so how do *you* manage your keys, then? part 3

Zooko Wilcox-O'Hearn zooko at
Wed Sep 2 15:50:21 EDT 2009

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  

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.



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

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