From 7cd252c6defc01f1f199bcdff57c3fb05a64eab6 Mon Sep 17 00:00:00 2001
From: pmvr <matthias.voegeler@hshl.de>
Date: Sun, 14 Jun 2020 16:30:49 +0200
Subject: [PATCH] small improvement for inversion

---
 README.md                           |  6 +++---
 mpy-modules/curve25519/arithmetic.c | 18 ++++++++----------
 2 files changed, 11 insertions(+), 13 deletions(-)

diff --git a/README.md b/README.md
index cd1acf6..02b7bf7 100644
--- a/README.md
+++ b/README.md
@@ -41,15 +41,15 @@ Python script `x25519.py`
 Python script `ed25519.py`
 
     Test 1: Length of message: 0 bytes
-    Computation time: 53 ms
+    Computation time: 52 ms
     Test 1 passed.
 
     Test 2: Length of message: 1 byte
-    Computation time: 53 ms
+    Computation time: 52 ms
     Test 2 passed.
 
     Test 3: Length of message: 2 bytes
-    Computation time: 53 ms
+    Computation time: 52 ms
     Test 3 passed.
 
     Test 4: Length of message: 1023 bytes
diff --git a/mpy-modules/curve25519/arithmetic.c b/mpy-modules/curve25519/arithmetic.c
index fb613f5..6d47298 100644
--- a/mpy-modules/curve25519/arithmetic.c
+++ b/mpy-modules/curve25519/arithmetic.c
@@ -104,7 +104,8 @@ STEP_4:
         shift_right(D);
     }
     sub9_zxy(u, u, v);
-    if ((u[8] & 0x80000000) == 0) {  // u >= v
+    if ((u[8] & 0x80000000) == 0) {
+        // u >= v
         sub9_zxy(B, B, D);
     }
     else {
@@ -112,16 +113,13 @@ STEP_4:
         sub9_zxy(v, v, u);
         sub9_zxy(D, D, B);
     }
-    uint32_t cmp = 0;
-    for (int i=0; i<9; i++) cmp |= u[i];
-    if (cmp == 0) {
-        if (D[8] & 0x80000000) {
-            add9_zxy(D, D, m);  // D < 0
-        }
-        for (uint32_t i=0; i<8; i++) y[i] = D[i];
-        return;
+    for (uint32_t i=0; i<9; i++) {
+        if (u[i] != 0) goto STEP_4;
     }
-    goto STEP_4;
+    if (D[8] & 0x80000000) {
+        add9_zxy(D, D, m);  // D < 0
+    }
+    for (uint32_t i=0; i<8; i++) y[i] = D[i];
 }