Zooko, Thanks for your patient explanation.
On Wed, Jun 01, 2011 at 04:53:35PM -0600, Zooko O'Whielacronx wrote: > On Wed, Jun 1, 2011 at 11:42 AM, toby cabot <[email protected]> wrote: > > I have a question about sharing files with other people and I can't > > find the answer on the site but I hope this isn't a FAQ. > > We should write a FAQ about this! But the answer might be long. Might > need to be its own wiki page. Any volunteers? I've taken a stab at this below. It *did* end up being longer than I'd hoped, but some ruthless editing can probably cut it down to size. Comments/criticism welcome. ===================================================================== [[http://tahoe-lafs.org/trac/tahoe-lafs][Tahoe-LAFS]] is described as "the first decentralized storage system with provider-independent security". Its name indicates that it's a "file system" but it's different than traditional file systems in ways that are important to understand before you start using it. This page will try to explain at a high level, in plain English, how Tahoe-LAFS works and provide links that will allow you to learn about it in detail. Before we go any further, please read the [[http://tahoe-lafs.org/trac/tahoe-lafs/browser/trunk/docs/about.rst][one-page summary]], then come back here. As you saw on that page, Tahoe-LAFS provides a guarantee that you can store your data on servers that you don't trust, and the administrators of those servers won't be able to read your data. It does this by encrypting the data before it stores it on those servers, so that all they see is random-looking bits and they can't recover the actual content of your files. Tahoe-LAFS also guards against the failure of the storage servers by storing the same data on more than one of them. Of course, this will use more disk storage than simply storing the file once, but you can decide how you'd like to trade off extra storage for fault-tolerance. Capabilities (vs. Access Control) Before we get into how Tahoe-LAFS stores files, it will be useful to recap how a traditional file system works. Traditional filesystems start at a well-known "root" and allow users to explore the filesystem from there. Because the root is well-known, you can go to it and list the files in it; you can also go "up" from any directory to its parent. Because users can explore file systems in this way, each user would be able to do anything they wanted unless there were some sort of inline permission check, so these filesystems implement "Access Control List" (ACL) permission checks. These checks specify which users are allowed to access each file and prevent users from doing things they can figure out how to do, but are not permitted to do. In other words, I can discover a directory's existence, and learn its name, but I might not be allowed to read from it. In order to implement these permission checks, though, the file system has to know who you are, so you need to log in. In order to prove that you are who you claim to be, you have to provide a password and/or other credentials. Then you need to specify who has what kind of access to each file and directory. This approach works well enough, but it is complex and because of that it's very difficult to ensure that it's secure. Tahoe-LAFS does away with the complexity inherent in the ACL approach and uses a much simpler approach, called "capabilities". Access to each file (and directory) in Tahoe-LAFS is allowed by a "capability" which is a string of characters that looks something like =URI:CHK:riplmjitnwh25ur3jomzyxrww4:et4gkxykswl7lstw5q4g5suf6y2xyyphvid5nn2r3ktvhytbs5da:3:10:3472=. A file can have different capabilities, for example, one capability might allow you to read the file but a different capability might allow you to read and write the file. Each capability contains the two things that you need to access the file: how to find the encrypted bits (the "storage index"), and how to decrypt them (the "encryption key"). Access to any given file is a simple yes/no proposition: if you know that file's capability then you'll be able to read it, if you don't then you won't be able to. It doesn't matter who you are, or what group you're in, or if you're a "superuser" or not. In fact, Tahoe-LAFS doesn't have any sense of "identity" at all: you don't have to sign in or provide credentials to prove who you are, because Tahoe-LAFS doesn't know or care. It's important to understand that a capability specifies the location of a file, but it's different than a traditional file system "path". Tahoe-LAFS has no well-known "root" so there's no way to poke around and try to discover things inside it. Each directory and file can be found only by its capability and can't be discovered in any other way. (How many bits in a capability, i.e. how hard would it be to guess?) A directory capability acts like a traditional file system directory in that users can browse down from it to see files in it and in the tree below it, but they can't browse "up" to see other directories within the same Tahoe-LAFS file system. It's as if each directory in Tahoe-LAFS is a root directory. Users cannot discover things that they're not supposed to know, so the in-line ACL checks implemented by traditional file systems are unnecessary. If you're curious about the capability model, it's worth taking some time to learn more about it: http://en.wikipedia.org/wiki/Capability-based_security Sharing As you can imagine, the Tahoe-LAFS capability model makes file sharing easy: just give the other person that file's capability string. Once you've done that, the other person will be able to do everything that the capability enables. If you share a read-only capability then the person you shared it with will be able to read the file but not write it. If you share a read-write capability then they will be able to both. The simplicity of Tahoe-LAFS capability model makes very fine-grained sharing control easy. In a traditional Unix filesystem, for example, you can control access to a file based on the person that owns the file, the group that owns the file, and "other" people. Since creating a group is something that only superusers can do, it's easy to imagine situations where a regular user would like to share a file with a small community of people but can't do it because she isn't allowed to create groups. A typical solution to this problem is to add another layer of complexity on top of the file system layer to provide this level of control. Tahoe's capability model provides this functionality easily. If I want to give one person read-write capability and give another four people read-only capability that's easy to do - just send the appropriate capability string to the appropriate people. Revoking Sharing is easy but "revoking" is much harder to do, for both traditional file systems and Tahoe-LAFS. The fundamental problem, which is the same in both cases, is that once you give someone else the ability to read some data, you can't prevent that person from re-distributing that data in ways that you might not intend. At first glance, it would appear that a traditional filesystem offers stronger protection for this case, but in fact the ACL approach and capability approach are similar in the ways in which data can "leak": both techniques prevent "outsiders" from seeing data that they're not supposed to, but both can be subverted by people who are given the ability to read some data, and then choose to export it from the system and distribute it outside the scope of the system. In both cases, then, it's important to be very sure that you trust the people that you're sharing data with, because once you share the data there's no going back. Links More info on Access Control Lists: http://en.wikipedia.org/wiki/Access_control_list<br/> A relevant mailing list discussion: http://tahoe-lafs.org/pipermail/tahoe-dev/2011-June/006388.html _______________________________________________ tahoe-dev mailing list [email protected] http://tahoe-lafs.org/cgi-bin/mailman/listinfo/tahoe-dev
