Re: AlgorithmIdentifier in encrypt/decrypt/sign/verify operations

On Fri, Mar 29, 2013 at 11:22 AM, Richard Barnes <rbarnes@bbn.com> wrote:

>
> On Mar 27, 2013, at 9:10 PM, Mark Watson <watsonm@netflix.com> wrote:
>
> > This may be related to ISSUE-12 and apologies again if this has been
> discussed before - it is coming up now frequently in implementation
> discussions.
> >
> > In the encrypt/decrypt/sign/verify operations, two AlgorithmIdentifier
> objects are provided as input, one as an explicit parameter and one which
> is associated with the Key object (and appears as the Key.algorithm
> attribute). Presumably it is an error if the "name" member of the
> dictionary does not match (after normalization), though I am not sure if
> this is clearly specified.
> >
> > In some cases, it is specified that the params member will have
> different types in these two places (I'm assuming that the Key.algorithm
> attribute takes the value that was provided to generateKey). For example
> for AES-CTR, the params in Key.algorithm contains the key length and params
> in encrypt/decrypt contains the IV.
> >
> > But for other cases things are very unclear. For example, for HMAC, the
> same AlgorithmParameters type is used, containing the hash algorithm. In
> this case it seems completely redundant to provide the same object twice to
> the sign/verify call (once in the method parameters and again in the
> Key.algorithm attribute).
> >
> > Am I missing something ? Does anyone else find this confusing ?
> >
> > I think the confusion could be resolved by
> > (i) replacing the AlgorithmIdentifier argument to
> sign/verify/encrypt/decrypt with AlgorithmParameters.
> > (ii) for HMAC, the params provided to sign/verify must be null, as the
> hash algorithm should have been provided when the Key was
> created/imported/unwrapped
>
> I agree that it could be made clearer.
>
> When I was implementing PolyCrypt, my read of the specification was as
> follows:
> 1. The algorithm provided as a parameter to encrypt() specifies the
> encryption (and parameters)
> 2. Throw an error if the Key.algorithm.name != algorithm.name
>
> That is, for the algorithm in the Key, everything besides the name is
> ignored.  This seems right to first order, but might be wrong, e.g., for
> HMAC, where you might want to compare the hash algorithm as well.
>

It seems ambiguous to me whether the hash algorithm is a property of the
key or a parameter to the operation. Another source of confusion.


>
> I'm leery of removing the algorithm parameter from encrypt(), if only
> because it seems really confusing and non-idiomatic.


What idiom, and why do you say it's confusing ? The algorithm is a property
of the key, so it's confusing that I need to re-specify it as a method
parameter. That only seems to introduce an unnecessary failure path and
give the incorrect impression to developer that they have some choice about
the algorithm here. They don't. It's implicit in the Key.


> I don't think it's terrible to have the algorithm specified in two places,
> as long as its clear how those two specifications relate to each other
>

It's not clear now. IIUC, anything in the method AlgorithmIdentifier that
is also a property of the Key must match. Anything else is a method
parameter.

A particularly confusing case is when there are both algorithm and method
parameters. For example, suppose a create an AES-CBC key with { name :
"AES-CBC", params: { length: 128 } }. Am I supposed to write

encrypt( { name: "AES-CBC", { "length" : 128, "iv" : iv } }, ... )

?

If I'm allowed to miss out the length here, why is it that it makes sense
to miss out some of the Key properties and not others (the algorithm name
itself) ?

This could be completely cleaned up by only specifying the "params" member
in the methods. Specifically by defining an OperationParameters for that
thing and derving the operation parameters objects from that. This would
provide a clear separation between algorithm and operation parameters.

...Mark


>
>
> > As a side note, I believe that to generate a HMAC key we need to specify
> the key length. At least according to FIPS 198-1 the key, K, can be of any
> length. So, either we require in WebCrypto that it is a particular length
> (say, the same size as the hash function), or we need a length parameter to
> generateKey for HMAC.
>
> +1 on adding a length parameter.
>
> --Richard
>
> >
> > ...Mark
>
>