More about patents than you wanted to know

[Nick Mathewson <nickm@alum.mit.edu>]

Summay: Even if no prior art exists to invalidate patent, it still
doesn't seem to cover our type-III remailer design.

On Thu, 2002-09-12 at 11:25, George Danezis wrote:
> Dear MixMinion crew,
> 
> I have just found (thanks to cryptome.org) a (pending?) U.S. patent that 
> seems to claim the monopoly on the creation of single use reply blocks. 
> The text can be found at:
>
> http://cryptome.org/intel-anon.htm

 [...]
> I would be very interested to hear other people's comments. We could 
> compile something and send it to cryptome.

IANAL, but according to my reading of the claims section, this patent
covers only systems where (A) Bob's software generates 'reply blocks'
automatically on request on from an incoming message, and (B) the
software encrypts Bob's address with the key of the first node that Bob
will _send_ the reply block to.

The only independent claims are #1, #14, #25, and #30.[NOTE 1]  Numbers
#25, and #30 are just make-it-a-device patent trickery, as George notes.

So we look at claim #1:
	
1. A cryptographic device comprising:
    a memory to contain at least one key; and
    
    a processor including circuitry to determine whether a response to
    an incoming electronic message is requested, to encrypt an address
    of a system associated with the cryptographic device with the key
    when the response is requested, and to place the encrypted address
    into an outgoing electronic message before transmission.
    
Mixminion doesn't look at incoming messages to determine whether a
response is requested.  Thus, I don't think we're covered by #1.

On to #14:

14. An anonymity system comprising:
    a system substrate; and
    
    a daughter card coupled to the system substrate, the daughter card
    including a cryptographic device (i) to analyze an incoming
    electronic message to determine whether or not a response to the
    incoming electronic message is requested, (ii) to encrypt an address
    of the anonymity system with a key associated with a system to
    receive an outgoing electronic message from the cryptographic
    device, and (iii) to place the encrypted address into the outgoing
    electronic message before transmission.

Here, (i) cover the check-if-a-reply-is-wanted thing that Mixminion
doesn't do. Also, (ii) says that when Alice's device generates a SURB,
it encrypts Alice's address with the key of the Mix that Alice is about
to send data to: Mixminion doesn't do that either.

Thus, --even if-- this patent is completely valid, and no prior art
exists (which I doubt), it doesn't seem to apply to our type-III
remailer design.

NOTES: 
    [1] About dependent and independent claims:  (IANAL, but I've had to
    work with one writing a set of patent claims.) When you write a
    patent, you try to write a set of claims that cover everything you
    invented.  You want these to be as broad as possible.  You do this
    by writing a few independent claims that cover the broadest possible
    interpretation of your invention, and then writing other claims that
    depend on them, narrowing down to the thing you actually built. 
    That way, you have a very broad patent, but if somebody finds prior
    art for your independent claims, you still have a patent covering
    what you specifically made.
    
    But if you're looking for see what the patent _covers_, it's
    sufficient to look _only_ at the independent claims.  
    
    Example: Suppose you've invented a nuclear self-cleaning spork.
      You could write your patent to cover:
    	1. A tool that cleans itself.
            2. The tool described in 1, where the tool uses nuclear 
               power.
            3. The tool described in 1, where the tool is a utensil.
    	4. The utensil described in 3, where the utensil is a 
               spork and so on and so on.  That way, even if 
               somebody else has invented a self-cleaning screwdriver,
               thus invalidating claim 1, you can still cover a 
               nuclear self-cleaning screwdriver, and all 
               self-cleaning utensils.
         But if somebody invent a nuclear spork that _isn't_   
         self-cleaning, the spork is _not_ covered by your patent,
         since it therefore isn't covered under your single 
         independent claim.

Of course, I could be wrong. :)

-- 
Nick