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

2009-09-08 Thread Zooko Wilcox-O'Hearn
[added Cc:, and I added on  
the whitelist so his posts will go through to tahoe-dev even if he  
isn't subscribed]

On Tuesday,2009-09-08, at 5:54 , Kevin Easton wrote:

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.

How do you prevent someone possessing the read-cap for a mutable  
file from rolling the file back to an earlier version that they  
have seen, without the consent of the write-cap possessor(s)?

You don't even need a read-cap to perform a roll-back attack -- if  
you can control the ciphertext that the reader gets, then you can  
give them a copy of an older ciphertext, even if you yourself are  
incapable of decrypting it.  This is a difficult attack to defend  
against.  In the current version of Tahoe-LAFS we already have one  
interesting defense -- we the reader is communicating with many  
different servers, and if *any* of the servers is honest and up-to- 
date and informs the reader about the existence of a newer version,  
then the reader knows that the older version that he can read is not  
the latest.  Readers in Tahoe-LAFS always download shares of the file  
from multiple servers, and all of the servers that it uses would have  
to agree on the version number.  Therefore, to perform a rollback  
attack you need to control at least that many servers as well as you  
have to control or deny-access-to any other servers which the reader  
would query and which would inform him about the newer version  
number.  See section 5 of [1].

Does that answer your question about rollback?

It would be interesting to build stronger defenses against rollback  
attack.  For starters, if the same reader reads the same file  
multiple times and gets new contents each time, he might as well  
remember the version number so that he will detect whether that file  
rolled back during his inspection of it.  Also, it would be  
interesting if a file handle to a mutable file included the version  
number that the mutable file was at when the file handle was  
created.  Building on that, it would be really cool if, in a future  
version of Tahoe-LAFS, we could make it so that you can take a cheap  
snapshot of the current contents and then give someone a file-handle  
which *both* securely identified the most recent version that you  
can find of this file and *also* the specific (immutable) version  
of this file that existed when I created this file-handle.

Also, am I correct in assuming that once write-caps have been  
distributed, they cannot be revoked, and a new file handle must be  

Currently, yes.  An improvement that I would like to make in the next  
version of Tahoe-LAFS is to allow the holder of a write-cap to revoke  
it.  While some kinds of revocation are tantamount to DRM (Digital  
Restrictions Management) and seem to be sufficiently difficult that  
we're not even going to try to implement them, the specific kind of  
revocation that you asked about seems to be quite doable.  Also, it  
happens to be the kind of revocation that I have already wanted for  
my own personal use of Tahoe-LAFS (to host my blog).  :-)

Here is a letter about that which explains why I needed this and how  
I envision it working: [2]

Stronger defenses against rollback attack, and revocation of write- 
caps -- these are only a few of the many possible extensions to the  
Tahoe-LAFS secure storage design.  We have a rich library of such  
designs documented on our issue tracker and our wiki.  We are  
currently having a detailed design discussion on the tahoe-dev list  
to which several cryptographers are contributing [e.g. 3, 4].  The  
primary goal for the next version of Tahoe-LAFS caps is to reduce the  
size of the caps and improve performance, but we're also cataloguing  
new features such as these to see if we can work them in.  Here is  
the wiki page where we're keeping our notes: [5].

If any smart cryptographer or hacker reading this wants to create  
secure, decentralized storage, please join us!  We could use the  
help!  :-)




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

2009-09-04 Thread Zooko Wilcox-O'Hearn

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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