From 782ccf05244eff14e9bc8f3a258c8f3210e85331 Mon Sep 17 00:00:00 2001
From: hasufell <hasufell@hasufell.de>
Date: Tue, 13 May 2014 00:20:46 +0200
Subject: [PATCH] POLY: improve error handling

---
 src/poly.c | 13 +++++++++----
 src/poly.h |  2 +-
 2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/src/poly.c b/src/poly.c
index 7f3aabd..5a152a5 100644
--- a/src/poly.c
+++ b/src/poly.c
@@ -300,11 +300,11 @@ void pb_xor(pb_poly *a,
  * Get the degree of the polynomial.
  *
  * @param poly the polynomial
- * @return the degree
+ * @return the degree, -1 if polynom is empty
  */
-unsigned int get_degree(pb_poly const * const poly)
+int get_degree(pb_poly const * const poly)
 {
-	unsigned int count = 0;
+	int count = -1;
 
 	for (int i = 0; i < poly->alloc; i++)
 		if (mp_iszero(&(poly->terms[i])) == MP_NO)
@@ -354,7 +354,7 @@ static void pb_mod2_to_modq(pb_poly * const a,
  * @param a polynomial to invert (is allowed to be the same as param Fq)
  * @param Fq polynomial [out]
  * @param ctx NTRU context
- * @return true/false for success/failure
+ * @return true if invertible, false if not
  */
 bool pb_inverse_poly_q(pb_poly * const a,
 		pb_poly *Fq,
@@ -395,6 +395,8 @@ bool pb_inverse_poly_q(pb_poly * const a,
 			MP_SET(&(f->terms[ctx->N]), 0);
 			MP_SET(&(c->terms[0]), 0);
 			k++;
+			if (get_degree(f) == -1)
+				return false;
 		}
 
 		if (get_degree(f) == 0)
@@ -411,6 +413,9 @@ bool pb_inverse_poly_q(pb_poly * const a,
 
 	k = k % ctx->N;
 
+	if (mp_cmp_d(&(b->terms[ctx->N]), 0) != MP_EQ)
+		return false;
+
 	/* Fq(x) = x^(N-k) * b(x) */
 	for (int i = ctx->N - 1; i >= 0; i--) {
 		j = i - k;
diff --git a/src/poly.h b/src/poly.h
index 7eacb21..6f675c6 100644
--- a/src/poly.h
+++ b/src/poly.h
@@ -202,7 +202,7 @@ void pb_xor(pb_poly *a,
 		pb_poly *c,
 		const size_t len);
 
-unsigned int get_degree(pb_poly const * const poly);
+int get_degree(pb_poly const * const poly);
 
 bool pb_inverse_poly_q(pb_poly *a,
 		pb_poly *Fq,