Re: AES CFB input parameter "s"

Ok, so I guess my question is what should we do in our specification ? At
least the version we put out for public comment.

Support only CFB-8, or let the caller choose in the AesCfbParams between 8,
64 and 128 ?

...Mark

Sent from my iPhone

On Feb 14, 2014, at 5:26 PM, Ryan Sleevi <sleevi@google.com> wrote:

If I were listing priorities of utility for applications and consistency of
implementation, 8, 128, 64.

I don't see value in arbitrary values of s, nor are there any
implementations that support it.
On Feb 14, 2014 5:20 PM, "Mark Watson" <watsonm@netflix.com> wrote:

> You mean support only CFB8 or support only CFB128 ?
>
> ...Mark
>
>
> On Fri, Feb 14, 2014 at 5:17 PM, Ryan Sleevi <sleevi@google.com> wrote:
>
>> I'm inclined to say a over b - outside of 8/128, usage is quite rare.
>>
>> It was added for the 8 case (GPG, IIRC)
>> On Feb 14, 2014 5:07 PM, "Mark Watson" <watsonm@netflix.com> wrote:
>>
>>> Ok, so the options for our specification are:
>>> (a) choose a fixed value e.g. support only CFB128
>>> (b) allow the caller to specify the value
>>> (c) remove the algorithm from the specification
>>>
>>> It seems from what you say that it is not the case that everyone
>>> supports just one value - so there must be applications for the different
>>> values. So, that would suggest we choose (b) if we don't choose (c).
>>>
>>> ...Mark
>>>
>>>
>>> On Fri, Feb 14, 2014 at 3:16 PM, Ryan Sleevi <sleevi@google.com> wrote:
>>>
>>>> I haven't heard any implementation movement on CFB.
>>>>
>>>> If using PKCS#11, only 8-bit, 64-bit, and 128-bit are defined
>>>> mechanisms. SP800-38a only provides vectors for 1, 8, 128.
>>>>
>>>> AFAICT, CNG supports CFB8 and CFB128 with AES. CFB128 isn't until Win8.
>>>>
>>>> OS X only supports CFB8 from what I can tell (at least, via the
>>>> CommonCrypto interface being used to implement in WebKit)
>>>> On Feb 14, 2014 3:04 PM, "Mark Watson" <watsonm@netflix.com> wrote:
>>>>
>>>>> All,
>>>>>
>>>>> Apologies in advance if this has been discussed before, or has a
>>>>> well-known answer.
>>>>>
>>>>> According to NIST SP800-38A [1] Section 6.3 which we reference for AES
>>>>> CFB, the input parameters to the encrypt/decrypt operations consist of an
>>>>> IV, the plaintext / ciphertext (respectively) and an additional parameter,
>>>>> s, which takes a value between 1 and 128, inclusive.
>>>>>
>>>>> What values of this shall we support in WebCrypto ?
>>>>>
>>>>> I assume that if we wish to support more than one value, then we must
>>>>> add a property to AesCfbParams for this.
>>>>>
>>>>> ...Mark
>>>>>
>>>>> [1] http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf
>>>>>
>>>>
>>>
>