RE: Use Cases | ACTION-13 Revisited [and latest API draft comments]

Ryan,
Yes, you are correct... The use case in 2.2 does cover both examples.

One level lower, however, my two use cases tried to break out how (in the first use case) the user's out-of-band private key must somehow be extracted from an external source, and/or how (in the second use case) the external device's own cryptographic functions (which are associated with the out-of-band) key are used to unwrap and decrypt the message.

This is related to other discussion threads today on key discovery. Since keys are internally bound to a cryptographic provider or module, how can we ensure that a particular out-of-band key is bound to a SUPPORTED cryptographic provider? My second use case shows the need to allow external cryptographic functions.



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-----Original Message-----
From: Ryan Sleevi [sleevi@google.com<mailto:sleevi@google.com>]
Sent: Monday, August 27, 2012 02:13 PM Eastern Standard Time
To: Davenport, James L.
Cc: Vijay Bharadwaj; Arun Ranganathan; public-webcrypto@w3.org; estark@mit.edu
Subject: Re: Use Cases | ACTION-13 Revisited [and latest API draft comments]


On Mon, Aug 27, 2012 at 6:11 AM, Davenport, James L. <jdavenpo@mitre.org> wrote:
> On the draft API, section 2 "Use Cases," I would also still like to see the previously mentioned out-"of-band provisioned" keys in the use cases:
>
>
> A national agency uploads each citizen's benefits summary report to the cloud. For privacy and security reasons, the reports are stored in an encrypted form. This national agency announces on their web site the availability of the reports. Each citizen can then open a browser, go to the web site for this national agency and log on, which takes them to their encrypted report. The encrypted report is fetched by the browser, along with some HTML and JavaScript.  The JavaScript requests the Crypto API to decrypt the report using an out-of-band provisioned key. The API returns the decrypted report and the JavaScript then inserts it into the HTML, which is then displayed on the browser screen.
>
> A financial brokerage firm generates and stores quarterly reports for each of its members to the cloud. For privacy and security reasons, the reports are stored in an encrypted form. Each member can then open a browser, go to the web site for the financial brokerage firm and log on, which takes them to their encrypted quarterly report. The encrypted report is fetched by the browser, along with some HTML and JavaScript.  The JavaScript requests the Crypto API to decrypt the report using the external decryption capabilities of the external device, using this device's out-of-band provisioned key. The API returns the decrypted report and the JavaScript then inserts it into the HTML, which is then displayed on the browser screen.
>
> --JD

James,

Apologies if I've misread, but are both examples you provide different
in the capabilities they require from the API, or are they simply
differences in how the API might be consumed?

Further, aren't these both examples of the currently documented
"Protected document exchange" use case (currently section 2.2)? Is
there a capability or concept required in these use cases that is not
documented in the existing use case?

>
> -----Original Message-----
> From: Vijay Bharadwaj [mailto:Vijay.Bharadwaj@microsoft.com]
> Sent: Monday, August 27, 2012 5:55 AM
> To: Arun Ranganathan; Ryan Sleevi
> Cc: public-webcrypto@w3.org; estark@mit.edu
> Subject: RE: Use Cases | ACTION-13 Revisited
>
> Perhaps there is a case for locally encrypted content when you combine it with a secure token.
>
> Take for example a web app that stores its local data encrypted to a smart card (provisioned out of band, like we have been assuming all trusted smart cards are). Then while the app is vulnerable if it is used after the user agent is compromised, at least it raises the bar by requiring the attacker to do a two-touch attack. An attacker who just compromises the user agent cannot decrypt the locally stored data, because the user agent itself cannot decrypt it without the token.
>
> To be more specific:
>
> Use case: encrypted local storage
>
> When caching sensitive data locally, an application may wish to ensure that this data cannot be compromised in an offline attack. In such a case, the application may leverage a key stored on a secure token distributed out of band (such as a smart card) to encrypt the local cache. Thus, the cache may only be decrypted by the application when the secure token is present; at other times (such as when an attacker has stolen the machine) the local cache is inaccessible and all operations will require online authentication to the application's web service.
>
> -----Original Message-----
> From: Arun Ranganathan [mailto:arun@mozilla.com]
> Sent: Friday, August 17, 2012 7:57 AM
> To: Ryan Sleevi
> Cc: public-webcrypto@w3.org; estark@mit.edu
> Subject: Re: Use Cases | ACTION-13 Revisited
>
> Ryan,
>
>
> On Aug 16, 2012, at 7:16 PM, Ryan Sleevi wrote:
>
>> On Thu, Aug 16, 2012 at 3:55 PM, Arun Ranganathan <arun@mozilla.com> wrote:
>>> While working through the use cases (per [ACTION-13]) with Wan-Teh
>>> (wtc), we came up with the following:
>>>
>
> <snip/>
>
>>> 1. The use cases rsleevi added to the draft [spec] are pretty solid;
>>> they are only missing a "local storage" scenario, first mentioned on
>>> the Wiki [cf. local].
>>> [cf. local]
>>> http://www.w3.org/community/webcryptoapi/wiki/Use_Cases#Storing_local

>>> _storage
>
>
>> I'm a little concerned about the "local storage" case, and wondering
>> whether it's something that would necessarily be in scope for this
>> group.
>>
>
>> Consider the example of IndexedDB, which uses "Keys" (IDB keys -
>> http://www.w3.org/TR/IndexedDB/#key-construct ) and returns "Values" (
>> http://www.w3.org/TR/IndexedDB/#value-construct ), and can
>> alternatively be accessed via indices (
>> http://www.w3.org/TR/IndexedDB/#index-concept ).
>>
>
>> A naieve assumption would be that this API would only protect the
>> Values - not the keys, nor the indices. However, as practically
>> deployed today, that wouldn't offer much protection, since both Keys
>> and Indices often reveal quite a bit of information.
>>
>> Further, by ciphering contents, it's a tradeoff between efficiency and
>> privacy. Perfect privacy (storing no relationships about keys/indices,
>> everything randomly distributed) is the worst efficiency, while
>> perfect efficiency (which is what is afforded by today's IndexedDB)
>> has no privacy/cryptography.
>>
>> A refinement might be to have the IndexedDB actually take a Key
>> (Crypto API key), that it can use to protect however the IndexedDB is
>> stored - keys, indices, everything. Call it an "EncryptedIndexedDB".
>> This is better, in that it allows the user agent to decrypt on the fly
>> (see caveat), and allows applications to use existing indices/keys.
>> The caveat, however, is that encryption requires defining an
>> encryption algorithm, and the choice of encryption algorithm directly
>> affects the efficiency of the API. For example, under today's
>> IndexedDB, a user agent can load data on the fly (eg: from disk), but
>> under EncryptedIndexedDB with say, a block cipher alg like AES, it
>> might have to read the entire DB into memory, then decrypt, in order
>> to be able to offer this functionality.
>>
>> Even more fundamentally though, is the question about what attack this
>> is trying to defend against. The arguments I've heard for encrypted
>> local storage seem to be about a remote server, serving a web
>> application, distrusting the client platform. If that's the case, it
>> doesn't seem like any level of cryptography will save them. As I noted
>> in the existing security considerations, it SHOULD be perfectly valid
>> for a user agent to store a key in plaintext on disk, so what actual
>> protections are afforded by this?
>
>
> You're right -- if the use case is primarily about an untrusted multi-user machine or virtual computing environment, we're only as safe as general user safety anyway.  This doesn't seem to be a use case we can salvage, nor one that should influence the API.  We should probably not include it.
>
> But:
>
>>
>> If something like EncryptedIndexedDB is what is meant here, then this
>> seems like something that would likely live in the Web Apps WG (since
>> it's about extending IndexedDB).
>>
>
> Maybe -- I doubt it's worth their while to solve for that use case either :).  Interestingly enough (and not to confuse matters, but) we've just heard from Facebook [FB-ScriptSigning] about localStorage (or IndexedDB) used as a script cache.  People are already using IndexedDB and localStorage in unsafe-ish ways.  Of course, we shouldn't confuse script signing with a general use case for protected/encrypted local storage, but perhaps if we jettison the "protected local storage" use case, we can bolster the "document signing" use case to explicitly refer to documents extracted from local storage for signature verification.
>
> This raises the sticky issue of types of documents.  We might naively say that a script is no ordinary document, and can be used by the relevant JSON primitive if it passes signature validity.
>
> In a nutshell, I'm saying: perhaps we cannot cater to an encrypted local store use case, but we may be able to flesh out the use case for signature verification, including extraction from local storage.  Our use cases should encourage patterns of behavior that we think are desirable.  We can't control or solve for undesirable patterns of behavior :)
>
>
>> I just want to make sure that we're carefully considering the use case
>> and the security implications before committing to them, as well as to
>> figure out what parts of the spec may need to change in order to
>> meaningfully implement them.
>
>
> +1.
>
> -- A*
>
> [FB-ScriptSigning] http://lists.w3.org/Archives/Public/public-webcrypto/2012Aug/0121.html

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