From 398edc729d936d6e3b7e320e06c894480c132b69 Mon Sep 17 00:00:00 2001
From: hasufell <hasufell@hasufell.de>
Date: Sat, 13 Sep 2014 18:26:31 +0200
Subject: [PATCH] POLY: fix substraction

Interestingly, the results and tests work with both variants.
However, this one is correct and matches the algorithm given in
NTRU Cryptosystems Tech Report #014.
---
 src/ntru_poly.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/ntru_poly.c b/src/ntru_poly.c
index 22f0759..283d67c 100644
--- a/src/ntru_poly.c
+++ b/src/ntru_poly.c
@@ -509,12 +509,12 @@ poly_inverse_poly_p(fmpz_poly_t Fp,
 
 			/* f = f - u * g mod p */
 			fmpz_poly_scalar_mul_fmpz(g_tmp, g_tmp, u);
-			fmpz_poly_sub(f, g_tmp, f);
+			fmpz_poly_sub(f, f, g_tmp);
 			fmpz_poly_mod_unsigned(f, params->p);
 
 			/* b = b - u * c mod p */
 			fmpz_poly_scalar_mul_fmpz(c_tmp, c_tmp, u);
-			fmpz_poly_sub(b, c_tmp, b);
+			fmpz_poly_sub(b, b, c_tmp);
 			fmpz_poly_mod_unsigned(b, params->p);
 
 			fmpz_clear(u);