RE: Non-extratability Tainting of Keys

I believe this is something that I desire.  I believe that this is probably
what Nexflix desires.

 

I believe that this is not going to make Ryan think that this is a problem
that needs to be solved.

 

Jim

 

 

From: Ryan Sleevi [mailto:sleevi@google.com] 
Sent: Friday, September 06, 2013 4:20 PM
To: Jim Schaad
Cc: public-webcrypto@w3.org
Subject: Re: Non-extratability Tainting of Keys

 

 

 

On Fri, Sep 6, 2013 at 3:41 PM, Jim Schaad <ietf@augustcellars.com> wrote:

This is a proposal of how to do tainting of keys with non-extractability.
As part of this proposal, there is a set of things that are restricted when
you have a key marked as non-extractable.  I do not believe that these
restrictions are burdensome, but others might.  This message is intended to
provide a workable solution to the non-extractible problem.  There is still
a need to determine if this work is needed or not.

My assumption is that when one starts with a non-extractable secret, then
all secrets that are derived from that secret need to be non-extractable as
well.

 

That's not something I desire. Is that something you desire, or is this an
interpretation of what Netflix desires?

 

I guess I read your preamble as "I'm not sure what problem I'm trying to
solve, but here's a possible solution", which leaves me a bit... confused?

 


Changes to the current document:

1.  There is a new restriction placed on algorithm descriptions so that the
usages [KeyWrap,KeyUnwrap] and [Encryption,Decryption] are mutually
exclusive.   Most of the time this will not be an issue, but for some
encryption algorithms it will mean that there will need to be two algorithms
defined.  One of these will be for content encryption and one will be for
key encryption.  This would be an issue for example with AES-GCM where a new
AES-GCM-KeyWrap algorithm would need to be defined which allows for key
wrapping to occur.

2.  When a key agreement operation is done with a key marked as
non-extractable, the DeriveBits key usage is removed on the resulting secret
so that only keys can be derived.

3.  KeyUnwrap, KeyAgreement and DeriveKey all propagate the non-extractable
bit if it is on the key that was used in the operation.

4.  It is assumed that a key encryption algorithm which unwraps a key
produces a content encryption key (i.e. the usage on the resulting key is
decryption not key unwrap). If one wants to do a two level key unwrap then a
new algorithm would need to be defined that went from a key wrap key to a
key wrap key rather than a content encryption key.  I would be surprised if
this was ever needed in practice.

The resulting restrictions:

1.  Only the algorithms defined in the UA will be available to a
non-extractible key and any secrets that are derived from it.  This is an
issue if one looks at some of the composite AEAD algorithms such as the
AES-CBC/HMAC-SHA-256 algorithm that the JOSE documents use.  If such an
algorithm is to be used, it must be in the UA and not provided by the script
even if all of the components of the composite algorithm are provided by the
UA.

2.  DeriveBits cannot be used to generate both key data and non-key data
when using a non-extractable key.   This makes it impossible, for example,
to derive both a key and an iv from the shared secret in the same way as the
TLS extractor done.  It would be possible to document how an extractor could
be defined where the DeriveBits would still be permitted, but the data
stream and the key stream would be required to be distinct by having the
extractor mix in data that indicates the stream that the data is being
generated for as part of the data generation process.

3.  Only those key formats that are supported by the UA can be used with a
non-extractable key.  Thus if one wants to use a JWK key format, then that
format must be supported by the UA or it cannot be used.

4.  If one assumes that bad code can exist, then one can only be truly
assured that this works for named keys (i.e. those keys that are populated
by the UA itself).  However there are some tricks that one can play with
trying to extract and test non-extractable keys where one can potentially
later detect that the key was not generated with the non-extractable bit
set.

Jim