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Require Crypto.Algebra.
Require Crypto.Util.GlobalSettings.
Require Import Crypto.Spec.WeierstrassCurve.
Module M.
Section MontgomeryCurve.
Import BinNat.
Context {F Feq Fzero Fone Fopp Fadd Fsub Fmul Finv Fdiv} {field:@Algebra.field F Feq Fzero Fone Fopp Fadd Fsub Fmul Finv Fdiv} {Feq_dec:Decidable.DecidableRel Feq} {char_gt_2:@Ring.char_gt F Feq Fzero Fone Fopp Fadd Fsub Fmul 2}.
Local Infix "=" := Feq : type_scope. Local Notation "a <> b" := (not (a = b)) : type_scope.
Local Infix "+" := Fadd. Local Infix "*" := Fmul.
Local Infix "-" := Fsub. Local Infix "/" := Fdiv.
Local Notation "- x" := (Fopp x).
Local Notation "x ^ 2" := (x*x) (at level 30). Local Notation "x ^ 3" := (x*x^2) (at level 30).
Local Notation "0" := Fzero. Local Notation "1" := Fone.
Local Notation "2" := (1+1). Local Notation "3" := (1+2).
Local Notation "'∞'" := unit : type_scope.
Local Notation "'∞'" := (inr tt) : core_scope.
Local Notation "( x , y )" := (inl (pair x y)).
Local Open Scope core_scope.
Context {a b: F} {b_nonzero:b <> 0}.
Definition point := { P | match P with
| (x, y) => b*y^2 = x^3 + a*x^2 + x
| ∞ => True
end }.
Definition coordinates (P:point) : (F*F + ∞) := proj1_sig P.
Import Crypto.Util.Tactics Crypto.Algebra.Field.
Ltac t :=
destruct_head' point; destruct_head' sum; destruct_head' prod;
break_match; simpl in *; break_match_hyps; trivial; try discriminate;
repeat match goal with
| |- _ /\ _ => split
| [H:@eq (sum _ _ ) _ _ |- _] => symmetry in H; injection H; intros; clear H
| [H:@eq (prod _ _ ) _ _ |- _] => symmetry in H; injection H; intros; clear H
end;
subst; try fsatz.
Program Definition add (P1 P2:point) : point :=
exist _
match coordinates P1, coordinates P2 return _ with
(x1, y1), (x2, y2) =>
if Decidable.dec (x1 = x2)
then if Decidable.dec (y1 = - y2)
then ∞
else (b*(3*x1^2+2*a*x1+1)^2/(2*b*y1)^2-a-x1-x1, (2*x1+x1+a)*(3*x1^2+2*a*x1+1)/(2*b*y1)-b*(3*x1^2+2*a*x1+1)^3/(2*b*y1)^3-y1)
else (b*(y2-y1)^2/(x2-x1)^2-a-x1-x2, (2*x1+x2+a)*(y2-y1)/(x2-x1)-b*(y2-y1)^3/(x2-x1)^3-y1)
| ∞, ∞ => ∞
| ∞, _ => coordinates P2
| _, ∞ => coordinates P1
end _.
Next Obligation. Proof. t. Qed.
Program Definition opp (P:point) : point :=
exist _
match P with
| (x, y) => (x, -y)
| ∞ => ∞
end _.
Next Obligation.
Proof. t. Qed.
Local Notation "4" := (1+3).
Local Notation "16" := (4*4).
Local Notation "9" := (3*3).
Local Notation "27" := (3*9).
Context {char_gt_27:@Ring.char_gt F Feq Fzero Fone Fopp Fadd Fsub Fmul 27}.
Let WeierstrassA := ((3-a^2)/(3*b^2)).
Let WeierstrassB := ((2*a^3-9*a)/(27*b^3)).
Local Notation Wpoint := (@W.point F Feq Fadd Fmul WeierstrassA WeierstrassB).
Program Definition MontgomeryOfWeierstrass (P:Wpoint) : point :=
exist
_
match W.coordinates P return _ with
| (x,y) => (b*x-a/3, b*y)
| _ => ∞
end
_.
Next Obligation.
Proof. subst WeierstrassA; subst WeierstrassB; destruct P; t. Qed.
Definition eq (P1 P2:point) :=
match coordinates P1, coordinates P2 with
| (x1, y1), (x2, y2) => x1 = x2 /\ y1 = y2
| ∞, ∞ => True
| _, _ => False
end.
Local Notation Wadd := (@W.add F Feq Fzero Fone Fopp Fadd Fsub Fmul Finv Fdiv field Feq_dec char_gt_2 WeierstrassA WeierstrassB).
Lemma MontgomeryOfWeierstrass_add P1 P2 :
eq (MontgomeryOfWeierstrass (W.add P1 P2))
(add (MontgomeryOfWeierstrass P1) (MontgomeryOfWeierstrass P2)).
Proof.
cbv [WeierstrassA WeierstrassB eq MontgomeryOfWeierstrass W.add add coordinates W.coordinates proj1_sig] in *; t.
Qed.
Section AddX.
Lemma homogeneous_x_differential_addition_releations P1 P2 :
match coordinates (add P2 (opp P1)), coordinates P1, coordinates P2, coordinates (add P1 P2) with
| (x, _), (x1, _), (x2, _), (x3, _) =>
if Decidable.dec (x1 = x2)
then x3 * (4*x1*(x1^2 + a*x1 + 1)) = (x1^2 - 1)^2
else x3 * (x*(x1-x2)^2) = (x1*x2 - 1)^2
| _, _, _, _ => True
end.
Proof. t. Qed.
Definition onCurve xy := let 'pair x y := xy in b*y^2 = x^3 + a*x^2 + x.
Definition xzpoint := { xz | let 'pair x z := xz in (z = 0 \/ exists y, onCurve (pair (x/z) y)) }.
Definition xzcoordinates (P:xzpoint) : F*F := proj1_sig P.
Program Definition toxz (P:point) : xzpoint :=
exist _
match coordinates P with
| (x, y) => pair x 1
| ∞ => pair 1 0
end _.
Next Obligation. t; [right; exists f0; t | left; reflexivity ]. Qed.
Definition sig_pair_to_pair_sig {T T' I I'} (xy:{xy | let 'pair x y := xy in I x /\ I' y})
: prod {x:T | I x} {y:T' | I' y}
:= let 'exist (pair x y) (conj pfx pfy) := xy in pair (exist _ x pfx) (exist _ y pfy).
(* From Explicit Formulas Database by Lange and Bernstein,
credited to 1987 Montgomery "Speeding the Pollard and elliptic curve
methods of factorization", page 261, fifth and sixth displays, plus
common-subexpression elimination, plus assumption Z1=1 *)
Context {a24:F} {a24_correct:4*a24 = a+2}.
Definition xzladderstep (X1:F) (P1 P2:xzpoint) : prod xzpoint xzpoint. refine (
sig_pair_to_pair_sig (exist _
match xzcoordinates P1, xzcoordinates P2 return _ with
pair X2 Z2, pair X3 Z3 =>
let A := X2+Z2 in
let AA := A^2 in
let B := X2-Z2 in
let BB := B^2 in
let E := AA-BB in
let C := X3+Z3 in
let D := X3-Z3 in
let DA := D*A in
let CB := C*B in
let X5 := (DA+CB)^2 in
let Z5 := X1*(DA-CB)^2 in
let X4 := AA*BB in
let Z4 := E*(BB + a24*E) in
(pair (pair X4 Z4) (pair X5 Z5))
end _) ).
Proof.
destruct P1, P2; cbv [onCurve xzcoordinates] in *. t; intuition idtac.
{ left. fsatz. }
{ left. fsatz. }
admit.
admit.
admit.
admit.
{ right.
admit. (* the following used to work, but slowly:
exists ((fun x1 y1 x2 y2 => (2*x1+x1+a)*(3*x1^2+2*a*x1+1)/(2*b*y1)-b*(3*x1^2+2*a*x1+1)^3/(2*b*y1)^3-y1) (f1/f2) x0 (f/f0) x).
Algebra.common_denominator_in H.
Algebra.common_denominator_in H0.
Algebra.common_denominator.
abstract Algebra.nsatz.
idtac.
admit.
admit.
admit.
admit.
admit. *) }
{ right.
(* exists ((fun x1 y1 x2 y2 => (2*x1+x2+a)*(y2-y1)/(x2-x1)-b*(y2-y1)^3/(x2-x1)^3-y1) (f1/f2) x0 (f/f0) x). *)
(* XXX: this case is probably not true -- there is not necessarily a guarantee that the output x/z is on curve if [X1] was not the x coordinate of the difference of input points as requored *)
Abort.
End AddX.
End MontgomeryCurve.
End M.
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