check for reasonable public DH values

4 files changed, 50 insertions, 12 deletions

diff --git a/usr.bin/ssh/kex.c b/usr.bin/ssh/kex.c
index cc2543a0ea1..2308c9398f5 100644
--- a/usr.bin/ssh/kex.c
+++ b/usr.bin/ssh/kex.c
@@ -28,7 +28,7 @@
  */
 
 #include "includes.h"
-RCSID("$Id: kex.c,v 1.2 2000/04/12 06:37:02 markus Exp $");
+RCSID("$Id: kex.c,v 1.3 2000/04/12 07:03:05 markus Exp $");
 
 #include "ssh.h"
 #include "ssh2.h"
@@ -74,8 +74,36 @@ kex_init(char *myproposal[PROPOSAL_MAX])
 
 /* diffie-hellman-group1-sha1 */
 
+int
+dh_pub_is_valid(DH *dh, BIGNUM *dh_pub)
+{
+	int i;
+	int n = BN_num_bits(dh_pub);
+	int bits_set = 0;
+
+	/* we only accept g==2 */
+	if (!BN_is_word(dh->g, 2)) {
+		log("invalid DH base != 2");
+		return 0;
+	}
+	if (dh_pub->neg) {
+		log("invalid public DH value: negativ");
+		return 0;
+	}
+	for (i = 0; i <= n; i++)
+		if (BN_is_bit_set(dh_pub, i))
+			bits_set++;
+	debug("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))
+		return 1;
+	log("invalid public DH value (%d/%d)", bits_set, BN_num_bits(dh->p));
+	return 0;
+}
+
 DH *
-new_dh_group1()
+dh_new_group1()
 {
 	static char *group1 =
 	    "FFFFFFFF" "FFFFFFFF" "C90FDAA2" "2168C234" "C4C6628B" "80DC1CD1"
@@ -85,19 +113,23 @@ new_dh_group1()
 	    "EE386BFB" "5A899FA5" "AE9F2411" "7C4B1FE6" "49286651" "ECE65381"
 	    "FFFFFFFF" "FFFFFFFF";
 	DH *dh;
-	int ret;
+	int ret, tries = 0;
 	dh = DH_new();
 	if(dh == NULL)
 		fatal("DH_new");
-	ret = BN_hex2bn(&dh->p,group1);
+	ret = BN_hex2bn(&dh->p, group1);
 	if(ret<0)
 		fatal("BN_hex2bn");
 	dh->g = BN_new();
 	if(dh->g == NULL)
 		fatal("DH_new g");
-	BN_set_word(dh->g,2);
-	if (DH_generate_key(dh) == 0)
-		fatal("DH_generate_key");
+	BN_set_word(dh->g, 2);
+	do {
+		if (DH_generate_key(dh) == 0)
+			fatal("DH_generate_key");
+		if (tries++ > 10)
+			fatal("dh_new_group1: too many bad keys: giving up");
+	} while (!dh_pub_is_valid(dh, dh->pub_key));
 	return dh;
 }
 
diff --git a/usr.bin/ssh/kex.h b/usr.bin/ssh/kex.h
index f9e79994888..5395ebc3f08 100644
--- a/usr.bin/ssh/kex.h
+++ b/usr.bin/ssh/kex.h
@@ -91,7 +91,8 @@ struct Kex {
 };
 
 Buffer	*kex_init(char *myproposal[PROPOSAL_MAX]);
-DH	*new_dh_group1();
+int	dh_pub_is_valid(DH *dh, BIGNUM *dh_pub);
+DH	*dh_new_group1();
 Kex 	*kex_choose_conf(char *cprop[PROPOSAL_MAX], char *sprop[PROPOSAL_MAX], int server);
 int	kex_derive_keys(Kex *k, unsigned char *hash, BIGNUM *shared_secret);
 void	bignum_print(BIGNUM *b);
diff --git a/usr.bin/ssh/sshconnect.c b/usr.bin/ssh/sshconnect.c
index 3391c33b640..e8b4f16b9ac 100644
--- a/usr.bin/ssh/sshconnect.c
+++ b/usr.bin/ssh/sshconnect.c
@@ -10,7 +10,7 @@
  */
 
 #include "includes.h"
-RCSID("$OpenBSD: sshconnect.c,v 1.62 2000/04/12 06:36:48 markus Exp $");
+RCSID("$OpenBSD: sshconnect.c,v 1.63 2000/04/12 07:03:06 markus Exp $");
 
 #include <ssl/bn.h>
 #include "xmalloc.h"
@@ -1393,7 +1393,7 @@ ssh_kex2(char *host, struct sockaddr *hostaddr)
 	debug("Sending SSH2_MSG_KEXDH_INIT.");
 
 	/* generate and send 'e', client DH public key */
-	dh = new_dh_group1();
+	dh = dh_new_group1();
 	packet_start(SSH2_MSG_KEXDH_INIT);
 	packet_put_bignum2(dh->pub_key);
 	packet_send();
@@ -1440,6 +1440,9 @@ ssh_kex2(char *host, struct sockaddr *hostaddr)
 	/* signed H */
 	signature = packet_get_string(&slen);
 
+	if (!dh_pub_is_valid(dh, dh_server_pub))
+		packet_disconnect("bad server public DH value");
+
 	klen = DH_size(dh);
 	kbuf = xmalloc(klen);
 	kout = DH_compute_key(kbuf, dh_server_pub, dh);
diff --git a/usr.bin/ssh/sshd.c b/usr.bin/ssh/sshd.c
index 7d8963ffa22..088da20c550 100644
--- a/usr.bin/ssh/sshd.c
+++ b/usr.bin/ssh/sshd.c
@@ -14,7 +14,7 @@
  */
 
 #include "includes.h"
-RCSID("$OpenBSD: sshd.c,v 1.100 2000/04/12 06:37:02 markus Exp $");
+RCSID("$OpenBSD: sshd.c,v 1.101 2000/04/12 07:03:06 markus Exp $");
 
 #include "xmalloc.h"
 #include "rsa.h"
@@ -1166,7 +1166,7 @@ do_ssh2_kex()
 #endif
 
 	/* generate DH key */
-	dh = new_dh_group1();			/* XXX depends on 'kex' */
+	dh = dh_new_group1();			/* XXX depends on 'kex' */
 
 #ifdef DEBUG_KEXDH
 	fprintf(stderr, "\np= ");
@@ -1177,6 +1177,8 @@ do_ssh2_kex()
 	bignum_print(dh->pub_key);
 	fprintf(stderr, "\n");
 #endif
+	if (!dh_pub_is_valid(dh, dh_client_pub))
+		packet_disconnect("bad client public DH value");
 
 	klen = DH_size(dh);
 	kbuf = xmalloc(klen);