Re: [hybi] It's time to ship

On Thu, Jan 13, 2011 at 06:15, Adam Barth <ietf@adambarth.com> wrote:

> On Wed, Jan 12, 2011 at 12:28 PM, Bjoern Hoehrmann <derhoermi@gmx.net>
> wrote:
> > * Adam Barth wrote:
> >>http://www.ietf.org/id/draft-abarth-thewebsocketprotocol-01.txt
> >
> > This says:
> >
> >  The masked-data is the clear-text frame encrypted under AES-128-CTR
> >  (see [TODO: Cite AES-128-CTR]) using the masking-key as the key and
> >  the initial counter value equal to the masking-nonce followed by 28
> >  zero octets.
> >
> >  For example, octet i of the masked-data is computed from octet i of
> >  the clear-text frame as follows:
> >
> >    initial-counter = masking-nonce << 96
> >    masked-octet-i = clear-text-octet-i XOR AES_k(initial-counter + i)
> >
> >  where AES_k is AES keyed with the masking-key.
> >
> > Could you give a reference for AES-128-CTR and explain the example?
>
> Here's an example RFC that uses AES-128-CTR
>
> http://www.ietf.org/rfc/rfc4344.txt
>
> I believe it's technically defined by combining
> http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf and
> http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf, the
> later of which contains test vectors.  (Careful readers will notice
> that my example is wrong.)
>
>
So, this part can be rewritten like this, I thought. do you think it's
correct?

----
...[snip]
   the initial counter value equal to the masking-nonce followed by 12

...[snip]

  initial-counter = masking-nonce << 96
  for i = 0 to size-of-clear-text - 1
    j = i MOD 16
    if j == 0
      frame-key = AES_k(initial-counter + i / 16)

    masked-octet-i = clear-text-octet-i XOR octet-j-of-frame-key
----

and, since masking-key is 20 octet length, it looks like we must
truncated/padded key to fit AES key size?

> Adam
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