Here's some trouble-making I'm doing on another list (one that believes in X.509 certs and CAs).... :) - Carl
Date: Tue, 02 Jul 1996 17:34:46 -0400 To: Greg.McPhee@Software.com (Greg McPhee) From: Carl Ellison <cme@cybercash.com> Subject: Re: Self-signed certificates Cc: ssl-talk@netscape.com
At 01:51 PM 7/2/96 -0700, Greg McPhee wrote:
If you have encountered an old friend of yours on the net and want to make sure that you can exchange keys with her without some active eavesdropper getting in the path and substituting keys, then a CA's cert is probably worthless to you. [I have a paper at this month's USENIX Security Symposium on this subject.]
I want to understand why the "CA's cert" above is worthless. Assuming the "CA's cert" is a self signed certificate identifying a CA, then is it worthless because it is an untrusted CA, or because my old friend and I don't have personal certificates signed by this CA?
Couldn't wait for the paper :-)
OK -- at the risk of boring the list... :)
There are many definitions for "identity". In this one case, I'm using the example of an old friend. We meet again on the net and want to trade keys, for private communications. Much of the loose talk over the years about certificates says that if she and I have certificates from a good CA, then we can be assured we aren't being spoofed. That statement isn't true.
To state it more formally, a CA's certificate in this case is neither necessary nor sufficient.
The CA binds a key to *its name for a person* -- trying to make that name globally unique and meaningful -- but all it can promise is to make the name unique. It can't promise to make it meaningful *to me*. The CA is not aware of my existence, much less of what I know about each person in the world. There might be 100 certificates for "Sue Robinson" -- with various other information to distinguish them from one another -- but when I knew her she was going under the name of Laura and I have no clue what her other distinguishing information is. I had lost touch with her.
I could ask her, over the net, and she would tell me all those new bits of information.
Trouble is, I need an authenticated channel to her in order to be sure I'm not being spoofed while she tells me her SNail address (or whatever makes her cert unique). I can't get an authenticated channel without the cert. Impasse.
Thus the cert from the CA is not sufficient.
It is also not necessary. The paper I'm presenting gives a protocol with which Sue and I can use our shared memories (what makes us old friends in the first place and, in a real sense, the *true* definition of "identity") to prove to each other that there is no eavesdropper over a confidential channel we create. Once we've done that, we then we can tell each other our keys and each issue a cert for the other's key. At that point, we have certified keys for each other without involving a CA. What's better, I have her certified key from a "CA" I can trust above all others -- myself.
[QED]
- Carl