From f940f80a2cb69ffcd1f0d608cbb1b0f06e0854d6 Mon Sep 17 00:00:00 2001
From: J08nY
Date: Thu, 18 Jan 2018 17:00:41 +0100
Subject: Add utilities to create random curve twists.

---
 src/math/twists.c           | 18 +++++++++++++++++
 src/math/twists.h           | 17 ++++++++++++++++
 src/util/random.c           | 12 ++++++++++++
 src/util/random.h           |  7 +++++++
 test/src/math/test_twists.c | 48 +++++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 102 insertions(+)
 create mode 100644 src/math/twists.c
 create mode 100644 src/math/twists.h
 create mode 100644 test/src/math/test_twists.c

diff --git a/src/math/twists.c b/src/math/twists.c
new file mode 100644
index 0000000..78481f4
--- /dev/null
+++ b/src/math/twists.c
@@ -0,0 +1,18 @@
+/*
+ * ecgen, tool for generating Elliptic curve domain parameters
+ * Copyright (C) 2017-2018 J08nY
+ */
+#include "twists.h"
+
+void curve_twist_rand(curve_t *to, const curve_t *of) {
+	to->field = gcopy(of->field);
+	GEN v = elltwist(of->curve, NULL);
+	to->curve = ellinit(v, to->field, -1);
+	if (typ(of->field) == t_INT) {
+		to->a = ell_get_a4(to->curve);
+		to->b = ell_get_a6(to->curve);
+	} else if (typ(of->field) == t_FFELT) {
+		to->a = ell_get_a2(to->curve);
+		to->b = ell_get_a6(to->curve);
+	}
+}
\ No newline at end of file
diff --git a/src/math/twists.h b/src/math/twists.h
new file mode 100644
index 0000000..3249047
--- /dev/null
+++ b/src/math/twists.h
@@ -0,0 +1,17 @@
+/*
+ * ecgen, tool for generating Elliptic curve domain parameters
+ * Copyright (C) 2017-2018 J08nY
+ */
+#ifndef ECGEN_TWIST_H
+#define ECGEN_TWIST_H
+
+#include "misc/types.h"
+
+/**
+ * @brief Twists the <code>of</code> curve randomly.
+ * @param to The result of the twist.
+ * @param of The curve to be twisted.
+ */
+void curve_twist_rand(curve_t *to, const curve_t *of);
+
+#endif  // ECGEN_TWIST_H
diff --git a/src/util/random.c b/src/util/random.c
index c754677..860cff0 100644
--- a/src/util/random.c
+++ b/src/util/random.c
@@ -61,3 +61,15 @@ GEN random_int(unsigned long bits) {
 	GEN range = mkvec2(int2n(bits - 1), int2n(bits));
 	return gerepilecopy(ltop, genrand(range));
 }
+
+GEN random_field_element(GEN field) {
+	switch (typ(field)) {
+		case t_INT:
+			return mkintmod(genrand(field), field);
+		case t_FFELT:
+			return genrand(field);
+		default:
+			pari_err_TYPE("", field);
+			return NULL;
+	}
+}
diff --git a/src/util/random.h b/src/util/random.h
index f78f991..56e2efd 100644
--- a/src/util/random.h
+++ b/src/util/random.h
@@ -36,4 +36,11 @@ GEN random_prime(unsigned long bits);
  */
 GEN random_int(unsigned long bits);
 
+/**
+ * @brief
+ * @param field
+ * @return
+ */
+GEN random_field_element(GEN field);
+
 #endif  // ECGEN_RANDOM_H
diff --git a/test/src/math/test_twists.c b/test/src/math/test_twists.c
new file mode 100644
index 0000000..30bc958
--- /dev/null
+++ b/test/src/math/test_twists.c
@@ -0,0 +1,48 @@
+/*
+ * ecgen, tool for generating Elliptic curve domain parameters
+ * Copyright (C) 2017 J08nY
+ */
+#include <criterion/criterion.h>
+#include "gen/point.h"
+#include "gen/field.h"
+#include "math/poly.h"
+#include "math/twists.h"
+#include "test/default.h"
+#include "util/random.h"
+
+TestSuite(twists, .init = default_setup, .fini = default_teardown);
+
+Test(twists, test_twist_rand_fp) {
+	random_init();
+	GEN a = mkintmodu(3, 23);
+	GEN b = mkintmodu(4, 23);
+	curve_t curve = {.a = a,
+	                 .b = b,
+	                 .field = stoi(23),
+	                 .curve = ellinit(mkvec2(a, b), stoi(23), -1)};
+
+	curve_t to = {0};
+	curve_twist_rand(&to, &curve);
+	cr_assert_not_null(to.a, );
+	cr_assert_not_null(to.b, );
+	cr_assert_not_null(to.field, );
+	cr_assert_not_null(to.curve, );
+}
+
+Test(twists, test_twist_rand_f2m) {
+	random_init();
+	GEN field = poly_find_gen(13);
+	GEN a = field_ielement(field, stoi(2));
+	GEN b = field_ielement(field, stoi(3));
+	curve_t curve = {.a = a,
+					 .b = b,
+					 .field = field,
+					 .curve = ellinit(mkvecn(5, gen_1, a, gen_0, gen_0, b), NULL, -1)};
+
+	curve_t to = {0};
+	curve_twist_rand(&to, &curve);
+	cr_assert_not_null(to.a, );
+	cr_assert_not_null(to.b, );
+	cr_assert_not_null(to.field, );
+	cr_assert_not_null(to.curve, );
+}
\ No newline at end of file
-- 
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