I'd like to bounce an idea off the list and get some feedback, especially on what I missed. say alice has some information, published e.g. on her website. she is afraid that said information may be a thought crime, aka decss or an sdmi hack. bob makes a copy of the information and wants to inform alice that he did so in case a court or some jackboots tell her to take it down, so she can link to him or do twist her dns records, whatever. so far, simple. the new twist is that I want bob to know when alice (or someone else) makes use of the information. at the lowest level, bob may want to simply know when his mirror is being used. or the information may be highly illegal and bob must know that he is now in the line of fire. or maybe he doesn't even actually provide the information until he gets the message that it is now required (think moving target). my solution is as follows: when bob copies the info, he sends a message to alice. the "outer shell" is encrypted to alice private key, to ensure that only alice can make use of it. it contains a URL, a public key and a second, encrypted part. the URL is under bob's control and provides the key to the second part. it is not linked from anywhere and reasonably complicated to make any "accidental" hit to it extremely unlikely. come day X where bob's mirror is needed. alice decrypts her mail and visits bob's URL to get the 2nd key. bob now knows (or at least can know, if he cares) that alice is in posession of the whole message (e.g. containing the URL of his mirror site) and can react accordingly. the 2nd key is also encrypted to alice, so eve can not learn about the whole message until and if alice decides to publish it (e.g. link to the new mirror site). the 2nd key is also signed with the key contained in the 1st message in order to assure alice that the information she now retrieves (possibly months or years later) is from the same source as the original message. problems I know about: legally, this surely puts bob and alice "in cooperation", so a legal injunction against alice will bind bob as well. any ideas on how to solve that would be great. it sure sounds a little heavy for a simple goal. I'm fairly sure there is a more general solution to the problem. it's just so common. "bob sends alice a message. bob needs to know when alice reads it." -- -- http://www.lemuria.org -- http://www.Nexus-Project.net --