[formerly_proven]

> If I'm unable to convey authorization by giving you the token, it is not a capability. If I am able to guess the token, it is not a capability.

You're taking "communicable" too literally. It doesn't mean "literally giving someone else this bytestring and now they can do it, too". It means "there is a way to give someone else the capability". SCM_RIGHTS is such a way to communicate capabilities between processes. Further up above you're also conflating files and file descriptors, which are very different things.

[staticassertion]

No, it does not mean there is "a way", it means that the name is the permission. Period. That is what a capability is. If the name is not the permission, it is not a capability.

[monocasa]

It can mean what the parent is talking about, and does in this case. It's an older morph of capability based security than the newer crypto enforced name concept, but equally valid.

You can read more about it in Capability Based Computer Systems by Henry M Levy. The Hydra MMP, GE-645, and iAPX 432 are older implementions of this concept in hardware.

In both cases (TCB table enforced and crypto one way function enforced) the name is the capability, but in a kernel (or hardware) table enforced method that's interpreted to mean that you have permission to access the object by having a simply having reference to it in your descriptor table.

You can see this also in KeyKOS, EROS, Coyotos, XOK, and SeL4 among many other capability based kernels. They don't give you a capability just because you got access to a global name to the backing object; the whole shtick for them is there is no global names for objects, only descriptor tables.

[staticassertion]

What you're referring to is how operating systems maintain the integrity of capabilities. This is addressed directly in 'Capability Based Computer Systems'. I would direct you to chapter 1 section 1.1.

> A capability thus provides addressing and access rights to an object.

> a program cannot access an object unless its capability list contains a suitably privileged capability for the object.

> Capability system integrity is usually maintained by prohibiting direct program modification of the capability list. The capability list is modified only by the operating system or the hardware.

> Thus, a program can execute direct control over the movement of capabilities and can share capabilities, and therefore, objects, with other programs and users.

You can think of capabilities lists as kernel managed namespaces if you'd like. Maybe that would help clear things up.

[monocasa]

In these non crypto enforced capability systems, you don't have any control over other processes' names for resources addressed through capabilities is the point I'm attempting to get across.

Crypto backed capabilities that merge the token with the capability itself are one morph of capability based systems, but that's not the scheme being described here. Leaking the name of a capability (the offset in the cap table) does not leak the capability itself in these TCB schemes. Capabilities are still communicable in these TCB schemes because they're explicitly supported in the IPC mechanisms like SCM_RIGHTS or seL4_Send along side normal data. The token naming a capability then is a per process concept and you can clone an identical capability into several tokens in the same cap table, unlike the crypto enforced scheme which are basically content addressed storage for a secret representing a capability.

[throwaway894345]

Semantics aside, is it accurate to say that schemes where the capability is represented as a signed token require care and diligence to mitigate leaks, and schemes where the capability is represented as a table entry (mapping a user/process to a permission) don't require any meaningful care/diligence?

[Rusky]

Yes, that's the idea.

Though, it is still worth thinking about how someone might accidentally leak a table-based capability via API misuse. For example, if table entries are reused over time you might get a sort of use-after-free situation where you mean to share an old capability but accidentally share the new one that replaced it.