More thoughts regarding the tokens vs. certs decision, and also multi-use:

* Client certs are a pain to turn on and off. If you select "ask me every time" before sending a client cert, you have to click half a dozen "OK"s per page. (This could be mitigated by having Wikipedia only use the SSL server for edits, since they're not blocking article viewing anyway, just editing.) If you tell the browser to send the certificate automatically and then forget about it, other SSL sites can silently request it, which is particularly bad if you're not using tor just then.

* Using tokens directly at site login time avoids the client cert hassles. However, evil web servers could then collect tokens (nyms) for use at other sites, suggesting that each server should run its own token server. But now each server has a (potentially short) list of client IPs, whereas a centralized token server would provide better concealment. Obviously, if wikipedia is the only site that ever bothers to use nym, this is a moot point.

* Lack of forward secrecy is indeed an issue, since our metaphorical Chinese dissident must keep around her cert to continue using it, which if discovered links her with all her past activities. This is a problem even if Wikipedia maps each client cert to a particular random value for public display, since the attackers can simply use the stolen cert to make an edit on wikipedia and then check to see if the identifier comes up the same.

If Wikipedia generates a new random ID for each edit, then attackers have to access Wikipedia internals to map the IDs back to the cert, but then, so do Wikipedia admins when they want to assess a user's pattern of (bad) behavior. Note that SSL does not (IIRC) encrypt certificates, so a passive network eavesdropper can associate client certs with the random IDs. (Do the ephemeral modes hide the certs?)

A related approach that thwarts the network eavesdropper would be to issue a series of certificates which expire one per interval (hour/day/whatever, trading privacy against the hassle of managing lots of certs). Then our dissident uses each cert in turn, securely deleting it after it expires. The CA keeps a list recording all the certs issued to the same user, and when Wikipedia wishes to ban a user, the CA revokes all the unexpired certs for that user. The CA also securely deletes expired certs from its lists, so that if compromised, it has merely the same list of certs found on the client machine, and is likewise devoid of any reference to certs used in prior transactions.

Of course, there are nifty cryptographic solutions to the problem of revoking repeat offenders without linking activities of good users. Private Credentials and Idemix are the two best known examples, but both are complicated and patent-ridden.


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