On Tue, Feb 20, 2001 at 12:48:09PM +0100, Johannes Geiger wrote:
> Wouldn't it be much easier and less error prone to actually disable the
> oracle, which is the real problem leading to the attack, instead of all
> this key regeneration stuff?
This is what OpenSSH-2.5.1 tries to do.
> So all you have to do is to always do both RSA operations, record a
> failure of the first but call fatal() only after the second. Or did I
> miss something?
OpenSSH checks whether the two calls to rsa_private_decrypt()
success and the resulting session keys has the correct size.
Otherwise it just uses a 'random' session key. Now the attacker no
longer can tell whether the RSA operations failed and the
oracle is (almost) closed. See
http://www.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/sshd.c?r1=1.158&r2=1.159
However, as Simon Tatham points out:
% It's just occurred to me what this means: if I send the same mp_int
% in two successive connections, the session key I get back will be
% _different_ if the decryption failed and the same if it didn't.
%
% This is _almost_ making the problem worse: if it wasn't for the
% random padding in the SSH1_SMSG_SUCCESS packet, I could still get
% the Bleichenbacher leakage by sending the same string twice and
% seeing if the encrypted data I got back was the same twice or not.
%
% Fortunately, it's not _that_ bad, because of the 3 bytes of random
% padding in the packet. But, correct me if I'm wrong, it's still
% theoretically possible to get the information out: I send 2^24+1
% copies of the same mp_int, and see if I get 2^24+1 different encrypted
% blocks back. If not, there's a good chance my string was valid.
This means that the oracle is not completely closed, but you
need about 2^23 connections (this is not possible because of
OpenSSH's MaxStartups option) if you want to know whether it's a
fake or a real encryption key.
Because of this problem future OpenSSH releases will include the
following change: The faked session keys is not 'random' but
depends on the values sent by the attacker:
dig1 = md5(cookie|session_key_int);
dig2 = md5(dig1|cookie|session_key_int);
fake_session_key = dig1|dig2;
where the 'cookie' is a re-generated at the same time as
the server key:
Index: sshd.c
===================================================================
RCS file: /home/markus/cvs/ssh/sshd.c,v
retrieving revision 1.168
diff -u -r1.168 sshd.c
--- sshd.c 2001/02/19 23:09:05 1.168
+++ sshd.c 2001/02/20 23:40:17
@@ -145,6 +145,7 @@
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;
/*
@@ -265,13 +266,23 @@
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
@@ -429,6 +440,7 @@
}
}
sensitive_data.ssh1_host_key = NULL;
+ memset(sensitive_data.ssh1_cookie, 0, SSH_SESSION_KEY_LENGTH);
}
Key *
load_private_key_autodetect(const char *filename)
@@ -1319,9 +1331,6 @@
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
@@ -1342,14 +1351,27 @@
}
}
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);
