2 files changed, 23 insertions, 3 deletions

diff --git a/ChangeLog b/ChangeLog
index d99a94176..2fe39cb25 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -5,6 +5,9 @@
      Prevent ssh from trying to open private keys with bad permissions more than
      once or prompting for their passphrases (which it subsequently ignores
      anyway), similar to a previous change in ssh-add.  bz #1186, ok djm@
+   - djm@cvs.openbsd.org 2006/05/04 14:55:23
+     [dh.c]
+     tighter DH exponent checks here too; feedback and ok markus@
 
 20060504
  - (dtucker) [auth-pam.c groupaccess.c monitor.c monitor_wrap.c scard-opensc.c
@@ -4602,4 +4605,4 @@
    - (djm) Trim deprecated options from INSTALL. Mention UsePAM
    - (djm) Fix quote handling in sftp; Patch from admorten AT umich.edu
 
-$Id: ChangeLog,v 1.4321 2006/05/06 07:41:51 dtucker Exp $
+$Id: ChangeLog,v 1.4322 2006/05/06 07:43:33 dtucker Exp $
diff --git a/dh.c b/dh.c
index b32a7efb5..b3f4ceef1 100644
--- a/dh.c
+++ b/dh.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: dh.c,v 1.35 2006/03/27 13:03:54 deraadt Exp $ */
+/* $OpenBSD: dh.c,v 1.36 2006/05/04 14:55:23 djm Exp $ */
 /*
  * Copyright (c) 2000 Niels Provos.  All rights reserved.
  *
@@ -181,19 +181,36 @@ dh_pub_is_valid(DH *dh, BIGNUM *dh_pub)
         int i;
         int n = BN_num_bits(dh_pub);
         int bits_set = 0;
+        BIGNUM *tmp;
 
         if (dh_pub->neg) {
                 logit("invalid public DH value: negativ");
                 return 0;
         }
+        if (BN_cmp(dh_pub, BN_value_one()) != 1) {      /* pub_exp <= 1 */
+                logit("invalid public DH value: <= 1");
+                return 0;
+        }
+
+        if ((tmp = BN_new()) == NULL)
+                return (-1);
+        if (!BN_sub(tmp, dh->p, BN_value_one()) ||
+            BN_cmp(dh_pub, tmp) != -1) {                /* pub_exp > p-2 */
+                BN_clear_free(tmp);
+                logit("invalid public DH value: >= p-1");
+                return 0;
+        }
+        BN_clear_free(tmp);
+
         for (i = 0; i <= n; i++)
                 if (BN_is_bit_set(dh_pub, i))
                         bits_set++;
         debug2("bits set: %d/%d", bits_set, BN_num_bits(dh->p));
 
         /* if g==2 and bits_set==1 then computing log_g(dh_pub) is trivial */
-        if (bits_set > 1 && (BN_cmp(dh_pub, dh->p) == -1))
+        if (bits_set > 1)
                 return 1;
+
         logit("invalid public DH value (%d/%d)", bits_set, BN_num_bits(dh->p));
         return 0;
 }