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Date: Tue, 28 Nov 1995 11:43:34 -0800 (PST) From: Raph Levien <raph@c2.org>
It would acquire a meaning by being associated with some message or set of messages:
a) an attribute testimony (signed by someone with known authority to specify such an attribute -- the equivalent of a certificate)
This is the induction case, not the base case. It assumes that you've already got a bunch of trusted public keys in your database. It also assumes the willingness of the ownsers of those public keys to sign new keys. See, now they've got the same problem of trying to determine whether the key is valid. Turtles all the way down.
Of course. This isn't the base case. We would have some keys which I sign based on my own personal knowledge; things handed to me by people I know; some possibly published in the paper where the real key owner would see the claim and be able to contest it. PGP today comes shipped with some keys to provide the base for a chain of key signatures, but there's no verification of PRZ's or JIS's keys. One has to prime the mesh somehow. I personally prime it by having some keys (or fingerprints) exchanged face to face with people I know -- and having others acquired by association with signed messages (b). I don't have any yet whose trust has been acquired by attribution (a), since we don't have that machinery set up yet. BTW -- PGP currently lacks a way for me to note, when I sign a key, how it is that I trust that key (by personal meeting, by attribution, by message association, ...). A signed attribute record would let me record that information for myself as well as for others.
b) a set of messages signed by the key in question (tying the key to the source material from which the user formed his/her impression of the sender)
There being no reason, of course, why Mallet couldn't just sign all that stuff with his own signature. Here, you're relying on the ability of data to authenticate itself.
Here I'm talking about people I "meet" and "get to know" based strictly on my own e-mail conversations with them. For such people, there is no other evidence. If it weren't for those messages, these people effectively wouldn't exist in my world. These messages define the people in question. Stated another way: I want to communicate with Alice. I don't want to communicate with Bob. I've never met Alice but I have an address for her and a public key. Alice --- Bob --- Carl shows Bob as an active eavesdropper, controlling all of Alice's channels, blocking release of her real key, announcing a key he controls under her name to the rest of the world. By contrast, Alice --- Bob --- Carl shows Bob as Alice's secretary, who has been given the job, by Alice, of reading all Alice's mail, choosing which to pass along to her and answering all the others. Alice has generated a key for herself and has given the private key to Bob so that he can sign for her and read all her mail. Alice could even have that key certified as hers within some massive X.509 hierarchy -- doing that before she gave the private key to Bob. I know of no crypto protocol which will distinguish one from the other unless I have a private channel to Alice at some time -- but that contradicts the original assumption that I've never met her. In both cases, the person I think of as Alice is really (Alice --- Bob) --- and that's the "person" I learn to trust or not to trust. That's the "person" for whom I attach an alias to the public key. - Carl +--------------------------------------------------------------------------+ |Carl M. Ellison cme@tis.com http://www.clark.net/pub/cme | |Trusted Information Systems, Inc. http://www.tis.com/ | |3060 Washington Road PGP 2.6.2: 61E2DE7FCB9D7984E9C8048BA63221A2| |Glenwood MD 21738 Tel:(301)854-6889 FAX:(301)854-5363 | +--------------------------------------------------------------------------+ -----BEGIN PGP SIGNATURE----- Version: 2.6.2 iQCVAwUBMLtxOlQXJENzYr45AQFw4gP/es2salfOBrsPW3X1d+NnsBTThKJSkTYP yCp7YZ9iIgBXnV/rQ3TcZg2Gbts/QwpUrqN7fQQ+tNazMxqomd3+Iz+5HPTU2jc7 5rW8p/dyq1vKGDgy+M4ohTLE9XXVJLJo3AwpUJeAhqd/SAUiJPTpdgggotnXfAeF wWovhe3nq+U= =jpzx -----END PGP SIGNATURE-----