So How Do You Manage Your Keys Then, part 3 of 5

In part one of this series [1] I described how Tahoe-LAFS combines decryption, integrity-checking, identification, and access into one bitstring, called an "immutable file read-cap" (short for "capability"). In part two [2] I described how users can build tree- like structures of files which contain caps pointing to other files, and how the cap pointing to the root of such a structure can reside on a different computer than the ciphertext. (Which is necessary if you want someone to store the ciphertext for you but you don't want to give them the ability to read the file contents.)

In this installment, consider the question of whether you can give someone a cap (which acts as a file handle) and then change the contents of the file that the cap points to, while preserving their ability to read with the original cap.

This would be impossible with the immutable file read-caps that we have been using so far, because each immutable file read cap uses a secure hash function to identify and integrity-check exactly one file's contents -- one unique byte pattern. Any change to the file contents will cause the immutable file read-cap to no longer match. This can be a desirable property if what you want is a permanent identifier of one specific, immutable file. With this property nobody -- not even the person who wrote the file in the first place -- is able to cause anyone else's read-caps to point to any file contents other than the original file contents.

But sometimes you want a different property, namely that an authorized writer *can* change the file contents and readers will be able to read the new file contents without first having to acquire a new file handle.

To accomplish this requires the use of public key cryptography, specifically digital signatures. Using digital signatures, Tahoe- LAFS implements a second kind of capability, in addition to the immutable-file capability, which is called a "mutable file capability". Whenever you create a new mutable file, you get *two* caps to it: a write-cap and a read-cap. (Actually you can always derive the read-cap from the write-cap, so for API simplicity you get just the write-cap to your newly created mutable file.)

Possession of the read-cap to the mutable file gives you two things: it gives you the symmetric encryption key with which you decrypt the file contents, and it gives you the public key with which you check a digital signature in order to be sure that the file contents were written by an authorized writer. The decryption and signature verification both happen automatically whenever you read data from that file handle (it downloads the digital signature which is stored with the ciphertext).

Possession of the write-cap gives two things: the symmetric key with which you can encrypt the ciphertext, and the private key with which you can sign the contents. Both are done automatically whenever you write data to that file handle.

The important thing about this scheme is that what we crypto geeks call "key management" is almost completely invisible to the users. As far as the users can tell, there aren't any "keys" here! The only objects in sight are the file handles, which they already use all the time.

All users need to know is that a write-cap grants write authority (only to that one file), and the read-cap grants read authority. They can conveniently delegate some of their read- or write- authority to another user, simply by giving that user a copy of that cap, without delegating their other authorities. They can bundle multiple caps (of any kind) together into a file and then use the capability to that file as a handle to that bundle of authorities.

At least, this is the theory that the object-capability community taught me, and I'm pleased to see that -- so far -- it has worked out in practice.

Programmers and end users appear to have no difficulty understanding the access control consequences of this scheme and then using the scheme appropriately to achieve their desired ends.

Installment 4 of this series will be about Tahoe-LAFS directories (those are the most convenient way to bundle together multiple caps -- put them all into a directory and then use the cap which points to that directory). Installment 5 will be about future work and new crypto ideas.



[1] # installment 1: immutable file caps [2] # installment 2: tree-like structure (like encrypted git)

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