1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
|
Require Import Crypto.Algebra.Field.
Require Import Crypto.Util.GlobalSettings Crypto.Util.Notations.
Require Import Crypto.Util.Sum Crypto.Util.Prod Crypto.Util.LetIn.
Require Import Crypto.Util.Decidable.
Require Import Crypto.Util.ForLoop.
Require Import Crypto.Spec.MontgomeryCurve Crypto.Curves.Montgomery.Affine.
Module M.
Section MontgomeryCurve.
Import BinNat.
Context {F Feq Fzero Fone Fopp Fadd Fsub Fmul Finv Fdiv}
{field:@Algebra.Hierarchy.field F Feq Fzero Fone Fopp Fadd Fsub Fmul Finv Fdiv}
{Feq_dec:Decidable.DecidableRel Feq}
{char_ge_5:@Ring.char_ge F Feq Fzero Fone Fopp Fadd Fsub Fmul 5}.
Delimit Scope F_scope with F.
Local Open Scope F_scope.
Local Infix "=" := Feq : type_scope. Local Notation "a <> b" := (not (a = b)) : type_scope.
Local Infix "+" := Fadd : F_scope. Local Infix "*" := Fmul : F_scope.
Local Infix "-" := Fsub : F_scope. Local Infix "/" := Fdiv : F_scope.
Local Notation "x ^ 2" := (x*x) : F_scope.
Local Notation "0" := Fzero : F_scope. Local Notation "1" := Fone : F_scope.
Local Notation "'∞'" := (inr tt) : core_scope.
Local Notation "( x , y , .. , z )" := (inl (pair .. (pair x y) .. z)) : F_scope.
Context {a b: F} {b_nonzero:b <> 0}.
Local Notation add := (M.add(b_nonzero:=b_nonzero)).
Local Notation opp := (M.opp(b_nonzero:=b_nonzero)).
Local Notation point := (@M.point F Feq Fadd Fmul a b).
Program Definition to_xz (P:point) : F*F :=
match M.coordinates P with
| (x, y) => (x, 1)%core
| ∞ => (1, 0)%core
end.
Let char_ge_3:@Ring.char_ge F Feq Fzero Fone Fopp Fadd Fsub Fmul (BinNat.N.succ_pos (BinNat.N.two)).
Proof. eapply Algebra.Hierarchy.char_ge_weaken; eauto; vm_decide. Qed.
(* From Curve25519 paper by djb, appendix B. Credited to Montgomery *)
Context {a24:F} {a24_correct:(1+1+1+1)*a24 = a-(1+1)}.
Definition xzladderstep (x1:F) (Q Q':F*F) : ((F*F)*(F*F)) :=
match Q, Q' with
pair x z, pair x' z' =>
dlet A := x+z in
dlet B := x-z in
dlet AA := A^2 in
dlet BB := B^2 in
dlet x2 := AA*BB in
dlet E := AA-BB in
dlet z2 := E*(AA + a24*E) in
dlet C := x'+z' in
dlet D := x'-z' in
dlet CB := C*B in
dlet DA := D*A in
dlet x3 := (DA+CB)^2 in
dlet z3 := x1*(DA-CB)^2 in
((x2, z2), (x3, z3))%core
end.
Context {cswap:bool->F*F->F*F->(F*F)*(F*F)}.
Local Notation xor := Coq.Init.Datatypes.xorb.
(* Ideally, we would verify that this corresponds to x coordinate
multiplication *)
Local Open Scope core_scope.
Definition montladder (bound : positive) (testbit:Z->bool) (u:F) :=
let '(P1, P2, swap) :=
for (int i = BinInt.Z.pos bound; i >= 0; i--)
updating ('(P1, P2, swap) = ((1%F, 0%F), (u, 1%F), false)) {{
dlet s_i := testbit i in
dlet swap := xor swap s_i in
let '(P1, P2) := cswap swap P1 P2 in
dlet swap := s_i in
let '(P1, P2) := xzladderstep u P1 P2 in
(P1, P2, swap)
}} in
let '((x, z), _) := cswap swap P1 P2 in
x * Finv z.
End MontgomeryCurve.
End M.
|
