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
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
|
Require Import Crypto.CompleteEdwardsCurve.Pre.
Require Import Crypto.CompleteEdwardsCurve.CompleteEdwardsCurveTheorems.
Require Import Crypto.ModularArithmetic.PrimeFieldTheorems.
Require Import Crypto.ModularArithmetic.FField.
Require Import Crypto.Tactics.VerdiTactics.
Require Import Util.IterAssocOp BinNat NArith.
Require Import Coq.Setoids.Setoid Coq.Classes.Morphisms Coq.Classes.Equivalence.
Local Open Scope equiv_scope.
Local Open Scope F_scope.
Section ExtendedCoordinates.
Context {prm:TwistedEdwardsParams}.
Local Opaque q a d prime_q two_lt_q nonzero_a square_a nonsquare_d. (* [F_field] calls [compute] *)
Existing Instance prime_q.
Add Field Ffield_p' : (@Ffield_theory q _)
(morphism (@Fring_morph q),
preprocess [Fpreprocess],
postprocess [Fpostprocess; try exact Fq_1_neq_0; try assumption],
constants [Fconstant],
div (@Fmorph_div_theory q),
power_tac (@Fpower_theory q) [Fexp_tac]).
Add Field Ffield_notConstant : (OpaqueFieldTheory q)
(constants [notConstant]).
(** [extended] represents a point on an elliptic curve using extended projective
* Edwards coordinates with twist a=-1 (see <https://eprint.iacr.org/2008/522.pdf>). *)
Record extended := mkExtended {extendedX : F q;
extendedY : F q;
extendedZ : F q;
extendedT : F q}.
Local Notation "'(' X ',' Y ',' Z ',' T ')'" := (mkExtended X Y Z T).
Definition twistedToExtended (P : (F q*F q)) : extended :=
let '(x, y) := P in (x, y, 1, x*y).
Definition extendedToTwisted (P : extended) : F q * F q :=
let '(X, Y, Z, T) := P in ((X/Z), (Y/Z)).
Definition rep (P:extended) (rP:(F q*F q)) : Prop :=
let '(X, Y, Z, T) := P in
extendedToTwisted P = rP /\
Z <> 0 /\
T = X*Y/Z.
Local Hint Unfold twistedToExtended extendedToTwisted rep.
Local Notation "P '~=' rP" := (rep P rP) (at level 70).
Ltac unfoldExtended :=
repeat progress (autounfold; unfold E.onCurve, E.add, E.add', rep in *; intros);
repeat match goal with
| [ p : (F q*F q)%type |- _ ] =>
let x := fresh "x" p in
let y := fresh "y" p in
destruct p as [x y]
| [ p : extended |- _ ] =>
let X := fresh "X" p in
let Y := fresh "Y" p in
let Z := fresh "Z" p in
let T := fresh "T" p in
destruct p as [X Y Z T]
| [ H: _ /\ _ |- _ ] => destruct H
| [ H: @eq (F q * F q)%type _ _ |- _ ] => invcs H
| [ H: @eq F q ?x _ |- _ ] => isVar x; rewrite H; clear H
end.
Ltac solveExtended := unfoldExtended;
repeat match goal with
| [ |- _ /\ _ ] => split
| [ |- @eq (F q * F q)%type _ _] => apply f_equal2
| _ => progress rewrite ?@F_add_0_r, ?@F_add_0_l, ?@F_sub_0_l, ?@F_sub_0_r,
?@F_mul_0_r, ?@F_mul_0_l, ?@F_mul_1_l, ?@F_mul_1_r, ?@F_div_1_r
| _ => solve [eapply @Fq_1_neq_0; eauto with typeclass_instances]
| _ => solve [eauto with typeclass_instances]
| [ H: a = _ |- _ ] => rewrite H
end.
Lemma twistedToExtended_rep : forall P, twistedToExtended P ~= P.
Proof.
solveExtended.
Qed.
Lemma extendedToTwisted_rep : forall P rP, P ~= rP -> extendedToTwisted P = rP.
Proof.
solveExtended.
Qed.
Definition extendedPoint := { P:extended | rep P (extendedToTwisted P) /\ E.onCurve (extendedToTwisted P) }.
Program Definition mkExtendedPoint : E.point -> extendedPoint := twistedToExtended.
Next Obligation.
destruct x; erewrite extendedToTwisted_rep; eauto using twistedToExtended_rep.
Qed.
Program Definition unExtendedPoint : extendedPoint -> E.point := extendedToTwisted.
Next Obligation.
destruct x; simpl; intuition.
Qed.
Definition extendedPoint_eq P Q := unExtendedPoint P = unExtendedPoint Q.
Global Instance Equivalence_extendedPoint_eq : Equivalence extendedPoint_eq.
Proof.
repeat (econstructor || intro); unfold extendedPoint_eq in *; congruence.
Qed.
Lemma unExtendedPoint_mkExtendedPoint : forall P, unExtendedPoint (mkExtendedPoint P) = P.
Proof.
destruct P; eapply E.point_eq; simpl; erewrite extendedToTwisted_rep; eauto using twistedToExtended_rep.
Qed.
Global Instance Proper_mkExtendedPoint : Proper (eq==>equiv) mkExtendedPoint.
Proof.
repeat (econstructor || intro); unfold extendedPoint_eq in *; congruence.
Qed.
Global Instance Proper_unExtendedPoint : Proper (equiv==>eq) unExtendedPoint.
Proof.
repeat (econstructor || intro); unfold extendedPoint_eq in *; congruence.
Qed.
Definition twice_d := d + d.
Section TwistMinus1.
Context (a_eq_minus1 : a = opp 1).
(** Second equation from <http://eprint.iacr.org/2008/522.pdf> section 3.1, also <https://www.hyperelliptic.org/EFD/g1p/auto-twisted-extended-1.html#addition-add-2008-hwcd-3> and <https://tools.ietf.org/html/draft-josefsson-eddsa-ed25519-03> *)
Definition unifiedAddM1' (P1 P2 : extended) : extended :=
let '(X1, Y1, Z1, T1) := P1 in
let '(X2, Y2, Z2, T2) := P2 in
let A := (Y1-X1)*(Y2-X2) in
let B := (Y1+X1)*(Y2+X2) in
let C := T1*twice_d*T2 in
let D := Z1*(Z2+Z2) in
let E := B-A in
let F := D-C in
let G := D+C in
let H := B+A in
let X3 := E*F in
let Y3 := G*H in
let T3 := E*H in
let Z3 := F*G in
(X3, Y3, Z3, T3).
Local Hint Unfold E.add.
Local Ltac tnz := repeat apply Fq_mul_nonzero_nonzero; auto using (@char_gt_2 q two_lt_q).
Lemma F_mul_2_l : forall x : F q, ZToField 2 * x = x + x.
intros. ring.
Qed.
Lemma unifiedAddM1'_rep: forall P Q rP rQ, E.onCurve rP -> E.onCurve rQ ->
P ~= rP -> Q ~= rQ -> (unifiedAddM1' P Q) ~= (E.add' rP rQ).
Proof.
intros P Q rP rQ HoP HoQ HrP HrQ.
pose proof (@edwardsAddCompletePlus _ _ _ _ two_lt_q nonzero_a square_a nonsquare_d).
pose proof (@edwardsAddCompleteMinus _ _ _ _ two_lt_q nonzero_a square_a nonsquare_d).
unfoldExtended; unfold twice_d; rewrite a_eq_minus1 in *; simpl in *. repeat rewrite <-F_mul_2_l.
repeat split; repeat apply (f_equal2 pair); try F_field; repeat split; auto;
repeat rewrite ?F_add_0_r, ?F_add_0_l, ?F_sub_0_l, ?F_sub_0_r,
?F_mul_0_r, ?F_mul_0_l, ?F_mul_1_l, ?F_mul_1_r, ?F_div_1_r;
field_nonzero tnz.
Qed.
Lemma unifiedAdd'_onCurve : forall P Q, E.onCurve P -> E.onCurve Q -> E.onCurve (E.add' P Q).
Proof.
intros; pose proof (proj2_sig (E.add (exist _ _ H) (exist _ _ H0))); eauto.
Qed.
Program Definition unifiedAddM1 : extendedPoint -> extendedPoint -> extendedPoint := unifiedAddM1'.
Next Obligation.
destruct x, x0; simpl; intuition.
- erewrite extendedToTwisted_rep; eauto using unifiedAddM1'_rep.
- erewrite extendedToTwisted_rep.
(* It would be nice if I could use eauto here, but it gets evars wrong :( *)
2: eapply unifiedAddM1'_rep. 5:apply H1. 4:apply H. 3:auto. 2:auto.
eauto using unifiedAdd'_onCurve.
Qed.
Lemma unifiedAddM1_rep : forall P Q, E.add (unExtendedPoint P) (unExtendedPoint Q) = unExtendedPoint (unifiedAddM1 P Q).
Proof.
destruct P, Q; unfold unExtendedPoint, E.add, unifiedAddM1; eapply E.point_eq; simpl in *; intuition.
pose proof (unifiedAddM1'_rep x x0 (extendedToTwisted x) (extendedToTwisted x0));
destruct (unifiedAddM1' x x0);
unfold rep in *; intuition.
Qed.
Global Instance Proper_unifiedAddM1 : Proper (equiv==>equiv==>equiv) unifiedAddM1.
Proof.
repeat (econstructor || intro).
repeat match goal with [H: _ === _ |- _ ] => inversion H; clear H end; unfold equiv, extendedPoint_eq.
rewrite <-!unifiedAddM1_rep.
destruct x, y, x0, y0; simpl in *; eapply E.point_eq; congruence.
Qed.
Lemma unifiedAddM1_0_r : forall P, unifiedAddM1 P (mkExtendedPoint E.zero) === P.
unfold equiv, extendedPoint_eq; intros.
rewrite <-!unifiedAddM1_rep, unExtendedPoint_mkExtendedPoint, E.add_0_r; auto.
Qed.
Lemma unifiedAddM1_0_l : forall P, unifiedAddM1 (mkExtendedPoint E.zero) P === P.
unfold equiv, extendedPoint_eq; intros.
rewrite <-!unifiedAddM1_rep, E.add_comm, unExtendedPoint_mkExtendedPoint, E.add_0_r; auto.
Qed.
Lemma unifiedAddM1_assoc : forall a b c, unifiedAddM1 a (unifiedAddM1 b c) === unifiedAddM1 (unifiedAddM1 a b) c.
Proof.
unfold equiv, extendedPoint_eq; intros.
rewrite <-!unifiedAddM1_rep, E.add_assoc; auto.
Qed.
Lemma testbit_conversion_identity : forall x i, N.testbit_nat x i = N.testbit_nat ((fun a => a) x) i.
Proof.
trivial.
Qed.
Definition scalarMultM1 := iter_op unifiedAddM1 (mkExtendedPoint E.zero) N.testbit_nat.
Definition scalarMultM1_spec :=
iter_op_spec unifiedAddM1 unifiedAddM1_assoc (mkExtendedPoint E.zero) unifiedAddM1_0_l
N.testbit_nat (fun x => x) testbit_conversion_identity.
Lemma scalarMultM1_rep : forall n P, unExtendedPoint (scalarMultM1 (N.of_nat n) P (N.size_nat (N.of_nat n))) = E.mul n (unExtendedPoint P).
intros; rewrite scalarMultM1_spec, Nat2N.id; auto.
induction n; [simpl; rewrite !unExtendedPoint_mkExtendedPoint; reflexivity|].
unfold E.mul; fold E.mul.
rewrite <-IHn, unifiedAddM1_rep; auto.
Qed.
End TwistMinus1.
Definition negateExtended' P := let '(X, Y, Z, T) := P in (opp X, Y, Z, opp T).
Program Definition negateExtended (P:extendedPoint) : extendedPoint := negateExtended' (proj1_sig P).
Next Obligation.
Proof.
unfold negateExtended', rep; destruct P as [[X Y Z T] H]; simpl. destruct H as [[[] []] ?]; subst.
repeat rewrite ?F_div_opp_1, ?F_mul_opp_l, ?F_square_opp; repeat split; trivial.
Qed.
Lemma negateExtended_correct : forall P, E.opp (unExtendedPoint P) = unExtendedPoint (negateExtended P).
Proof.
unfold E.opp, unExtendedPoint, negateExtended; destruct P as [[]]; simpl; intros.
eapply E.point_eq; repeat rewrite ?F_div_opp_1, ?F_mul_opp_l, ?F_square_opp; trivial.
Qed.
End ExtendedCoordinates.
|
