- (djm) OpenBSD CVS Sync:

- markus@cvs.openbsd.org  2001/01/29 12:47:32
     [rsa.c rsa.h ssh-agent.c sshconnect1.c sshd.c]
     handle rsa_private_decrypt failures; helps against the Bleichenbacher
     pkcs#1 attack

ChangeLog

@@ -6,6 +6,10 @@
    - markus@cvs.openbsd.org  2001/01/29 12:42:35
      [canohost.c canohost.h channels.c clientloop.c]
      add get_peer_ipaddr(socket), x11-fwd in ssh2 requires ipaddr, not DNS
+   - markus@cvs.openbsd.org  2001/01/29 12:47:32
+     [rsa.c rsa.h ssh-agent.c sshconnect1.c sshd.c]
+     handle rsa_private_decrypt failures; helps against the Bleichenbacher
+     pkcs#1 attack
 
 20000129
  - (stevesk) sftp-server.c: use %lld vs. %qd

rsa.c

@@ -60,7 +60,7 @@
  */
 
 #include "includes.h"
-RCSID("$OpenBSD: rsa.c,v 1.19 2001/01/21 19:05:54 markus Exp $");
+RCSID("$OpenBSD: rsa.c,v 1.20 2001/01/29 19:47:30 markus Exp $");
 
 #include "rsa.h"
 #include "log.h"
@@ -94,7 +94,7 @@ rsa_public_encrypt(BIGNUM *out, BIGNUM *in, RSA *key)
 	xfree(inbuf);
 }
 
-void
+int
 rsa_private_decrypt(BIGNUM *out, BIGNUM *in, RSA *key)
 {
 	u_char *inbuf, *outbuf;
@@ -108,13 +108,14 @@ rsa_private_decrypt(BIGNUM *out, BIGNUM *in, RSA *key)
 	BN_bn2bin(in, inbuf);
 
 	if ((len = RSA_private_decrypt(ilen, inbuf, outbuf, key,
-	    RSA_PKCS1_PADDING)) <= 0)
+	    RSA_PKCS1_PADDING)) <= 0) {
-		fatal("rsa_private_decrypt() failed");
+		error("rsa_private_decrypt() failed");
-
+	} else {
-	BN_bin2bn(outbuf, len, out);
+		BN_bin2bn(outbuf, len, out);
-
+	}
 	memset(outbuf, 0, olen);
 	memset(inbuf, 0, ilen);
 	xfree(outbuf);
 	xfree(inbuf);
+	return len;
 }

rsa.h

@@ -11,7 +11,7 @@
  * called by a name other than "ssh" or "Secure Shell".
  */
 
-/* RCSID("$OpenBSD: rsa.h,v 1.9 2000/11/12 19:50:38 markus Exp $"); */
+/* RCSID("$OpenBSD: rsa.h,v 1.10 2001/01/29 19:47:30 markus Exp $"); */
 
 #ifndef RSA_H
 #define RSA_H
@@ -20,6 +20,6 @@
 #include <openssl/rsa.h>
 
 void rsa_public_encrypt __P((BIGNUM * out, BIGNUM * in, RSA * prv));
-void rsa_private_decrypt __P((BIGNUM * out, BIGNUM * in, RSA * prv));
+int rsa_private_decrypt __P((BIGNUM * out, BIGNUM * in, RSA * prv));
 
 #endif				/* RSA_H */

ssh-agent.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: ssh-agent.c,v 1.48 2001/01/25 08:06:33 deraadt Exp $	*/
+/*	$OpenBSD: ssh-agent.c,v 1.49 2001/01/29 19:47:31 markus Exp $	*/
 
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
@@ -37,7 +37,7 @@
  */
 
 #include "includes.h"
-RCSID("$OpenBSD: ssh-agent.c,v 1.48 2001/01/25 08:06:33 deraadt Exp $");
+RCSID("$OpenBSD: ssh-agent.c,v 1.49 2001/01/29 19:47:31 markus Exp $");
 
 #include <openssl/evp.h>
 #include <openssl/md5.h>
@@ -198,7 +198,8 @@ process_authentication_challenge1(SocketEntry *e)
 	private = lookup_private_key(key, NULL, 1);
 	if (private != NULL) {
 		/* Decrypt the challenge using the private key. */
-		rsa_private_decrypt(challenge, challenge, private->rsa);
+		if (rsa_private_decrypt(challenge, challenge, private->rsa) <= 0)
+			goto failure;
 
 		/* The response is MD5 of decrypted challenge plus session id. */
 		len = BN_num_bytes(challenge);

sshconnect1.c

@@ -13,7 +13,7 @@
  */
 
 #include "includes.h"
-RCSID("$OpenBSD: sshconnect1.c,v 1.20 2001/01/22 23:06:40 markus Exp $");
+RCSID("$OpenBSD: sshconnect1.c,v 1.21 2001/01/29 19:47:31 markus Exp $");
 
 #include <openssl/bn.h>
 #include <openssl/evp.h>
@@ -163,14 +163,17 @@ respond_to_rsa_challenge(BIGNUM * challenge, RSA * prv)
 	int i, len;
 
 	/* Decrypt the challenge using the private key. */
-	rsa_private_decrypt(challenge, challenge, prv);
+	/* XXX think about Bleichenbacher, too */
+	if (rsa_private_decrypt(challenge, challenge, prv) <= 0)
+		packet_disconnect(
+		    "respond_to_rsa_challenge: rsa_private_decrypt failed");
 
 	/* Compute the response. */
 	/* The response is MD5 of decrypted challenge plus session id. */
 	len = BN_num_bytes(challenge);
 	if (len <= 0 || len > sizeof(buf))
-		packet_disconnect("respond_to_rsa_challenge: bad challenge length %d",
+		packet_disconnect(
-				  len);
+		    "respond_to_rsa_challenge: bad challenge length %d", len);
 
 	memset(buf, 0, sizeof(buf));
 	BN_bn2bin(challenge, buf + sizeof(buf) - len);

sshd.c

@@ -40,7 +40,7 @@
  */
 
 #include "includes.h"
-RCSID("$OpenBSD: sshd.c,v 1.158 2001/01/28 10:37:26 markus Exp $");
+RCSID("$OpenBSD: sshd.c,v 1.159 2001/01/29 19:47:31 markus Exp $");
 
 #include <openssl/dh.h>
 #include <openssl/bn.h>
@@ -1186,6 +1186,7 @@ do_ssh1_kex(void)
 {
 	int i, len;
 	int plen, slen;
+	int rsafail = 0;
 	BIGNUM *session_key_int;
 	u_char session_key[SSH_SESSION_KEY_LENGTH];
 	u_char cookie[8];
@@ -1296,7 +1297,7 @@ do_ssh1_kex(void)
 	 * with larger modulus first).
 	 */
 	if (BN_cmp(sensitive_data.server_key->rsa->n, sensitive_data.ssh1_host_key->rsa->n) > 0) {
-		/* Private key has bigger modulus. */
+		/* Server key has bigger modulus. */
 		if (BN_num_bits(sensitive_data.server_key->rsa->n) <
 		    BN_num_bits(sensitive_data.ssh1_host_key->rsa->n) + SSH_KEY_BITS_RESERVED) {
 			fatal("do_connection: %s: server_key %d < host_key %d + SSH_KEY_BITS_RESERVED %d",
@@ -1305,10 +1306,12 @@ do_ssh1_kex(void)
 			    BN_num_bits(sensitive_data.ssh1_host_key->rsa->n),
 			    SSH_KEY_BITS_RESERVED);
 		}
-		rsa_private_decrypt(session_key_int, session_key_int,
+		if (rsa_private_decrypt(session_key_int, session_key_int,
-		    sensitive_data.server_key->rsa);
+		    sensitive_data.server_key->rsa) <= 0)
-		rsa_private_decrypt(session_key_int, session_key_int,
+			rsafail++;
-		    sensitive_data.ssh1_host_key->rsa);
+		if (rsa_private_decrypt(session_key_int, session_key_int,
+		    sensitive_data.ssh1_host_key->rsa) <= 0)
+			rsafail++;
 	} else {
 		/* Host key has bigger modulus (or they are equal). */
 		if (BN_num_bits(sensitive_data.ssh1_host_key->rsa->n) <
@@ -1319,10 +1322,12 @@ do_ssh1_kex(void)
 			    BN_num_bits(sensitive_data.server_key->rsa->n),
 			    SSH_KEY_BITS_RESERVED);
 		}
-		rsa_private_decrypt(session_key_int, session_key_int,
+		if (rsa_private_decrypt(session_key_int, session_key_int,
-		    sensitive_data.ssh1_host_key->rsa);
+		    sensitive_data.ssh1_host_key->rsa) < 0)
-		rsa_private_decrypt(session_key_int, session_key_int,
+			rsafail++;
-		    sensitive_data.server_key->rsa);
+		if (rsa_private_decrypt(session_key_int, session_key_int,
+		    sensitive_data.server_key->rsa) < 0)
+			rsafail++;
 	}
 
 	compute_session_id(session_id, cookie,
@@ -1337,15 +1342,29 @@ do_ssh1_kex(void)
 	 * least significant 256 bits of the integer; the first byte of the
 	 * key is in the highest bits.
 	 */
-	BN_mask_bits(session_key_int, sizeof(session_key) * 8);
+	if (!rsafail) {
-	len = BN_num_bytes(session_key_int);
+		BN_mask_bits(session_key_int, sizeof(session_key) * 8);
-	if (len < 0 || len > sizeof(session_key))
+		len = BN_num_bytes(session_key_int);
-		fatal("do_connection: bad len from %s: session_key_int %d > sizeof(session_key) %d",
+		if (len < 0 || len > sizeof(session_key)) {
-		    get_remote_ipaddr(),
+			error("do_connection: bad session key len from %s: "
-		    len, sizeof(session_key));
+			    "session_key_int %d > sizeof(session_key) %d",
-	memset(session_key, 0, sizeof(session_key));
+			    get_remote_ipaddr(), len, sizeof(session_key));
-	BN_bn2bin(session_key_int, session_key + sizeof(session_key) - len);
+			rsafail++;
-
+		} else {
+			memset(session_key, 0, sizeof(session_key));
+			BN_bn2bin(session_key_int,
+			    session_key + sizeof(session_key) - len);
+		}
+	}
+	if (rsafail) {
+		log("do_connection: generating a fake encryption key");
+		for (i = 0; i < SSH_SESSION_KEY_LENGTH; i++) {
+			if (i % 4 == 0)
+				rand = arc4random();
+			session_key[i] = rand & 0xff;
+			rand >>= 8;
+		}
+	}
 	/* Destroy the decrypted integer.  It is no longer needed. */
 	BN_clear_free(session_key_int);