So How Do You Manage Your Keys Then, part 3 of 5
In part one of this series  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  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
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
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
# installment 1: immutable file caps
# installment 2: tree-like structure (like encrypted git)
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