Dr. Mike wrote, patiently, persistently and truthfully:

On Fri, 16 Aug 2002, AARG! Anonymous wrote:

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Here are some more thoughts on how cryptography could be used to enhance user privacy in a system like TCPA. Even if the TCPA group is not receptive to these proposals, it would be useful to have an understanding of the security issues. And the same issues arise in many other kinds of systems which use certificates with some degree of anonymity, so the discussion is relevant even beyond TCPA.

OK, I'm going to discuss it from a philosophical perspective. i.e. I'm just having fun with this.

Fine, but let me put this into perspective. First, although the discussion is in terms of a centralized issuer, the same issues arise if there are multiple issuers, even in a web-of-trust situation. So don't get fixated on the fact that my analysis assumed a single issuer - that was just for simplicity in what was already a very long message. The abstract problem to be solved is this: given that there is some property which is being asserted via cryptographic certificates (credentials), we want to be able to show possession of that property in an anonymous way. In TCPA the property is "being a valid TPM". Another example would be a credit rating agency who can give out a "good credit risk" credential. You want to be able to show it anonymously in some cases. Yet another case would be a state drivers license agency which gives out an "over age 21" credential, again where you want to be able to show it anonymously. This is actually one of the oldest problems which proponents of cryptographic anonymity attempted to address, going back to David Chaum's seminal work. TCPA could represent the first wide-scale example of cryptographic credentials being shown anonymously. That in itself ought to be of interest to cypherpunks. Unfortunately TCPA is not going for full cryptographic protection of anonymity, but relying on Trusted Third Parties in the form of Privacy CAs. My analysis suggests that although there are a number of solutions in the cryptographic literature, none of them are ideal in this case. Unless we can come up with a really strong solution that satisfies all the security properties, it is going to be hard to make a case that the use of TTPs is a mistake.

I don't like the idea that users *must* have a "certificate". Why can't each person develop their own personal levels of trust and associate it with their own public key? Using multiple channels, people can prove their key is their word. If any company wants to associate a certificate with a customer, that can have lots of meanings to lots of other people. I don't see the usefullness of a "permanent certificate". Human interaction over electronic media has to deal with monkeys, because that's what humans are :-)

A certificate is a standardized and unforgeable statement that some person or key has a particular property, that's all. The kind of system you are talking about, of personal knowledge and trust, can't really be generalized to an international economy.

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Actually, in this system the Privacy CA is not really protecting anyone's privacy, because it doesn't see any identities. There is no need for multiple Privacy CAs and it would make more sense to merge the Privacy CA and the original CA that issues the permanent certs. That way there would be only one agency with the power to forge keys, which would improve accountability and auditability.

I really, REALLY, *REALLY*, don't like the idea of one entity having the ability to create or destroy any persons ability to use their computer at whim. You are suggesting that one person (or small group) has the power to create (or not) and revoke (or not!) any and all TPM's!

I don't know how to describe my astoundment at the lack of comprehension of history.

Whoever makes a statement about a property should have the power to revoke it. I am astounded that you think this is a radical notion. If one or a few entities become widely trusted to make and revoke statements that people care about, it is because they have earned that trust. If the NY Times says something is true, people tend to believe it. If Intel says that such-and-such a key is in a valid TPM, people may choose to believe this based on Intel's reputation. If Intel later determines that the key has been published on the net and so can no longer be presumed to be a TPM key, it revokes its statement. This does not mean that Intel would destroy any person's ability to use their computer on a whim. First, having the TPM cert revoked would not destroy your ability to use your computer; at worst you could no longer persuade other people of your trustworthiness. And second, Intel would not make these kind of decision on a whim, any more than the NY Times would publish libelous articles on a whim; doing so would risk destroying the company's reputation, one of its most valuable assets. I can't really respond to the remainder of the message. It doesn't seem to have anything to do with the real issues. Hopefully my introduction above will have put the problem into perspective. I suggest you educate yourself on cryptographic technologies for anonymity. You might start with David Chaum's early CACM article, http://www.chaum.com/articles/Security_Wthout_Identification.htm.