Re: On Crypto API Safety in the Hands of Unskilled Developers

Mark,

Richard's points about defaults gave been repeatedly discussed in the group
and numerous examples provided for how they fundamentally represent bad API
design for a low-level API, but are very much a part of a high-level API.

Further, it has also been discussed with how the argument that crypto is
somehow "special" or inherently more dangerous than other APIs is a
fundamentally flawed assertion. The ability for CSRF, cookie leaking, or
XSS all represent vastly more serious and real threats than bad
cryptography.

Developers can write extremely poor performant WebGL, but you don't see us
discussing VRML as the solution. Developers can write absolutely horribly
performing CSS, but you don't see us discussing ways to provide a default
set of "site templates," ala GeoCities of yore, to have code blessed by the
design priesthood.

The argument that Crypto can or is used to implement "security," a
fundamentally amorphous term that van mean completely contradictory things
depending on personal views (e.g. DRM) is irrelevant to the discussion of
the API - but it makes up the fundamental argument of Richard's position.

There is unfortunately nothing here that has not already been discussed at
length, and with numerous examea provided as to its unsuitablity for a
low-level API.

Discussions of parameters and serialization are EXACTLY the issue that a
high-level API tries to solve; when you say 'let's combine primitives and
serialization/persistence', you are no longer discussing a low-level API.

I did not say Richard's points were wrong - in the context of a high-level
API, they're an imminently sensible reminder of objectives. But in the
context of a low-level API, but especially in the context of this group,
its a retread of points not applicable.
On Mar 28, 2013 8:42 AM, "Mark Watson" <watsonm@netflix.com> wrote:

> Ryan,
>
> I thought Richard's piece was well written, made some good points and was
> clearly targeted at the low-level API on which we are working. His
> suggested remediation for the problems is straightforward. IMHO, at the
> very least, it deserves a more considered response.
>
> ...Mark
>
> On Thu, Mar 28, 2013 at 8:39 AM, Ryan Sleevi <sleevi@google.com> wrote:
>
>> Thank you for your description of a high-level API.
>>
>> At this time, the WG is pursuing a low-level API.
>> On Mar 28, 2013 8:21 AM, "Richard Barnes" <rbarnes@bbn.com> wrote:
>>
>>> The SubtleCrypto thread reminded me that I'd been meaning to send out
>>> some notes I wrote down about unskilled developers.
>>>
>>> Brief essay follows.  Comments welcome.
>>>
>>> --Richard
>>>
>>>
>>>
>>> On Crypto API Safety in the Hands of Unskilled Developers
>>> =========================================================
>>>
>>> I. What is the problem (general)?
>>> ---------------------------------
>>>
>>> The current API's approach of exposing unmitigated complexity to the
>>> developer -- no defaults, no help from the browser -- is only plausible if
>>> we assume that the only people who will use the API are experienced
>>> cryptographers.  This assumption is clearly not true.  Any API that is
>>> supplied in the DOM will be exposed to, and get used by, a much wider
>>> variety of developers than we ever intend.
>>>
>>> That's true of any DOM API, whether it's crypto, geolocation, canvas,
>>> etc.  But crypto is special.
>>>
>>> -- Bad crypto design leads to worse consequences
>>> -- Bad crypto design is hard to detect
>>>
>>> The whole point of having a crypto API is to protect sensitive things.
>>>  So by definition, if you screw up your usage of the crypto API, you are
>>> exposing sensitive things.  Moreover, if this happens, you are likely not
>>> to notice it.  If you screw up your WebGL rendering code, things will look
>>> bad.  If you re-use the same nonce twice in GCM, nothing is obviously
>>> different.
>>>
>>> So in its current state, the API makes it likely for bad things to
>>> happen.  It would be irresponsible of this group to release an API in this
>>> state.  We need to think seriously about how to make the default mode of
>>> the API less likely to lead to pain, while still allowing for full
>>> generality.
>>>
>>> Think of this like consumer protection.  You can't ship a lawn mower
>>> that doesn't have a guard around the blade.  Someone can buy a lawn mower,
>>> take off the guard, and use the motor and blade in new and creative ways,
>>> at the risk of injuring himself.  Even if someone isn't doing something
>>> advanced, they can still stick their hand under the guard and get cut.  But
>>> by default, in most use cases, the lawn mower is safe to use.
>>>
>>>
>>>
>>> II. What is the problem (specific)?
>>> -----------------------------------
>>>
>>> Conceptually, there are two classes of CryptoOperation: "Plain to
>>> ciphertext" operations that convert plaintext to data with cryptographic
>>> structure, and "Cipher to plaintext" operations that do the reverse.
>>>
>>> P2C       C2P
>>> -----------------
>>> sign      verify
>>> encrypt   decrypt
>>> digest
>>>
>>> The difference is this: P2C operations can meaningfully be done with
>>> many different choices of parameters.  C2P operations can only be done with
>>> a specific set of parameters.
>>>
>>> Both of these create problems for developers.
>>>
>>> For P2C operations, the developer must choose how to set multiple
>>> parameters, choices that are likely not obvious to someone not skilled in
>>> the art.
>>> For C2P operations, the developer needs to make sure that they keep all
>>> the relevant parameters together with protected information.
>>>
>>> So we have two problems:
>>> P2C: How to help developers make good choices
>>> C2P: How to help developers keep ciphertext associated to parameters
>>>
>>>
>>>
>>> III. What would a solution look like?
>>> -------------------------------------
>>>
>>> On the face of it, the P2C problem -- choosing parameters -- seems easy
>>> to solve.  If there are multiple valid sets of parameters, just have the
>>> browser / API implementation make the choice on behalf of the developer.
>>>
>>> However, this exacerbates the C2P problem, because there are now many
>>> ways for the ciphertext to be separated from its parameters.  If a web app
>>> does not store the parameters with which the ciphertext was computed
>>> (relying on the browser's defaults), then if the browser changes defaults,
>>> then the app will be unable to decrypt the ciphertext (or validate the
>>> signature).  Even if the app stores the parameters, then it needs to make
>>> sure that the ciphertext is always associated with the correct parameters;
>>> the app cannot, for example, send the ciphertext for storage on a server,
>>> but not the parameters.
>>>
>>> So in order to solve the P2C problem, we also need to solve the C2P
>>> problem.  Namely, we need to make it easy by default for apps to keep
>>> parameters and ciphertext together.  In API terms, that would seem to
>>> indicate that the results of a crypto operation should be provided as an
>>> object that contains all the relevant parameters (as indeed,
>>> CryptoOperation already does).  In addition, it would be helpful if this
>>> object had a default serialization, to address the issue of parameters
>>> getting lost when the object is stored or sent someplace else.
>>>
>>> This gives us two solutions to match the two problems:
>>> P2C: Provide browser-chosen defaults
>>> C2P: Provide results in an object with parameters and a serialization
>>>
>>> These don't prevent developers from running into problems -- choosing
>>> bad IVs, or deleting default parameters from the object -- but it
>>> encourages a default life-cycle that should be problem free:
>>> * Process plaintext, get ciphertext+parameters
>>> * Store ciphertext+parameters
>>> * Process ciphertext+parameters, get plaintext
>>>
>>> These solutions also donít get in the way of more advanced developers.
>>>  You can still specify all the parameters, and still use whatever parts of
>>> the object you want.
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>

Received on Thursday, 28 March 2013 15:52:48 UTC