Add DL_Group functions to verify elements

2 files changed, 46 insertions, 1 deletions

diff --git a/src/lib/pubkey/dl_group/dl_group.cpp b/src/lib/pubkey/dl_group/dl_group.cpp
index c96dea677..7d2ad15e9 100644
--- a/src/lib/pubkey/dl_group/dl_group.cpp
+++ b/src/lib/pubkey/dl_group/dl_group.cpp
@@ -292,6 +292,36 @@ const DL_Group_Data& DL_Group::data() const
    throw Invalid_State("DL_Group uninitialized");
    }
 
+bool DL_Group::verify_public_element(const BigInt& y) const
+   {
+   const BigInt& p = get_p();
+   const BigInt& q = get_q();
+
+   if(y <= 1 || y >= p)
+      return false;
+
+   if(q.is_zero() == false)
+      {
+      if(power_mod(y, q, p) != 1)
+         return false;
+      }
+
+   return true;
+   }
+
+bool DL_Group::verify_element_pair(const BigInt& y, const BigInt& x) const
+   {
+   const BigInt& p = get_p();
+
+   if(y <= 1 || y >= p || x <= 1 || x >= p)
+      return false;
+
+   if(y != power_g_p(x))
+      return false;
+
+   return true;
+   }
+
 /*
 * Verify the parameters
 */
diff --git a/src/lib/pubkey/dl_group/dl_group.h b/src/lib/pubkey/dl_group/dl_group.h
index 2bd79e3dd..b9a7bb992 100644
--- a/src/lib/pubkey/dl_group/dl_group.h
+++ b/src/lib/pubkey/dl_group/dl_group.h
@@ -135,7 +135,22 @@ class BOTAN_PUBLIC_API(2,0) DL_Group final
       * @param strong whether to perform stronger by lengthier tests
       * @return true if the object is consistent, false otherwise
       */
-      bool verify_group(RandomNumberGenerator& rng, bool strong) const;
+      bool verify_group(RandomNumberGenerator& rng, bool strong = true) const;
+
+      /**
+      * Verify a public element, ie check if y = g^x for some x.
+      *
+      * This is not a perfect test. It verifies that 1 < y < p and (if q is set)
+      * that y is in the subgroup of size q.
+      */
+      bool verify_public_element(const BigInt& y) const;
+
+      /**
+      * Verify a pair of elements y = g^x
+      *
+      * This verifies that 1 < x,y < p and that y=g^x mod p
+      */
+      bool verify_element_pair(const BigInt& y, const BigInt& x) const;
 
       /**
       * Encode this group into a string using PEM encoding.