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(* ========================================================================= *)
(* Specific formulas for evaluating (X,Z)-only projective point operations. *)
(* ========================================================================= *)
needs "EC/xzprojective.ml";;
(* ------------------------------------------------------------------------- *)
(* Montgomery ladder step, XZ-coordinate differential addition and doubling. *)
(* *)
(* Source: Montgomery [1987] "Speeding the Pollard and elliptic curve..." *)
(* ------------------------------------------------------------------------- *)
(***
***
http://hyperelliptic.org/EFD/g1p/auto-code/montgom/xz/ladder/mladd-1987-m.op3
***)
let mladd_1987_m = new_definition
`mladd_1987_m (f:A ring,a:A,b:A) a24 X1 (X2,Z2) (X3,Z3) =
let A = ring_add f X2 Z2 in
let AA = ring_pow f A 2 in
let B = ring_sub f X2 Z2 in
let BB = ring_pow f B 2 in
let E = ring_sub f AA BB in
let C = ring_add f X3 Z3 in
let D = ring_sub f X3 Z3 in
let DA = ring_mul f D A in
let CB = ring_mul f C B in
let t0 = ring_add f DA CB in
let X5 = ring_pow f t0 2 in
let t1 = ring_sub f DA CB in
let t2 = ring_pow f t1 2 in
let Z5 = ring_mul f X1 t2 in
let X4 = ring_mul f AA BB in
let t3 = ring_mul f a24 E in
let t4 = ring_add f BB t3 in
let Z4 = ring_mul f E t4 in
(X4,Z4),(X5,Z5)`;;
let MLADD_1987_M = prove
(`!f (a:A) b a24 X1 X2 Z2 X3 Z3.
field f /\ ~(ring_char f = 2) /\
a IN ring_carrier f /\ b IN ring_carrier f /\
a24 IN ring_carrier f /\ X1 IN ring_carrier f /\
X2 IN ring_carrier f /\ Z2 IN ring_carrier f /\
X3 IN ring_carrier f /\ Z3 IN ring_carrier f /\
ring_mul f (ring_of_num f 4) a24 = ring_add f a (ring_of_num f 2)
==> mladd_1987_m (f,a,b) a24 X1 (X2,Z2) (X3,Z3) =
(montgomery_xzdouble (f,a,b) (X2,Z2),
montgomery_xzdiffadd (f,a,b) (X1,ring_1 f) (X2,Z2) (X3,Z3))`,
REWRITE_TAC[FIELD_CHAR_NOT2] THEN
REWRITE_TAC[mladd_1987_m; montgomery_xzdouble; montgomery_xzdiffadd] THEN
REPEAT STRIP_TAC THEN CONV_TAC(TOP_DEPTH_CONV let_CONV) THEN
REWRITE_TAC[PAIR_EQ] THEN FIELD_TAC THEN
NOT_RING_CHAR_DIVIDES_TAC);;
(* ------------------------------------------------------------------------- *)
(* Recovering y coordinate within the projective representation. *)
(* *)
(* Source: Okeya and Sakurai [2001] "Efficient Elliptic Curve...", Alg. 1. *)
(* ------------------------------------------------------------------------- *)
let okeya_sakurai_1 = new_definition
`okeya_sakurai_1 (f:A ring,a:A,b:A) (x,y) (X1,Z1) (X2,Z2) =
let a2 = ring_add f a a
and b2 = ring_add f b b in
let t1 = ring_mul f x Z1 in
let t2 = ring_add f X1 t1 in
let t3 = ring_sub f X1 t1 in
let t3 = ring_mul f t3 t3 in
let t3 = ring_mul f t3 X2 in
let t1 = ring_mul f a2 Z1 in
let t2 = ring_add f t2 t1 in
let t4 = ring_mul f x X1 in
let t4 = ring_add f t4 Z1 in
let t2 = ring_mul f t2 t4 in
let t1 = ring_mul f t1 Z1 in
let t2 = ring_sub f t2 t1 in
let t2 = ring_mul f t2 Z2 in
let y' = ring_sub f t2 t3 in
let t1 = ring_mul f b2 y in
let t1 = ring_mul f t1 Z1 in
let t1 = ring_mul f t1 Z2 in
let x' = ring_mul f t1 X1 in
let z' = ring_mul f t1 Z1 in
(x',y',z')`;;
(*** Note the overarching assumption that the initial point is non-trivial
*** and has nonzero y coordinate, although we do handle degeneracy in the
*** result point.
***)
let OKEYA_SAKURAI_1 = prove
(`!f (a:A) b x y p X1 Z1 X2 Z2.
field f /\ ~(ring_char f = 2) /\
a IN ring_carrier f /\ b IN ring_carrier f /\ ~(b = ring_0 f) /\
montgomery_curve (f,a,b) (SOME(x,y)) /\ ~(y = ring_0 f) /\
montgomery_curve (f,a,b) p /\
montgomery_xz f p (X1,Z1) /\
montgomery_xz f (montgomery_add(f,a,b) (SOME(x,y)) p) (X2,Z2)
==> let x',y',z' = okeya_sakurai_1 (f,a,b) (x,y) (X1,Z1) (X2,Z2) in
p = if z' = ring_0 f then
(if Z1 = ring_0 f then NONE else SOME(x,ring_neg f y))
else SOME(ring_div f x' z',ring_div f y' z')`,
MAP_EVERY X_GEN_TAC [`f:A ring`; `a:A`; `b:A`; `x:A`; `y:A`] THEN
REWRITE_TAC[FIELD_CHAR_NOT2; FORALL_OPTION_THM; FORALL_PAIR_THM] THEN
REWRITE_TAC[montgomery_curve; montgomery_xz; okeya_sakurai_1] THEN
CONJ_TAC THENL
[REPEAT GEN_TAC THEN
REPEAT(DISCH_THEN(CONJUNCTS_THEN2 STRIP_ASSUME_TAC MP_TAC)) THEN
CONV_TAC(TOP_DEPTH_CONV let_CONV) THEN
SPEC_TAC(`montgomery_add (f,a:A,b) (SOME(x,y)) NONE`,`q:(A#A)option`) THEN
REWRITE_TAC[FORALL_OPTION_THM; FORALL_PAIR_THM; montgomery_xz] THEN
REPEAT STRIP_TAC THEN
ASM_SIMP_TAC[RING_MUL_LZERO; RING_MUL_RZERO; RING_0; RING_ADD; RING_MUL];
ALL_TAC] THEN
MAP_EVERY X_GEN_TAC [`x1:A`; `y1:A`; `X1:A`; `Z1:A`; `X2:A`; `Z2:A`] THEN
REPEAT(DISCH_THEN(CONJUNCTS_THEN2 STRIP_ASSUME_TAC MP_TAC)) THEN
CONV_TAC(TOP_DEPTH_CONV let_CONV) THEN
ASM_SIMP_TAC[FIELD_MUL_EQ_0; RING_ADD; RING_MUL] THEN
ASM_CASES_TAC `(X2:A) IN ring_carrier f /\ Z2 IN ring_carrier f` THENL
[FIRST_X_ASSUM(CONJUNCTS_THEN ASSUME_TAC) THEN
ASM (CONV_TAC o GEN_SIMPLIFY_CONV TOP_DEPTH_SQCONV (basic_ss []) 5)
[FIELD_MUL_EQ_0; RING_ADD; RING_MUL; RING_OF_NUM; RING_OF_NUM_EQ_0;
RING_RULE `ring_add f b b:A = ring_mul f (ring_of_num f 2) b`];
ASM_REWRITE_TAC[montgomery_add; LET_DEF; LET_END_DEF] THEN
REPEAT(COND_CASES_TAC THEN ASM_REWRITE_TAC[montgomery_xz]) THEN
ASM_REWRITE_TAC[CONJ_ASSOC]] THEN
REWRITE_TAC[montgomery_add; LET_DEF; LET_END_DEF] THEN
REPEAT(COND_CASES_TAC THEN
ASM_REWRITE_TAC[montgomery_xz; option_DISTINCT; option_INJ; PAIR_EQ])
THENL [FIELD_TAC THEN NOT_RING_CHAR_DIVIDES_TAC; FIELD_TAC; ALL_TAC] THEN
SUBGOAL_THEN `~(ring_of_num f 2:A = ring_0 f)` ASSUME_TAC THENL
[FIELD_TAC; RING_PULL_DIV_TAC THEN DISCH_THEN SUBST1_TAC] THEN
CONJ_TAC THENL [FIELD_TAC; ALL_TAC] THEN
SUBGOAL_THEN `~(ring_sub f x1 x:A = ring_0 f)` ASSUME_TAC THENL
[FIELD_TAC; RING_PULL_DIV_TAC THEN FIELD_TAC]);;
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