Re: Key wrap/unwrap/import/export open issue

On Thu, Jul 18, 2013 at 7:09 PM, Ryan Sleevi <sleevi@google.com> wrote:

> On Mon, Jul 8, 2013 at 5:12 PM, Mark Watson <watsonm@netflix.com> wrote:
> > Returning to the subject of the original post, and to start off the
> > discussion.
> >
> > Ryan has mentioned two other possibilities for solving this problem, so
> I'd
> > like to take a moment to describe my understanding of those.
> >
> > (1) Implicit unwrap semantics in pre-provisioned keys
> >
> > A pre-provisioned key with usage unwrap could be imbued with behaviors
> that
> > dictate the extractable and usage attributes of keys that it unwraps or
> even
> > that imbue the unwrapped keys with other such properties. The former
> would
> > be sufficient for "single step" key wrapping, where the final key to be
> used
> > for encryption, decryption, signature or signature verification is
> wrapped
> > directly with the pre-provisioned key. The special property of the
> > pre-provisioned key ensures that the final key has extractable = false.
> >
> > If you want to have two steps, for example the key you are transferring
> is
> > encrypted using a temporary Content Encryption Key (as in JWE) and then
> this
> > CEK is wrapped using the pre-provisioned key, then you not only need the
> > pre-provisioned key to force extractable = false and usage = unwrap on
> the
> > CEK, but it must also transfer a special property to the CEK, so that
> when
> > this in turn is used for unwrapping the resultant key always has
> extractable
> > = false.
>
> Correct. The "Named Pre-provisioned keys" is already imbued with
> special properties by definition, so this is consistent.
>
> JWK is not unique in this 'two step' form - consider multi-party
> RSA-KEM - you have the RSA key, the derived per-party KEK, and the
> shared, protected key.
>
> >
> > (2) Explicit attributes on wrapping keys
> >
> > A key with usage "unwrap" also has properties which dictate the
> attributes
> > of keys that it unwraps. Let's call these properties "unwrap-extractable"
> > and "unwrap-usages". Whenever a key, W, is used to perform an unwrap
> > operation, the unwrapped key, K, gets it's attributes set as follows:
> >
> > K.extractable = W.unwrap-extractable
> > K.usages = W.unwrap-usages
> >
> > Again, this is sufficient for single-step unwrapping. When the wrapping
> key
> > W is generated, the unwrap-extractable and unwrap-usages properties are
> set
> > to 'false' and the intended usages of the expected wrapped key,
> > respectively, When it comes to unwrapping the unwrapped key, K, gets the
> > appropriate properties.
>
> Correct.
>
> This matches PKCS#11's CKA_WRAP_TEMPLATE and CKA_UNWRAP_TEMPLATE
> properties, for which the smart card and secure element industry have
> long since embraced as sufficient for a variety of high-security needs
> (eg: eID cards, as a number of members have pointed out)
>
> >
> > However, if the intended usage of the key K is also for unwrapping (as in
> > the two-step key wrapping described above), we need a way to set
> > K.unwrap-extractable and K.unwrap-usages.
> >
> > Theoretically, we could go down the path of having unwrap-extractable and
> > unwrap-usages each be an array, popping the first value on each unwrap
> > operation, i.e.
> >
> > K.extractable = W.unwrap-extractable[ 0 ]
> > K.usages = W.unwrap-usages[ 0 ]
> > K.unwrap-extractable = W.unwrap-extractable[ 1 : ]
> > K.unwrap-usages = W.unwrap-usages[ 1 : ]
> >
> > (using python-like slice notation)
> >
> > It may not be necessary to explicitly expose these attributes on the Key
> > object: it may be sufficient to have them settable at key creation time.
> >
> > The other option is to have the extractable and usage attributes carried
> > securely with the wrapped key, as I have proposed.
>
> Note: This solution ONLY works with JWE-protected-JWK keys - it does
> not and cannot work with 'raw' or 'pkcs8'/spki. The smart card / HSM /
> SE industry certainly seems to recognize that mixing/matching as you
> propose only really works in an implementation-specific manner - see
> the CKM_SEAL_KEY proposal in the OASIS TC to see how the very nature
> of 'opaque' key blobs is left up to implementations because of this.
>
> You missed the third option though - which is that the (JavaScript)
> caller specifies the policy.
>

As you explain below, that's not an option that maintains the
extractability functionality. In this mail, I was exploring options which
do that.


>
> If I can sum up the discussion so far, the two objections against this
> last point (eg: what is currently specified) are:
> 1) It allows end-users to manipulate variables (eg: in the Javascript
> console) to circumvent this
> 2) In the event of an XSS, an attacker can unwrap a key and set
> extractable to false.
>   2.1) The first attack requires the attacker has previously observed
> a wrapped key in transit (eg: MITM) before an XSS, then later XSSes
> and replays the original key with 'extractable' as true.
>   2.2) The second attack requires the attacker have XSSed the site,
> the server send a wrapped key, and the XSS change 'extractable' to
> true.
>
> I see #1 as an explicit non-goal for a general web spec - it's a
> feature, not a bug.
>

I don't see it as consistent with the existing extractable attribute
though. We should be consistent. Following your approach, we should remove
the extractable attribute (not that I am proposing this).


> #2.1 can (and should) be mitigated via HTTPS and related.
> #2.2 can (and should) be mitigated via CSP and related.
>

There are many ways in which the Javascript running on the users machine
may not be the Javascript that either the user or the service provider
expects. The extractability attribute provides some protection against such
scripts obtaining the raw keying material once it has been installed,
provided the browser itself is not compromised. We're not in a position to
do security engineering for every possible application here, we're
providing tools and extractability is a useful one.

Given the above, it's completely reasonable to want to maintain this
property with wrapped keys.


>
> Finally, the Structured Clonability of Key objects permits other
> creative uses that have strong parallels to existing software such as
> middleware, for example, by having a 'trusted' origin perform the
> unwrapping, and then postMessaging() to the untrusted origin (which,
> for example, may not be able to support strict CSP policies), while
> still preserving attributes.
>

Sure, but you are making a bunch of assumptions or imposing a bunch of
constraints on how applications are designed. What I can say is that for
our application, this wouldn't work. Our security analysis suggests that we
should in all cases attach a different level of trust to the Javascript
code than we do to the browser code. Both can be compromised, of course,
but the ways in which the Javascript can be attacked are more numerous and
varied.

...Mark