+/*-
+ * Set s := p, r := 2p.
+ *
+ * For doubling we use Formula 3 from Izu-Takagi "A fast parallel elliptic curve
+ * multiplication resistant against side channel attacks" appendix, as described
+ * at
+ * https://hyperelliptic.org/EFD/g1p/auto-shortw-xz.html#doubling-dbl-2002-it-2
+ *
+ * The input point p will be in randomized Jacobian projective coords:
+ * x = X/Z**2, y=Y/Z**3
+ *
+ * The output points p, s, and r are converted to standard (homogeneous)
+ * projective coords:
+ * x = X/Z, y=Y/Z
+ */
+int ec_GFp_simple_ladder_pre(const EC_GROUP *group,
+ EC_POINT *r, EC_POINT *s,
+ EC_POINT *p, BN_CTX *ctx)
+{
+ BIGNUM *t1, *t2, *t3, *t4, *t5, *t6 = NULL;
+
+ t1 = r->Z;
+ t2 = r->Y;
+ t3 = s->X;
+ t4 = r->X;
+ t5 = s->Y;
+ t6 = s->Z;
+
+ /* convert p: (X,Y,Z) -> (XZ,Y,Z**3) */
+ if (!group->meth->field_mul(group, p->X, p->X, p->Z, ctx)
+ || !group->meth->field_sqr(group, t1, p->Z, ctx)
+ || !group->meth->field_mul(group, p->Z, p->Z, t1, ctx)
+ /* r := 2p */
+ || !group->meth->field_sqr(group, t2, p->X, ctx)
+ || !group->meth->field_sqr(group, t3, p->Z, ctx)
+ || !group->meth->field_mul(group, t4, t3, group->a, ctx)
+ || !BN_mod_sub_quick(t5, t2, t4, group->field)
+ || !BN_mod_add_quick(t2, t2, t4, group->field)
+ || !group->meth->field_sqr(group, t5, t5, ctx)
+ || !group->meth->field_mul(group, t6, t3, group->b, ctx)
+ || !group->meth->field_mul(group, t1, p->X, p->Z, ctx)
+ || !group->meth->field_mul(group, t4, t1, t6, ctx)
+ || !BN_mod_lshift_quick(t4, t4, 3, group->field)
+ /* r->X coord output */
+ || !BN_mod_sub_quick(r->X, t5, t4, group->field)
+ || !group->meth->field_mul(group, t1, t1, t2, ctx)
+ || !group->meth->field_mul(group, t2, t3, t6, ctx)
+ || !BN_mod_add_quick(t1, t1, t2, group->field)
+ /* r->Z coord output */
+ || !BN_mod_lshift_quick(r->Z, t1, 2, group->field)
+ || !EC_POINT_copy(s, p))
+ return 0;
+
+ r->Z_is_one = 0;
+ s->Z_is_one = 0;
+ p->Z_is_one = 0;
+
+ return 1;
+}
+
+/*-
+ * Differential addition-and-doubling using Eq. (9) and (10) from Izu-Takagi
+ * "A fast parallel elliptic curve multiplication resistant against side channel
+ * attacks", as described at
+ * https://hyperelliptic.org/EFD/g1p/auto-shortw-xz.html#ladder-ladd-2002-it-4
+ */
+int ec_GFp_simple_ladder_step(const EC_GROUP *group,
+ EC_POINT *r, EC_POINT *s,
+ EC_POINT *p, BN_CTX *ctx)
+{
+ int ret = 0;
+ BIGNUM *t0, *t1, *t2, *t3, *t4, *t5, *t6, *t7 = NULL;
+
+ BN_CTX_start(ctx);
+ t0 = BN_CTX_get(ctx);
+ t1 = BN_CTX_get(ctx);
+ t2 = BN_CTX_get(ctx);
+ t3 = BN_CTX_get(ctx);
+ t4 = BN_CTX_get(ctx);
+ t5 = BN_CTX_get(ctx);
+ t6 = BN_CTX_get(ctx);
+ t7 = BN_CTX_get(ctx);
+
+ if (t7 == NULL
+ || !group->meth->field_mul(group, t0, r->X, s->X, ctx)
+ || !group->meth->field_mul(group, t1, r->Z, s->Z, ctx)
+ || !group->meth->field_mul(group, t2, r->X, s->Z, ctx)
+ || !group->meth->field_mul(group, t3, r->Z, s->X, ctx)
+ || !group->meth->field_mul(group, t4, group->a, t1, ctx)
+ || !BN_mod_add_quick(t0, t0, t4, group->field)
+ || !BN_mod_add_quick(t4, t3, t2, group->field)
+ || !group->meth->field_mul(group, t0, t4, t0, ctx)
+ || !group->meth->field_sqr(group, t1, t1, ctx)
+ || !BN_mod_lshift_quick(t7, group->b, 2, group->field)
+ || !group->meth->field_mul(group, t1, t7, t1, ctx)
+ || !BN_mod_lshift1_quick(t0, t0, group->field)
+ || !BN_mod_add_quick(t0, t1, t0, group->field)
+ || !BN_mod_sub_quick(t1, t2, t3, group->field)
+ || !group->meth->field_sqr(group, t1, t1, ctx)
+ || !group->meth->field_mul(group, t3, t1, p->X, ctx)
+ || !group->meth->field_mul(group, t0, p->Z, t0, ctx)
+ /* s->X coord output */
+ || !BN_mod_sub_quick(s->X, t0, t3, group->field)
+ /* s->Z coord output */
+ || !group->meth->field_mul(group, s->Z, p->Z, t1, ctx)
+ || !group->meth->field_sqr(group, t3, r->X, ctx)
+ || !group->meth->field_sqr(group, t2, r->Z, ctx)
+ || !group->meth->field_mul(group, t4, t2, group->a, ctx)
+ || !BN_mod_add_quick(t5, r->X, r->Z, group->field)
+ || !group->meth->field_sqr(group, t5, t5, ctx)
+ || !BN_mod_sub_quick(t5, t5, t3, group->field)
+ || !BN_mod_sub_quick(t5, t5, t2, group->field)
+ || !BN_mod_sub_quick(t6, t3, t4, group->field)
+ || !group->meth->field_sqr(group, t6, t6, ctx)
+ || !group->meth->field_mul(group, t0, t2, t5, ctx)
+ || !group->meth->field_mul(group, t0, t7, t0, ctx)
+ /* r->X coord output */
+ || !BN_mod_sub_quick(r->X, t6, t0, group->field)
+ || !BN_mod_add_quick(t6, t3, t4, group->field)
+ || !group->meth->field_sqr(group, t3, t2, ctx)
+ || !group->meth->field_mul(group, t7, t3, t7, ctx)
+ || !group->meth->field_mul(group, t5, t5, t6, ctx)
+ || !BN_mod_lshift1_quick(t5, t5, group->field)
+ /* r->Z coord output */
+ || !BN_mod_add_quick(r->Z, t7, t5, group->field))
+ goto err;
+
+ ret = 1;