.  It is an optimization technique.  It aims to do only
  enough calculation to avoid overflow errors.

  For interoperability, field elements need to be fully reduced,
  because partial reduction means the elements still have multiple
  different representations.

  Pseudocode that aims for final reduction is the following:

  #define FIX(j,r,k) {q=x[j]>>r;\
   x[j]-=q<<r; x[(j+1)%5]+=q*k;}
  fix(f x) {
   i j,q,t=2;
   for(;t--;) for(j=0;j<5;j++) FIX(j,(j<4?55:53),(j<4?1:-5));
   x[0]+=q*5; x[4]+=q>>53;

C.8.  Scalar point multiplication

  Work in progress.

  A recommended method of scalar point multiplication is the
  Montgomery ladder.  However, the curve 2y^2=x^3+x has an efficient
  endomorphism.  So, this can be used to speed-up scalar point
  multiplication, as suggested by Gallant, Lambert and Vanstone.

  Combining both GLV and Montgomery is also possible, such as
  suggested as by Bernstein.

    Note: The following pseudocode is not entirely consistent with
    previous pseudocode examples.

    Note and Warning: The following pseudocode uses secret indices to
    access (small) arrays.  This has a risk of cache-timing attacks.

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  typedef f p[2];
  typedef struct rung {i x0; i x1; i y; i z;} k[137];
  monty_2d (f ps,k sk,f px) {
    i j,h; f z; p w[3],x[3],y[2]={{{},{1}}},z[2];
    endo_i(y[1],y[0]); endo_i(z[1],z[0]);
    copy(x[1],y[0]); copy(x[2],z[0]);
     double_xz(w[0],     x[sk[j].x0 /* cache attack here? */ ]);
     diff_add (w[1],x[1],x[sk[j].x1],y[sk[j].y]);
     diff_add (2[2],x[2],x[0],       z[sk[j].z]);
     for(h=0;h<3;h++) {copy(x[h],w[h]);}

    Note: The pseudocode uses some other functions not defined here,
    but whose meaning can be inferred by ECC experts.

    Note: The pseudocode uses a specialized format for the scalar.
    Normal scalars would have to be re-coded into this format, and
    re-coding has non-negligible run-time.  Perhaps in
    Diffie--Hellman, re-coding is not necessary if one can ensure that
    uniformly selection of coded scalars is not a security risk.

  TO DO:
  -  Define the functions used by monty_2d.
  -  Prove that these function avoid overflow.
  -  Define functions to re-code scalars for monty_2d.

C.9.  Diffie--Hellman pseudocode

  To be completed.

  This pseudocode would show how to use to scalar multiplication,
  combined with point validation, and so on.

C.10.  Elligator i

  To be completed.

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  This pseudocode would show how to implement to the Elligator i map
  from byte strings to points.

  Pseudocode (to be verified):

  typedef f xy[2] ;
  #define X p[0]
  #define Y p[1]
  lift(xy p, f r) {
    f t ; i b ;
    squ(t,r);        // r^2
    mul(t,I,t);      // ir^2
    sub(t,(f){1},t); // 1-ir^2
    inv(t,t);        // 1/(1-ir^2)
    mal(t,3,t);      // 3/(1-ir^2)
    mul(t,I,t);      // 3i/(1-ir^2)
    sub(X,I,t);      // i-3i/(1-ir^2)
    b = get_y(t,X);
    mal(t,1-b,I);    // (1-b)i
    add(X,X,t);      // EITHER  x  OR  x + i
    mal(Y,2*b-1,Y);  // (-1)^(1-b)""
    fix(X);  fix(Y);

  drop(f r, xy p)
    f t ; i b,h ;
    fix(X); fix(Y);
    sub(t,X,t);   // EITHER x or x-i
    sub(t,I,t);   // i-x
    inv(t,t);     // 1/(i-x)
    mal(t,3,t);   // 3/(i-x)
    add(t,I,t);   // i+ 3/(i-x)
    mal(t,-1,t);  // -i-3/(i-x)) = (1-3i/(i-x))/i
    b = root(r,t) ;
    h = (r[4]<(1LL<<52)) ;

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  elligator(xy p,c b) {f r; feed(r,b); lift(p,r);}

  crocodile(c b,xy p) {f r; drop(r,p); bite(b,r);}

D. Primality proofs and certificates

  Recent work of Albrecht and others [AMPS] has shown the combination
  of adversarially chosen prime and improper probabilistic primality
  tests can result in attacks.

  The two primes involved for 2y^2=x^3+x/GF(8^91+5) should already
  resist [AMPS] because compact representation of these primes.

  For further assurance, this section provides Pratt primality
  certificates for the two primes.

    Note: Recall that every prime p has a unique Pratt certificate,
    which consists of the factorization of p into primes, and,
    recursively, Pratt primality certificates for each of those
    primes.  Verification of a Pratt certificates is also recursive:
    it uses the factorization data to conduct Fermat and Lehmer
    tests, which together verify primality.

D.1.  Pratt certificate for the field size 8^91+5

  Define 52 positive integers, a,b,c,...,z,A,...,Z as follows:

   a=2 b=1+a c=1+aa d=1+ab e=1+ac f=1+aab g=1+aaaa h=1+abb i=1+ae
   j=1+aaac k=1+abd l=1+aaf m=1+abf n=1+aacc o=1+abg p=1+al q=1+aaag
   r=1+abcc s=1+abbbb t=1+aak u=1+abbbc v=1+ack w=1+aas x=1+aabbi
   y=1+aco z=1+abu A=1+at B=1+aaaadh C=1+acu D=1+aaav E=1+aeff F=1+aA
   G=1+aB H=1+aD I=1+acx J=1+aaacej K=1+abqr L=1+aabJ M=1+aaaaaabdt
   N=1+abdpw O=1+aaaabmC P=1+aabeK Q=1+abcfgE R=1+abP S=1+aaaaaaabcM
   T=1+aIO U=1+aaaaaduGS V=1+aaaabbnuHT W=1+abffLNQR X=1+afFW
   Y=1+aaaaauX Z=1+aabzUVY.

    Note: variable concatenation is used to indicate multiplication.
    For example, f = 1+aab = 1+2*2*(1+2) = 13.

    Note: Writing % for modular reduction (with lower precedence than
    exponentiation ^), a first step in verifying the Pratt certificate
    is a Fermat pseudoprime test for each prime in the list, meaning
    the all the numbers below are 1:

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     2^(b-1)%b, 2^(c-1)%c, 3^(d-1)%d, 2^(e-1)%e, 2^(f-1)%f, 3^(g-1)%g,
     2^(h-1)%h, 5^(i-1)%i, 6^(j-1)%j, 3^(k-1)%k, 2^(l-1)%l, 3^(m-1)%m,
     2^(n-1)%n, 5^(o-1)%o, 2^(p-1)%p, 3^(q-1)%q, 6^(r-1)%r, 2^(s-1)%s,
     2^(t-1)%t, 6^(u-1)%u, 7^(v-1)%v, 2^(w-1)%w, 2^(x-1)%x, 14^(y-1)%y,
     3^(z-1)%z, 5^(A-1)%A, 3^(B-1)%B, 7^(C-1)%C, 3^(D-1)%D, 7^(E-1)%E,
     5^(F-1)%F, 2^(G-1)%G, 2^(H-1)%H, 2^(I-1)%I, 3^(J-1)%J, 2^(K-1)%K,
     2^(L-1)%L, 10^(M-1)%M, 5^(N-1)%N, 10^(O-1)%O, 2^(P-1)%P,
     10^(Q-1)%Q, 6^(R-1)%R, 7^(S-1)%S, 5^(T-1)%T, 3^(U-1)%U, 5^(V-1)%V,
     2^(W-1)%W, 2^(X-1)%X, 3^(Y-1)%Y, 7^(Z-1)%Z.

    Note: Each Fermat base above was chosen as the minimal possible
    value.  These bases can be deduced from b,c,...,Z by searching
    bases 2,3,4,... until a Fermat is found.  The results of these
    search are included above for convenience.

    Note: A second step to verifying a Pratt certificate is to apply
    Lehmer's theorem to each Fermat psuedoprime.  To prove b,c,d,...,Z
    are prime, it now suffices to verify that all of following 154
    modular exponentiations result in a value different from 1.

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     2^((b-1)/a)%b, 2^((c-1)/a)%c, 3^((d-1)/a)%d, 3^((d-1)/b)%d,
     2^((e-1)/a)%e, 2^((e-1)/c)%e, 3^((f-1)/a)%f, 3^((f-1)/b)%f,
     3^((g-1)/a)%g, 2^((h-1)/a)%h, 2^((h-1)/b)%h, 5^((i-1)/a)%i,
     5^((i-1)/e)%i, 6^((j-1)/a)%j, 6^((j-1)/c)%j, 3^((k-1)/a)%k,
     2^((l-1)/a)%l, 2^((l-1)/f)%l, 3^((m-1)/a)%m, 3^((m-1)/b)%m,
     3^((m-1)/f)%m, 2^((n-1)/a)%n, 2^((n-1)/c)%n, 5^((o-1)/a)%o,
     5^((o-1)/b)%o, 5^((o-1)/f)%o, 2^((p-1)/a)%p, 2^((p-1)/l)%p,
     3^((q-1)/a)%q, 3^((q-1)/g)%q, 6^((r-1)/a)%r, 6^((r-1)/a)%r,
     2^((s-1)/a)%s, 2^((s-1)/b)%s, 2^((t-1)/a)%t, 2^((t-1)/k)%t,
     6^((u-1)/a)%u, 6^((u-1)/b)%u, 6^((u-1)/c)%u, 7^((v-1)/a)%v,
     7^((v-1)/c)%v, 7^((v-1)/k)%v, 2^((w-1)/a)%w, 2^((w-1)/s)%w,
     2^((x-1)/a)%x, 2^((x-1)/b)%x, 2^((x-1)/i)%x, 14^((y-1)/a)%y,
     14^((y-1)/c)%y, 14^((y-1)/o)%y, 3^((z-1)/a)%z, 3^((z-1)/b)%z,
     3^((z-1)/u)%z, 5^((A-1)/a)%A, 5^((A-1)/t)%A, 3^((B-1)/a)%B,
     3^((B-1)/d)%B, 3^((B-1)/h)%B, 7^((C-1)/a)%C, 7^((C-1)/c)%C,
     7^((C-1)/u)%C, 3^((D-1)/a)%D, 3^((D-1)/v)%D, 7^((E-1)/a)%E,
     7^((E-1)/e)%E, 7^((E-1)/f)%E, 5^((F-1)/a)%F, 5^((F-1)/A)%F,
     2^((G-1)/a)%G, 2^((G-1)/B)%G, 2^((H-1)/a)%H, 2^((H-1)/D)%H,
     2^((I-1)/a)%I, 2^((I-1)/c)%I, 2^((I-1)/x)%I, 3^((J-1)/a)%J,
     3^((J-1)/c)%J, 3^((J-1)/e)%J, 3^((J-1)/j)%J, 2^((K-1)/a)%K,
     2^((K-1)/b)%K, 2^((K-1)/q)%K, 2^((K-1)/r)%K, 2^((L-1)/a)%L,
     2^((L-1)/b)%L, 2^((L-1)/J)%L, 10^((M-1)/a)%M, 10^((M-1)/b)%M,
     10^((M-1)/d)%M, 10^((M-1)/t)%M, 5^((N-1)/a)%N, 5^((N-1)/b)%N,
     5^((N-1)/d)%N, 5^((N-1)/p)%N, 5^((N-1)/w)%N, 10^((O-1)/a)%O,
     10^((O-1)/b)%O, 10^((O-1)/m)%O, 10^((O-1)/C)%O, 2^((P-1)/a)%P,
     2^((P-1)/b)%P, 2^((P-1)/e)%P, 2^((P-1)/K)%P, 10^((Q-1)/a)%Q,
     10^((Q-1)/b)%Q, 10^((Q-1)/c)%Q, 10^((Q-1)/f)%Q, 10^((Q-1)/g)%Q,
     10^((Q-1)/E)%Q, 6^((R-1)/a)%R, 6^((R-1)/b)%R, 6^((R-1)/P)%R,
     7^((S-1)/a)%S, 7^((S-1)/b)%S, 7^((S-1)/c)%S, 7^((S-1)/M)%S,
     5^((T-1)/a)%T, 5^((T-1)/I)%T, 5^((T-1)/O)%T, 3^((U-1)/a)%U,
     3^((U-1)/d)%U, 3^((U-1)/u)%U, 3^((U-1)/G)%U, 3^((U-1)/S)%U,
     5^((V-1)/a)%V, 5^((V-1)/b)%V, 5^((V-1)/n)%V, 5^((V-1)/u)%V,
     5^((V-1)/H)%V, 5^((V-1)/T)%V, 2^((W-1)/a)%W, 2^((W-1)/b)%W,
     2^((W-1)/f)%W, 2^((W-1)/L)%W, 2^((W-1)/N)%W, 2^((W-1)/Q)%W,
     2^((W-1)/R)%W, 2^((X-1)/a)%X, 2^((X-1)/f)%X, 2^((X-1)/F)%X,
     2^((X-1)/W)%X, 3^((Y-1)/a)%Y, 3^((Y-1)/u)%Y, 3^((Y-1)/X)%Y,
     7^((Z-1)/a)%Z, 7^((Z-1)/b)%Z, 7^((Z-1)/z)%Z, 7^((Z-1)/U)%Z,
     7^((Z-1)/V)%Z, 7^((Z-1)/Y)%Z.

    Note: The verifier should verify that each base in the Fermat and
    Lehmer test are equal.  For example, the Fermat base for Z was 7,
    and the factorization of Z-1 was aabzUVY, so the Lehmer theorem
    test 7^((Z-1)/a)%Z, 7^((Z-1)/b)%Z, ..., 7^((Z-1)/Y)%Z.

    Note: The final step is to verify that Z=8^91+5.

Brown                2y^2=x^3+x over 8^91+5                 [Page 31]

Internet-Draft                                             2019-04-04

    Note: The decimal values for a,b,c,...,Y are given by: a=2, b=3,
    c=5, d=7, e=11, f=13, g=17, h=19, i=23, j=41, k=43, l=53, m=79,
    n=101, o=103, p=107, q=137, r=151, s=163, t=173, u=271, v=431,
    w=653, x=829, y=1031, z=1627, A=2063, B=2129, C=2711, D=3449,
    E=3719, F=4127, G=4259, H=6899, I=8291, J=18041, K=124123,
    L=216493, M=232513, N=2934583, O=10280113, P=16384237, Q=24656971,
    R=98305423, S=446424961, T=170464833767, U=115417966565804897,
    V=4635260015873357770993, W=1561512307516024940642967698779,

D.2.  Pratt certificate for subgroup order

  Define 56 variables a,b,...,z,A,B,...,Z,!,@,#,$, with new

   a=2 b=1+a c=1+a2 d=1+ab e=1+ac f=1+a2b g=1+a4 h=1+ab2 i=1+ae
   j=1+a2d k=1+a3c l=1+abd m=1+a2f n=1+acd o=1+a3b2 p=1+ak q=1+a5b
   r=1+a2c2 s=1+am t=1+ab2d u=1+abi v=1+ap w=1+a2l x=1+abce y=1+a5e
   z=1+a2t A=1+a3bc2 B=1+a7c C=1+agh D=1+a2bn E=1+a7b2 F=1+abck
   G=1+a5bf H=1+aB I=1+aceg J=1+a3bc3 K=1+abA L=1+abD M=1+abcx N=1+acG
   O=1+aqs P=1+aqy Q=1+abrv R=1+ad2eK S=1+a3bCL T=1+a2bewM U=1+aijsJ
   V=1+auEP W=1+agIR X=1+a2bV Y=1+a2cW Z=1+ab3oHOT !=1+a3SUX @=1+abNY!
   #=1+a4kzF@ $=1+a3QZ#

    Note: numeral after variable names represent powers.  For example,
    f = 1 + a2b = 1 + 2^2 * 3 = 13.

    Note: The same process (Fermat and Lehmer tests) verifies that $
    is prime.  This process includes search for bases for each of the
    prime.  These bases have not been included in the certificate.

  The last variable, $, is the order of the base point, and the order
  of the curve is 72$.

    Note: Punctuation used for variable names !,@,#,$, would not scale
    for larger primes.  For larger primes, a similar format might work
    by using a prefix-free set of multil-letter variable names.
    E.g. replace, Z,!,@,#,$ by Za,Zb,Zc,Zd,Ze:

Brown                2y^2=x^3+x over 8^91+5                 [Page 32]

Internet-Draft                                             2019-04-04

    a=2 b=1+a c=1+a2 d=1+ab e=1+ac f=1+a2b g=1+a4 h=1+ab2 i=1+ae
    j=1+a2d k=1+a3c l=1+abd m=1+a2f n=1+acd o=1+a3b2 p=1+ak q=1+a5b
    r=1+a2c2 s=1+am t=1+ab2d u=1+abi v=1+ap w=1+a2l x=1+abce y=1+a5e
    z=1+a2t A=1+a3bc2 B=1+a7c C=1+agh D=1+a2bn E=1+a7b2 F=1+abck
    G=1+a5bf H=1+aB I=1+aceg J=1+a3bc3 K=1+abA L=1+abD M=1+abcx N=1+acG
    O=1+aqs P=1+aqy Q=1+abrv R=1+ad2eK S=1+a3bCL T=1+a2bewM U=1+aijsJ
    V=1+auEP W=1+agIR X=1+a2bV Y=1+a2cW Za=1+ab3oHOT Zb=1+a3SUX
    Zc=1+abNYZb Zd=1+a4kzFZc Ze=1+a3QZaZd

   When parsing this, say 2nd last item Zd=1+a4kzFZc, the string Zd on
   the left of =  defines a new variables, while the string on the
   right = gives its value. The string a4kzFZc can be unambiguously
   parsed as a^4*k*z*F*Zc, scanning from left to right for previous
   variables, or integers as powers.


  Thanks to John Goyo and various other BlackBerry employees for past
  technical review, to Gaelle Martin-Cocher for encouraging submission
  of this I-D.  Thanks to David Jacobson for sending Pratt primality

Author's Address

  Dan Brown
  4701 Tahoe Blvd.
  BlackBerry, 5th Floor
  Mississauga, ON

Brown                2y^2=x^3+x over 8^91+5                 [Page 33]