Re: Non-extratability Tainting of Keys

On Mon, Sep 9, 2013 at 3:45 PM, Jim Schaad <ietf@augustcellars.com> wrote:

> Ok – I remember that as being discussed, but I did not think it was on the
> table as a solution and I believe that this is not a solution that solves
> the Netflix problem.****
>
> ** **
>
> The problem statement that I believe that Netflix has is:****
>
> ** **
>
> It is required that one can chain from one non-extractable key (KeyA) to a
> second non-extractable key (KeyB) in such a way as to prevent an active
> script attacker from obtaining the value of KeyB while being able to use
> KeyA for “normal” purposes.****
>
> ** **
>
> The solution of having an attribute in a JWK fails for the following
> reasons:****
>
> ** **
>
> **1.       ** You may not be able to wrap a JWK in an RSA key due to size
> constraints.  This solution only works if you are assuming that KeyA is a
> symmetric key and not if it is an asymmetric key (either RSA or ECDH).
>
If you use something like JWE-encapsulated-JWK, then the JWK is always
wrapped using a symmetric key and only the (symmetric) Content Master Key
is wrapped using the asymmetric key. I believe the numbers work out such
that the size constraints are not a problem.

Nevertheless, in this case you need the Content Master Key to also be a JWK
in order to carry its own attributes. (I'm assuming we use the existing
WebCrypto API where unwrap = decrypt + export and not the originally
proposed idea where JWE-encapsulated-JWK was natively supported as an
algorithm in its own right).

And you do need the usages in the JWK also to be respected, or for
encryption and wrap usages to be distinct algorithms (and I mean distinct
in that one cannot be performed using the other, like the difference
between AES-CBC and AES-KW, not just that they have different WebCrypto
names but do the same thing.)

This is also an example of an application where you unwrap a key which must
then only be usable for unwrapping.

Strictly, for our purposes, the constraint on JWK size that comes with an
asymmetric wrapping key is acceptable.


> ****
>
> **2.       **If the active script can capture the encrypted JWK which is
> being passed from the server to the client, then it can call the decrypt
> function on KeyA to decrypt the JWK and then parse the JWK to get KeyB
> independent of the presence of an attribute in the JWK string.
>
Yes, the usage on KeyA would include unwrap but not decrypt.

Then, in the case that KeyB is actually the Content Master Key for a
JWE-encapsulated-JWK, the usage attribute of KeyB would also indicate only
unwrap.

...Mark 




> ****
>
> ** **
>
> Jim****
>
> ** **
>
> ** **
>
> *From:* Ryan Sleevi [mailto:sleevi@google.com]
> *Sent:* Monday, September 09, 2013 11:57 AM
>
> *To:* Jim Schaad
> *Cc:* public-webcrypto@w3.org
> *Subject:* Re: Non-extratability Tainting of Keys****
>
> ** **
>
> ** **
>
> ** **
>
> On Sat, Sep 7, 2013 at 7:17 PM, Jim Schaad <ietf@augustcellars.com> wrote:
> ****
>
> If there is a solution to the non-extractability on the table, I have
> completely missed it.****
>
>  ****
>
> I was unable to come up with a good solution to the non-extractability
> problem that was not as draconian as the one that I proposed except for
> some very limited cases.  I would love to get a pointer to the Netflix
> solution so that I can see if I think it really works or if it is just a
> very small subset of what is needed.  Does the solution work just for the
> RSA key transport problem or does it work for the ECDH problem as well?***
> *
>
>  ****
>
> Jim****
>
>  ****
>
> ** **
>
> The Netflix solution: Only applies to JWK, through the introduction of a
> new attribute. Any JWKs unwrapped with this attribute supercede the
> attributes specified by the caller.****
>
> ** **
>
> My proposal: unextractability is specified by the caller.****
>
> ** **
>
> It's extremely important that you elaborate on your threat model when
> unwrapping keys as well as your expectations on what 'unextractable' means.
> In the discussions so far, it's in the context of "key provisioning", which
> is such a complex can of worms (see Anders' proposals to understand just
> *some* of what is commonly requested) that I would much rather not try to
> boil the ocean to solve that yet, and I'm convinced that the current
> Netflix proposal is NOT something that is web-extendable in the future when
> we DO try to solve those.****
>
> ** **
>
>  ****
>
>  ****
>
> *From:* Ryan Sleevi [mailto:sleevi@google.com]
> *Sent:* Friday, September 06, 2013 4:55 PM****
>
>
> *To:* Jim Schaad
> *Cc:* public-webcrypto@w3.org
> *Subject:* Re: Non-extratability Tainting of Keys****
>
>  ****
>
> Jim,****
>
>  ****
>
> Could you state then what problem you're trying to solve?****
>
>  ****
>
> We have solutions for non-extractability on the table. You've proposed yet
> another. The question is what set of problems you believe they solve that
> the other solutions do not solve or solve inadequately that required yet
> another proposal.****
>
>  ****
>
> That is, you enumerated your proposal, highlighting a number of
> disadvantages compared to any of the existing proposals, but unless I'm
> mistaken, you failed to highlight what (new) problems were being solved.**
> **
>
>  ****
>
> Given that Netflix has proposed at least one solution that they feel meets
> their needs in a much less restrictive manner, I'm curious what problem
> space you feel needs an even more restrictive API space (eg: one that
> prevents polyfills).****
>
>  ****
>
> Cheers****
>
>  ****
>
> On Fri, Sep 6, 2013 at 4:50 PM, Jim Schaad <ietf@augustcellars.com> wrote:
> ****
>
> 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****
>
>  ****
>
>  ****
>
> ** **
>