From 34e6b723ffeceebe87f9cf89f4e73ba93d566d51 Mon Sep 17 00:00:00 2001
From: Steffen Jaeckel <s@jaeckel.eu>
Date: Wed, 25 Nov 2015 21:24:21 +0100
Subject: [PATCH] Implement handling of special case (0 | 1)

---
 bn_mp_jacobi.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/bn_mp_jacobi.c b/bn_mp_jacobi.c
index 76596a6..6a1155d 100644
--- a/bn_mp_jacobi.c
+++ b/bn_mp_jacobi.c
@@ -17,6 +17,8 @@
 
 /* computes the jacobi c = (a | n) (or Legendre if n is prime)
  * HAC pp. 73 Algorithm 2.149
+ * HAC is wrong here, as the special case of (0 | 1) is not
+ * handled correctly.
  */
 int mp_jacobi (mp_int * a, mp_int * n, int *c)
 {
@@ -29,10 +31,15 @@ int mp_jacobi (mp_int * a, mp_int * n, int *c)
      return MP_VAL;
   }
 
-  /* step 1.  if a == 0, return 0 */
+  /* step 1. handle case of a == 0 */
   if (mp_iszero (a) == MP_YES) {
-    *c = 0;
-    return MP_OKAY;
+     /* special case of a == 0 and n == 1 */
+     if (mp_cmp_d (n, 1) == MP_EQ) {
+       *c = 1;
+     } else {
+       *c = 0;
+     }
+     return MP_OKAY;
   }
 
   /* step 2.  if a == 1, return 1 */