From 274f2b52ca8f5f45de6dffd0df7345fd1f23507d Mon Sep 17 00:00:00 2001
From: Loup Vaillant <loup@loup-vaillant.fr>
Date: Fri, 21 Feb 2020 00:21:21 +0100
Subject: [PATCH] Elligator script: added fast_from_edwards

---
 tests/gen/elligator.py | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/tests/gen/elligator.py b/tests/gen/elligator.py
index fa59588..df9c394 100755
--- a/tests/gen/elligator.py
+++ b/tests/gen/elligator.py
@@ -240,6 +240,17 @@ def fast_scalarmult(point, scalar):
 def fast_scalarbase(scalar):
     return fast_scalarmult(edwards_base, scalar)
 
+def fast_from_edwards(point):
+    sqA = sqrt(fe(-486664)) # constant
+    x = point[0]
+    y = point[1]
+    z = point[2]
+    u  = z + y
+    zu = z - y
+    v  = u * z * sqA
+    zv = zu * x
+    return (u*zv, v*zu, zu*zv)
+
 # Explicit formula for hash_to_curve
 # We don't need the v coordinate for X25519, so it is omited
 def explicit_hash_to_curve(r):
-- 
2.47.3