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Re: Missing requirements

From: Bennet Yee <bsy@cs.ucsd.edu>
Date: Tue, 21 May 1996 19:53:57 -0700
Message-Id: <199605220254.TAA26484@work.ucsd.edu>
To: Charles Watt <watt@sware.com>
Cc: ietf-tls@w3.org

In message <9605201546.AA01595@mordred.sware.com>, Charles Watt writes:
> ....  The group's focus seems to be "let's slam something together 
> ASAP so all of our browsers will interoperate, and maybe what we wind up 
> with will also be useful to other applications".  Although I too would 
> like to see security deployed ASAP, I find this approach short sighted.  
> In order to truly support electronic commerce applications, we shall 
> need secure systems, not just secure applications.  

To build secure systems, we would need to do a lot more work.  Do you
mean Orange Book B level at least?  I doubt that you meant to replace
Windows / MacOS / Unix with an alternative OS.  In this context, what
do you mean by "secure system"?

> As an admittedly biased example, SecureWare's Hannah product 
> (see http://www.secureware.com/papers for a white paper and protocol
> specifications) creates a secure system by providing transport 
> layer security for ALL network communications in a manner that is 
> transparent to the applications -- i.e., existing applications are 
> secured without modification.  It then controls access to transport 
> layer networking (who can access which applications, what security 
> services must be enforced, which algorithms and key sizes must be 
> used, etc...) through ties to the system's underlying security policy.  
> Although a security-aware user or application can request specific 
> security services and algorithms, such requests cannot override 
> security policy.

From the white paper, it appears that Hannah "creates a secure system"
only in the sense that it tries to mediate in all network traffic and
provides some node management functionality.  It is not multi-level
security, obviously, since there's no data labelling and the node
management functionality apparently provide basic, low level access
control for network connections.

I briefly looked at http://www.secureware.com/papers/pakmp/pakmp.ps,
and have some questions.  It is a 65 page document that dives quickly
into the layout of the packets and does not seem to include a high
level description of the cryptographic protocols used.

On page 13, it states that Family C "provides key management using
Diffie-Hellman", and that "authentication is achieved by having each
side encrypt a nonce with the public key of the peer and having the
peer return the decrypted value."  Presumably with RSA, since that
paragraph mentions RSA later, and presumably the nonce, both
RSA-encrypted and not, are actually sent over a channel encrypted with
the key derived in the D-H exchange.  Later sections that I found on
Family C had only "TBD" (4.4, pg 46).

Now, this sort of authentication is subject to a man-in-the-middle
attack, unless the D-H keys are somehow certified and the RSA
certificates are linked to the D-H certificates.  This is a very
important requirement that does not seem to be stated anywhere.
Now, pg 22 mentions PKCS3, so it does not appear that you're using
the pair-wise fixed keys scheme but rather D-H with global p,g to
obtain a shared secret the value of which neither side controls.

Without something stronger than plain D-H, we can have the following
scenario: Alice and Bob wishes to communite.  Guy is the man in the
middle.  G pretends to be B to A (denoted G_B), and at the same time
pretends to be A to B (G_A).  G performs two independent D-H key
exchanges with A and B (as G_B and G_A resp) to obtain two shared
secrets k_a = g^{r_a r_{g_b}} mod p and k_b = g^{r_b r_{g_a}} mod p
respectively.  Now, suppose A wishes G_B to authenticate his identity
as B.  G_B can not do this directly, since he lacks Bob's RSA private
key (d_b).  However, when A sends her nonce r_n^{e_b} mod n_b, it is
over the channel encrypted with k_a -- the D-H derived key Alice
believes that she shares with Bob.  Guy can easily obtain r_n^{e_b}
mod n_b, of course, since he has k_a.  Now G re-encrypts this value
with k_b and sends it to B (acting as G_A).  B decrypts using secret
key cryptosystem key k_b, decrypts using RSA to obtain r_n, and
replies with r_n over his secret-key protected channel.  Straight to
G, who can forward it to A.

This is a simple, standard textbook-example MITM attack that I'd
expect any of my students to understand and avoid.  Perhaps your
document is in error and does not actually describe the real
cryptographic protocol family C as originally designed.

In any case, a clearer document that describes the protocols involved
rather than a which-byte-goes-where document (which is more useful to
an implementer than a cryptographer) would be nice.

I haven't looked at any of the other families A-G (or whatever the
highest letter is); C just jumped out at me.

> In order to provide such pervasive security, Hannah implements
> transport security services within the protocol stack.  This can best 
> be achieved when there is clean separation between the actual transport
> layer security protocol and its supporting key management and 
> authentication -- the approach taken by SDNS's SP4, OSI's TLSP, and 
> other previous attempts to "standardize" transport layer security.

Maybe Microsoft wouldn't care (;-), but Hannah's implementation -- as
described in the white paper -- requires in-kernel modifications on
Unix hosts (Hannah is only available HPUX and SCO -- for Windows
Hannah takes over the Winsock DLL to control the network connections).
This is contradictory to one of the (perhaps implicit) goals of being
usable on all popular systems.  Granted, the Hannah protocols may be
implemented as a user-level library just like SSL/PCT/etc, but in that
case the advantage of being pervasive (and thus can handle all network
connections by mandate) disappears.


p.s.: the Hannah protocol description appears to be missing a
requirement to verify that the cert matches the DNS name somehow.  For
web use, we wouldn't want a certified but malicious web server to be
providing Web documents that are purportedly from e.g. SecureWare
using DNS spoofing tricks and its own certificate(s), which the users
(or users' Web browsers) would not be able to examine in a completely
transparent scheme.

Bennet S. Yee		Phone: +1 619 534 4614	    Email: bsy@cs.ucsd.edu

Web:	http://www-cse.ucsd.edu/users/bsy/
USPS:	Dept of Comp Sci and Eng, 0114, UC San Diego, La Jolla, CA 92093-0114
Received on Tuesday, 21 May 1996 22:54:32 EDT

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