From cbf74a7082ee3dd176217298c886ef48bf4ccd9a Mon Sep 17 00:00:00 2001
From: Dusan Klinec <dusan.klinec@gmail.com>
Date: Fri, 17 Aug 2018 03:04:01 +0200
Subject: [PATCH] xmr: bp - detect which modular inversion is usable

---
 src/apps/monero/xmr/bulletproof.py | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/src/apps/monero/xmr/bulletproof.py b/src/apps/monero/xmr/bulletproof.py
index 891f123bc..c7994c6e8 100644
--- a/src/apps/monero/xmr/bulletproof.py
+++ b/src/apps/monero/xmr/bulletproof.py
@@ -91,6 +91,14 @@ def mul_inverse(x, n):
     return pow(x, n - 2, n)
 
 
+mul_inverse_used = mul_inverse_egcd
+try:
+    pow(2, 5, 7)
+    mul_inverse_used = mul_inverse_egcd
+except NotImplementedError:
+    pass
+
+
 def invert(dst, x):
     """
     Modular inversion mod curve order.
@@ -104,7 +112,7 @@ def invert(dst, x):
     dst = _ensure_dst_key(dst)
     xint = 0
     xint = xint.from_bytes(x, "little")
-    xinv = mul_inverse(xint, ED25519_ORD)
+    xinv = mul_inverse_used(xint, ED25519_ORD)
     buff = xinv.to_bytes(32, "little")
     copy_key(dst, buff)
     return dst