| > If the subject can utter the action, then it can perform it. This sounds like another layer of weird terminology that doesn't mean anything for someone who is not familiar with whatever capability system you're thinking of. Say I am a user who can see a particular directory on a shared setup. I try to upload a file in this directory, using the same method that worked on another directory. The question of AuthZ is: will I be allowed to do it or not? In the plain sense of the words, I can absolutely "utter the action", I have all of the "verbs" (upload) and "nouns" (the file, the destination path). Still, I should not be allowed to perform the action if I was only given read-only access here. Now sure, you can say that "upload to dirA" is a different verb than "upload to dirB". But this is just confusing terminology, it doesn't enlighten anything. |
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| ▲ | black_knight 4 hours ago | parent [-] | | You seem to understand it just fine. Your accessor, dirB, should not contain the “upload files” verb, while your dirA accessor (noun) should. My favorite example is the home directory and the file picker. Why should a program have access to all your files by default then politely ask you which file it should read/write to? It would make more sense if the file picker was something the operating system ran when a program wants to edit a file, and what came back to the program after you selected was the accessor for that file (with read and/or write verbs). So the program only have access to those files you have it access to. It cannot even ask the question to open another file, because it only has opaque accessors to those files it has been given. | | |
| ▲ | uncommoncense 2 hours ago | parent | next [-] | | What you're describing is essentially what the authorization system would need to do in order to answer the question "can this subject perform this action on this object?". If you're suggesting that the program should receive a list a priori, then there are potential scale issues since that list would need to be exhaustive of both nouns and verbs, which can be a large set. | | |
| ▲ | black_knight an hour ago | parent [-] | | The point is to flip the burden of proof. Instead of an authorisation system trying to find a reason to give you permission, you have to carry the proof in the form of a “verb”. Which you use when you perform the action. | | |
| ▲ | uncommoncense an hour ago | parent [-] | | Right, but where do you get the proof to begin with? Using your OS example, it seems like the OS would need to precompute all of the possible accesses for the file picker? In this case, the OS is an authorization system. Do you mean that the directory should not be responsible for making this decision and there should be a central authorization authority? | | |
| ▲ | black_knight 39 minutes ago | parent [-] | | There a sort of “might makes right” principle here. If you can do it, you can mint a capability for it, which means you will perform the action when someone with that capability requests it. A central authority is not a requirement. What is required is some way making sure capabilities are unforgeable. This can be a central authority, which then has a completely mechanical task of registering capabilities and their ownership. But it can also be ensured “cryptographically” with a key. The OS already has a capability system called “file descriptors”. Which works quite well, within its limited scope. This could be expanded out to more areas. | | |
| ▲ | simiones 30 minutes ago | parent [-] | | The question is - how do you get the ability to write to pathA and to pathB but not to pathC, while another user gets the ability to write to pathA and pathC but not to pathB? Do you get a huge list of capability keys when you log into the system, one for each path? Do you ask a service for a capability when you want to perform the action? |
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| ▲ | nmadden 4 hours ago | parent | prev | next [-] | | I used to use the example of Dropbox’s chooser API to illustrate this: https://www.dropbox.com/developers/chooser If you use this API (via a simple widget library) then the user simply picks a file in their dropbox and the app gets access to that one file. Vs OAuth where you grant the app broad access to the whole dropbox (or maybe some sub-folder). | | | |
| ▲ | simiones 2 hours ago | parent | prev [-] | | First of all, this necessitates a certain data model, where instead of a "UploadFile(file, destPath)" operation, I have to have a "destPath.UploadFile(file)" operation. This would be ok for this case, but not all operations can be expressed in this simple parent -> child relationship. Furthermore, even here, this doesn't cover another case: what if I am allowed to add files to destPath, but I'm not allowed to modify a specific file? This API still has to fail if `destPath/file.Name` already exists and I'm not allowed to modify it (or it at least has to do something different than when `destPath/file.Name` doesn't already exist). And even if we accept that we can only ever write things in this way, this still leaves the problem of terminology intact. Depending on the technology, it's simply not true that I can't "utter this phrase" if I don't have the capability. For example, if this is an HTTP API, then I can always do a `POST /dest-path/upload-file` with the file I want, regardless of whether I have the authorization to access that or not. Sure, if it's a HATEOAS-style API, the `GET /dest-path` might not return a link to `./upload-file` at all, but that doesn't mean that I can't utter that sentence - i.e. issue that HTTP request. | | |
| ▲ | black_knight an hour ago | parent [-] | | I have designed capability based HTTP APIs before, it took some work but the end result was ergonomic. Of course over the web the capabilities must be secured in some way. I opted for keys to prove that you can perform a given action. So, “utter” there means make a valid request with a key. On both the client side and the server side the keys and validations were invisible to the business logic, they just carried objects as usual. You created capabilities by registering a handler for the operation, and got back a token object you could hand out and even send over the api to those who were meant to use them. And the clients got these objects which they could just manipulate and keep around for making API requests. The cool part was that you could never forget to do an authorisation check. The keys were automatically checked when the request came in. An API request handler would have no privilege itself, it would only call the key-validated handlers created when the capability was minted. | | |
| ▲ | simiones 39 minutes ago | parent [-] | | Oh, I'm sure it can be done, and what you're describing sounds quite nice. All I take issue with is the claim that this is a way to make the unauthorized actions "impossible to utter". The reality is that, at least at some level, you always have to evaluate a request and, based on some cryptography related to user identity, decide if you'll honor it or refuse it. That may be checking a cookie to look up the user and then checking a separate place to see if the user is authorized to perform the action (perhaps with an extra step of finding a role, etc), or it can be checking a "pre-approval" signature obtained at some earlier point as you're describing here, but it's ultimately the same concept, and isn't "implicitly handled" in one case anymore than the other. | | |
| ▲ | black_knight 28 minutes ago | parent [-] | | I would hate to argue over semantics. But in addition to the keys being checked automatically at run time, the type system ensured that, unless the capability had been revoked, illegal requests would not type-check at compile time. Which I think is pretty close to unutterable. The keys were there to stop an attacker, the type-checker helps the good guys stay in line. |
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| ▲ | formerly_proven 3 hours ago | parent | next [-] | | They clearly mean capabilities. https://en.wikipedia.org/wiki/Capability-based_security > Capabilities achieve their objective of improving system security by being used in place of forgeable references. A forgeable reference (for example, a path name) identifies an object, but does not specify which access rights are appropriate for that object and the user program which holds that reference. Consequently, any attempt to access the referenced object must be validated by the operating system, based on the ambient authority of the requesting program, typically via the use of an access-control list (ACL). > Instead, in a system with capabilities, the mere fact that a user program possesses that capability entitles it to use the referenced object in accordance with the rights that are specified by that capability. In theory, a system with capabilities removes the need for any access control list or similar mechanism by giving all entities all and only the capabilities they will actually need. | | | |
| ▲ | black_knight 4 hours ago | parent | prev [-] | | I chose those words here because they are not programming language specific. For the OOPers, I guess you can imagine I said “objects” and “methods”. | | |
| ▲ | williamdclt 22 minutes ago | parent [-] | | Not particularly an OOPer, but regardless tou're only making it less clear I'm afraid! "objects" and "methods" instead of what? I'll make an educated guess that you mean "objects and methods" instead of "subject and action" or "noun and verb" respectively. But it's really the "utter" and "transfer" terminology that I have no idea what it might mean |
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