Re: Hooking nym to wikipedia
I'm a bit concerned by this scheme. I'm not clear at the moment whether you're proposing imposing it on all wikipedia users or just those that want to access via Tor? On Mon, Oct 03, 2005 at 11:48:48AM +, Jason Holt wrote: * 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. There's a big useability issue with client certs, in that they are part of a particular PC browser profile and are fiddly to move between PCs; while being moved (e.g. USB key) or at rest on the disk they are vulnerable to raids by the security services. I'd expect the mythical Chinese dissident to be using netcafes rather than his/her home PC which will have a keylogger installed on it / be taken as evidence in raids. (e.g. http://gizmonaut.net/bits/suspect.html ) (Also, I'd expect any serious repressive regimes to simply have anyone found using Tor taken out and shot; has this been addressed?) Pete -- Peter Clay | Campaign for _ _| .__ | Digital / / | | | Rights! \_ \_| | | http://www.ukcdr.org - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
Re: Hooking nym to wikipedia
On 10/3/05, Jason Holt [EMAIL PROTECTED] wrote: More thoughts regarding the tokens vs. certs decision, and also multi-use: This is a good summary of the issues. With regard to turning client certs on and off: from many years of experience with anonymous and pseudonymous communication, the big usability problem is remembering which mode you are in - whether you are identified or anonymous. This relates to the technical problem of preventing data from one mode from leaking over into the other. The best solution is to use separate logins for the two modes. This prevents any technical leakage such as cookies or certificates. Separate desktop pictures and browser skins can be selected to provide constant cues about the mode. Using this method it would not be necessary to be asked on every certificate usage, so that problem with certs would not arise. (As far as the Chinese dissident using net cafes, if they are using Tor at all it might be via a USB token like the one (formerly?) available from virtualprivacymachine.com. The browser on the token can be configured to hold the cert, making it portable.) Network eavesdropping should not be a major issue for a pseudonym server. Attackers would have little to gain for all their work. The user is accessing the server via Tor so their anonymity is still protected. Any solution which waits for Wikimedia to make changes to their software will probably be long in coming. When Jimmy Wales was asked whether their software could allow logins for trusted users from otherwise blocked IPs, he didn't have any idea. The technical people are apparently in a separate part of the organization. Even if Jimmy endorsed an idea for changing Wikipedia, he would have to sell it to the technical guys, who would then have to implement and test it in their Wiki code base, then it would have to be deployed in Wikipedia (which is after all their flagship product and one which they would want to be sure not to break). Even once this happened, the problem is only solved for that one case (possibly also for other users of the Wiki code base). What about blogs or other web services that may decide to block Tor? It would be better to have a solution which does not require customization of the web service software. That approach tries to make the Tor tail wag the Internet dog. The alternative of running a pseudonym based web proxy that only lets good users pass through will avoid the need to customize web services on an individual basis, at the expense of requiring a pseudonym quality administrator who cancels nyms that misbehave. For forward secrecy, this service would expunge its records of which nyms had been active, after a day or two (long enough to make sure no complaints are going to come back). As far as the Unlinkable Serial Transactions proposal, the gist of it is to issue a new blinded token whenever one is used. That's a clever idea but it is not adequate for this situtation, because abuse information is not available until after the fact. By the time a complaint arises the miscreant will have long ago received his new blinded token and the service will have no way to stop him from continuing to use it. I could envision a complicated system whereby someone could use a token on Monday to access the net, then on Wednesday they would become eligible to exchange that token for a new one, provided that it had not been black-listed due to complaints in the interim. This adds considerable complexity, including the need to supply people with multiple initial tokens so that they could do multiple net accesses while waiting for their tokens to be eligible for exchange; the risk that exchange would often be followed immediately by use of the new token, harming unlinkability; the difficulty in fully black-listing a user who has multiple independent tokens, when each act of abuse essentially just takes one of his tokens away from him. Overall this would be too cumbersome and problematic to use for this purpose. Providing forward secrecy by having the nym-based web proxy erase its records every two days is certainly less secure than doing it by cryptographic means, but at the same time it is more secure than trusting every web service out there to take similar actions to protect its clients. Until a clean and unemcumbered technological approach is available, this looks like a reasonable compromise. CP - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
Re: Hooking nym to wikipedia
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 OKs 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. -J - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
Re: Hooking nym to wikipedia (fwd)
-- Forwarded message -- Date: Mon, 3 Oct 2005 08:32:44 -0400 From: Paul Syverson [EMAIL PROTECTED] Reply-To: [EMAIL PROTECTED] To: [EMAIL PROTECTED] Cc: cryptography@metzdowd.com Subject: Re: Hooking nym to wikipedia Hi Jason et al, On Mon, Oct 03, 2005 at 11:48:48AM +, Jason Holt wrote: More thoughts regarding the tokens vs. certs decision, and also multi-use: [snip] 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. You might want to have a look at our UST (Unlinkable Serial Transactions) It was published in ACM TISSEC http://chacs.nrl.navy.mil/publications/CHACS/1999/1999stubblebine-serial.pdf (or .ps) There was also an earlier version published at Financial Crypto. (It lacks the proofs and some improvements to the protocols) http://chacs.nrl.navy.mil/publications/CHACS/1997/1997goldschlag-FC97.pdf (or .ps) 1. I think it is much less complicated than the other things you raised, but of course has other tradeoffs. 2. The papers are it. There is no current code worth looking at. 3. Thanks for the reminder. It too is patented if not patent-ridden, but we should be able to cope with that. Basically you shouldn't put huge work in assuming that there are no encumbrances to address, but if you are interested given 1 and 2 after you look at it, let me know. I can then explain the issues regarding the patent situation. aloha, Paul - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
