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Require Crypto.Algebra Crypto.Algebra.Field.
Require Crypto.Util.GlobalSettings.
Require Crypto.Util.Tactics Crypto.Util.Sum Crypto.Util.Prod.
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_ge_3:@Ring.char_ge F Feq Fzero Fone Fopp Fadd Fsub Fmul (BinNat.N.succ_pos (BinNat.N.two))}.
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 : F*F+∞ | 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.
Program Definition zero : point := ∞.
Definition eq (P1 P2:point) :=
match coordinates P1, coordinates P2 with
| (x1, y1), (x2, y2) => x1 = x2 /\ y1 = y2
| ∞, ∞ => True
| _, _ => False
end.
Program Definition add (P1 P2:point) : point :=
match coordinates P1, coordinates P2 return F*F+∞ with
(x1, y1), (x2, y2) =>
if Decidable.dec (x1 = x2)
then if Decidable.dec (y1 = - y2)
then ∞
else let k := (3*x1^2 + 2*a*x1 + 1)/(2*b*y1) in
(b*k^2 - a - x1 - x1, (2*x1 + x1 + a)*k - b*k^3 - y1)
else let k := (y2 - y1)/(x2-x1) in
(b*k^2 - a - x1 - x2, (2*x1 + x2 + a)*k - b*k^3 - y1)
| ∞, ∞ => ∞
| ∞, _ => coordinates P2
| _, ∞ => coordinates P1
end.
Next Obligation.
Proof.
repeat match goal with
| _ => solve [ trivial ]
| _ => progress Tactics.DestructHead.destruct_head' @point
| _ => progress Tactics.DestructHead.destruct_head' @prod
| _ => progress Tactics.DestructHead.destruct_head' @sum
| _ => progress Sum.inversion_sum
| _ => progress Prod.inversion_prod
| _ => progress Tactics.BreakMatch.break_match_hyps
| _ => progress Tactics.BreakMatch.break_match
| _ => progress subst
| _ => progress cbv [coordinates proj1_sig] in *
| |- _ /\ _ => split
| |- _ => Field.fsatz
end.
Qed.
End MontgomeryCurve.
End M.
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