From cb50b81a3d7098a864b99832354f9e2cdbbca965 Mon Sep 17 00:00:00 2001
From: Never <neverhub@users.noreply.github.com>
Date: Mon, 19 Dec 2016 13:32:01 +0100
Subject: Improved DL_Group verification. The group is invalid, if g^q mod p
 !=1 and increased number of Miller-Rabin iterations, if strong is set (we
 pass 128 as prob in make_prm.cpp).

---
 src/lib/pubkey/dl_group/dl_group.cpp | 24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

(limited to 'src/lib/pubkey/dl_group')

diff --git a/src/lib/pubkey/dl_group/dl_group.cpp b/src/lib/pubkey/dl_group/dl_group.cpp
index 40660e62a..5ca07eae2 100644
--- a/src/lib/pubkey/dl_group/dl_group.cpp
+++ b/src/lib/pubkey/dl_group/dl_group.cpp
@@ -12,6 +12,7 @@
 #include <botan/ber_dec.h>
 #include <botan/pem.h>
 #include <botan/workfactor.h>
+#include <botan/pow_mod.h>
 
 namespace Botan {
 
@@ -149,15 +150,28 @@ bool DL_Group::verify_group(RandomNumberGenerator& rng,
 
    if(m_g < 2 || m_p < 3 || m_q < 0)
       return false;
-   if((m_q != 0) && ((m_p - 1) % m_q != 0))
-      return false;
 
-   const size_t prob = (strong) ? 56 : 10;
+   const size_t prob = (strong) ? 128 : 10;
 
+   if(m_q != 0)
+      {
+      if((m_p - 1) % m_q != 0)
+         {
+         return false;
+         }
+      if(power_mod(m_g, m_q, m_p) != 1)
+         {
+         return false;
+         }
+      if(!is_prime(m_q, rng, prob))
+         {
+         return false;
+         }
+      }
    if(!is_prime(m_p, rng, prob))
+      {
       return false;
-   if((m_q > 0) && !is_prime(m_q, rng, prob))
-      return false;
+      }
    return true;
    }
 
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