From cfdddb2a57ad77f485eb4be1a52efe5ffe19a220 Mon Sep 17 00:00:00 2001
From: J08nY
Date: Fri, 19 Jan 2018 23:34:17 +0100
Subject: Compute twist order, as it is obviously known.

---
 src/math/twists.c | 11 ++++++++++-
 src/math/twists.h |  4 ++--
 2 files changed, 12 insertions(+), 3 deletions(-)

(limited to 'src/math')

diff --git a/src/math/twists.c b/src/math/twists.c
index 3f1296d..043594f 100644
--- a/src/math/twists.c
+++ b/src/math/twists.c
@@ -13,16 +13,25 @@ void twist_rand_to(curve_t *to, const curve_t *of) {
 	if (typ(of->field) == t_INT) {
 		to->a = ell_get_a4(to->curve);
 		to->b = ell_get_a6(to->curve);
+		if (of->order != NULL) {
+			GEN q = addis(mulis(of->field, 2), 2);
+			to->order = subii(q, of->order);
+			obj_insert_shallow(to->curve, 1, to->order);
+		}
 	} else if (typ(of->field) == t_FFELT) {
 		to->a = ell_get_a2(to->curve);
 		to->b = ell_get_a6(to->curve);
+		if (of->order != NULL) {
+			GEN q = addis(mulis(int2n(degree(FF_mod(of->field))), 2), 2);
+			to->order = subii(q, of->order);
+			obj_insert_shallow(to->curve, 1, to->order);
+		}
 	}
 }
 
 void twist_rand(curve_t *what) {
 	twist_rand_to(what, what);
 	seed_free(&what->seed);
-	what->order = NULL;
 	points_free_deep(&what->points, what->npoints);
 	points_free_deep(&what->generators, what->ngens);
 }
\ No newline at end of file
diff --git a/src/math/twists.h b/src/math/twists.h
index dcbdb31..d68bb27 100644
--- a/src/math/twists.h
+++ b/src/math/twists.h
@@ -15,8 +15,8 @@
 void twist_rand_to(curve_t *to, const curve_t *of);
 
 /**
- * @brief
- * @param what
+ * @brief Twists the <code>what</code> curve randomly, inplae.
+ * @param what The curve to be twisted, also the result.
  */
 void twist_rand(curve_t *what);
 
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