Add EC_Group::a_is_zero

3 files changed, 19 insertions, 1 deletions

diff --git a/src/lib/pubkey/ec_group/ec_group.cpp b/src/lib/pubkey/ec_group/ec_group.cpp
index 1fb762e4b..d5a94c90c 100644
--- a/src/lib/pubkey/ec_group/ec_group.cpp
+++ b/src/lib/pubkey/ec_group/ec_group.cpp
@@ -43,7 +43,8 @@ class EC_Group_Data final
          m_oid(oid),
          m_p_bits(p.bits()),
          m_order_bits(order.bits()),
-         m_a_is_minus_3(a == p - 3)
+         m_a_is_minus_3(a == p - 3),
+         m_a_is_zero(a.is_zero())
          {
          }
 
@@ -79,6 +80,7 @@ class EC_Group_Data final
       const PointGFp& base_point() const { return m_base_point; }
 
       bool a_is_minus_3() const { return m_a_is_minus_3; }
+      bool a_is_zero() const { return m_a_is_zero; }
 
       BigInt mod_order(const BigInt& x) const { return m_mod_order.reduce(x); }
 
@@ -108,6 +110,7 @@ class EC_Group_Data final
       size_t m_p_bits;
       size_t m_order_bits;
       bool m_a_is_minus_3;
+      bool m_a_is_zero;
    };
 
 class EC_Group_Data_Map final
@@ -399,6 +402,11 @@ bool EC_Group::a_is_minus_3() const
    return data().a_is_minus_3();
    }
 
+bool EC_Group::a_is_zero() const
+   {
+   return data().a_is_zero();
+   }
+
 size_t EC_Group::get_p_bits() const
    {
    return data().p_bits();
diff --git a/src/lib/pubkey/ec_group/ec_group.h b/src/lib/pubkey/ec_group/ec_group.h
index 031e5cc34..8bb1a3044 100644
--- a/src/lib/pubkey/ec_group/ec_group.h
+++ b/src/lib/pubkey/ec_group/ec_group.h
@@ -132,6 +132,11 @@ class BOTAN_PUBLIC_API(2,0) EC_Group final
       bool a_is_minus_3() const;
 
       /**
+      * Return if a == 0 mod p
+      */
+      bool a_is_zero() const;
+
+      /**
       * Return the size of p in bits (same as get_p().bits())
       */
       size_t get_p_bits() const;
diff --git a/src/tests/unit_ecc.cpp b/src/tests/unit_ecc.cpp
index 6ec9be3e8..c980d4a24 100644
--- a/src/tests/unit_ecc.cpp
+++ b/src/tests/unit_ecc.cpp
@@ -298,6 +298,11 @@ class EC_Group_Tests : public Test
             else
                result.test_ne("Group " + group_name + " A does not equal -3", group.get_a(), group.get_p() - 3);
 
+            if(group.a_is_zero())
+               result.test_eq("Group A is zero", group.get_a(), BigInt(0));
+            else
+               result.test_ne("Group " + group_name + " A does not equal zero", group.get_a(), BigInt(0));
+
             // get a valid point
             Botan::PointGFp p = group.get_base_point() * Test::rng().next_nonzero_byte();