- markus@cvs.openbsd.org 2001/02/23 18:15:13

     [sshd.c]
     the random session key depends now on the session_key_int
     sent by the 'attacker'
             dig1 = md5(cookie|session_key_int);
             dig2 = md5(dig1|cookie|session_key_int);
             fake_session_key = dig1|dig2;
     this change is caused by a mail from anakin@pobox.com
     patch based on discussions with my german advisor niels@openbsd.org

1 files changed, 43 insertions, 20 deletions

diff --git a/sshd.c b/sshd.c
index bd90d0e56..2669a935e 100644
--- a/sshd.c
+++ b/sshd.c
@@ -40,7 +40,7 @@
  */
 
 #include "includes.h"
-RCSID("$OpenBSD: sshd.c,v 1.168 2001/02/19 23:09:05 deraadt Exp $");
+RCSID("$OpenBSD: sshd.c,v 1.169 2001/02/23 18:15:13 markus Exp $");
 
 #include <openssl/dh.h>
 #include <openssl/bn.h>
@@ -154,6 +154,7 @@ struct {
         Key     **host_keys;            /* all private host keys */
         int     have_ssh1_key;
         int     have_ssh2_key;
+        u_char  ssh1_cookie[SSH_SESSION_KEY_LENGTH];
 } sensitive_data;
 
 /*
@@ -274,13 +275,23 @@ grace_alarm_handler(int sig)
 void
 generate_empheral_server_key(void)
 {
+        u_int32_t rand = 0;
+        int i;
+
         log("Generating %s%d bit RSA key.", sensitive_data.server_key ? "new " : "",
             options.server_key_bits);
         if (sensitive_data.server_key != NULL)
                 key_free(sensitive_data.server_key);
         sensitive_data.server_key = key_generate(KEY_RSA1, options.server_key_bits);
-        arc4random_stir();
         log("RSA key generation complete.");
+
+        for (i = 0; i < SSH_SESSION_KEY_LENGTH; i++) {
+                if (i % 4 == 0)
+                        rand = arc4random();
+                sensitive_data.ssh1_cookie[i] = rand & 0xff;
+                rand >>= 8;
+        }
+        arc4random_stir();
 }
 
 void
@@ -438,6 +449,7 @@ destroy_sensitive_data(void)
                 }
         }
         sensitive_data.ssh1_host_key = NULL;
+        memset(sensitive_data.ssh1_cookie, 0, SSH_SESSION_KEY_LENGTH);
 }
 Key *
 load_private_key_autodetect(const char *filename)
@@ -1338,14 +1350,6 @@ do_ssh1_kex(void)
                     sensitive_data.server_key->rsa) < 0)
                         rsafail++;
         }
-
-        compute_session_id(session_id, cookie,
-            sensitive_data.ssh1_host_key->rsa->n,
-            sensitive_data.server_key->rsa->n);
-
-        /* Destroy the private and public keys.  They will no longer be needed. */
-        destroy_sensitive_data();
-
         /*
          * Extract session key from the decrypted integer.  The key is in the
          * least significant 256 bits of the integer; the first byte of the
@@ -1363,24 +1367,43 @@ do_ssh1_kex(void)
                         memset(session_key, 0, sizeof(session_key));
                         BN_bn2bin(session_key_int,
                             session_key + sizeof(session_key) - len);
+
+                        compute_session_id(session_id, cookie,
+                            sensitive_data.ssh1_host_key->rsa->n,
+                            sensitive_data.server_key->rsa->n);
+                        /*
+                         * Xor the first 16 bytes of the session key with the
+                         * session id.
+                         */
+                        for (i = 0; i < 16; i++)
+                                session_key[i] ^= session_id[i];
                 }
         }
         if (rsafail) {
+                int bytes = BN_num_bytes(session_key_int);
+                char *buf = xmalloc(bytes);
+                MD5_CTX md;
+
                 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;
-                }
+                BN_bn2bin(session_key_int, buf);
+                MD5_Init(&md);
+                MD5_Update(&md, buf, bytes);
+                MD5_Update(&md, sensitive_data.ssh1_cookie, SSH_SESSION_KEY_LENGTH);
+                MD5_Final(session_key, &md);
+                MD5_Init(&md);
+                MD5_Update(&md, session_key, 16);
+                MD5_Update(&md, buf, bytes);
+                MD5_Update(&md, sensitive_data.ssh1_cookie, SSH_SESSION_KEY_LENGTH);
+                MD5_Final(session_key + 16, &md);
+                memset(buf, 0, bytes);
+                xfree(buf);
         }
+        /* Destroy the private and public keys.  They will no longer be needed. */
+        destroy_sensitive_data();
+
         /* Destroy the decrypted integer.  It is no longer needed. */
         BN_clear_free(session_key_int);
 
-        /* Xor the first 16 bytes of the session key with the session id. */
-        for (i = 0; i < 16; i++)
-                session_key[i] ^= session_id[i];
-
         /* Set the session key.  From this on all communications will be encrypted. */
         packet_set_encryption_key(session_key, SSH_SESSION_KEY_LENGTH, cipher_type);