Re: [TLS] Length of a variable-length vector: Could it be an odd multiple?

[Benjamin Kaduk <[e-mail]>]

On 01/22/2016 03:26 PM, =JeffH wrote:
> On 01/22/2016 12:29 PM, =JeffH wrote:
>> [ fixed pitch font advised here ]
>
> the below is corrected to use "byte count" rather than "index" or
> "indicies" (and to ditch the tabs)..
>
>
> > On 01/22/2016 09:42 AM, =JeffH wrote:
> > > [ resending from different account - my work addr ends up in spam
> > > bucket for many it seems ]
> > >
> > > On 1/20/16, 11:01 AM, "Benjamin Kaduk" <[e-mail]> wrote:
> > > >On 01/20/2016 12:47 PM, Hodges, Jeff wrote:
> > > >> On 1/13/16, 12:53 PM, "Benjamin Kaduk" <[e-mail]> wrote:
> > > >>> On 01/13/2016 02:44 PM, Jong-Shian Wu wrote:
> > > >>>> I have a question about the even-vs-odd restrictions on the
> > > length of
> > > >>>> a valid variable-length vector defined in TLS specification
> after
> > > >>>> reading the section 4.3 of RFC 5246 [1] which states that:
> > > >>>> "The length of an encoded vector must be an even multiple of the
> > > >>>>length
> > > >>>> of a single element (for example, a 17-byte vector of uint16
> > > would be
> > > >>>> illegal)."
> > > >>>>
> > > >>> It means "whole-number" as opposed to fractional, i.e., there
> should
> > > >>>not
> > > >>> be unused "junk bytes" at the end.
> > > >> In case it's helpful, here's a suggested re-write of that quoted
> > > >>sentence
> > > >> above..
> > > >>
> > > >>   The length of an encoded variable-length vector must be an
> > > >>   exact multiple of the length of a single element. For example,
> > > >>   an encoded 17-byte vector of uint16 would be illegal, and an
> > > >>   encoded variable-length vector of four 32 byte elements,
> > > >>   having a ceiling of 2^16-1, will be 130 bytes long overall
> > > >>   (2 byte length field followed by 128 bytes of data).
> > > >
> > > >Wouldn't the ceiling more properly be 2^16-4 in that case?
> > >
> > > hm, I'm not sure -- what would be the rationale?  The exact multiple
> > > criteria?  but 2^16 / 32 = 2048  while  (2^16-4) / 32 = 2047.875
> >
> > Ah, I seem to have conflated bits and bytes due to reading too quickly
> > and should have said (2^16-32), as Ilari alluded to with "or rounding
> > thereof to integral multiple of
> > elements".
>
> hm, but in this case it seems that a variable-length vector declared
> with a length range of <0..2^16-1> would exactly accommodate up to
> 2048 32-byte elements..
>
>   opaque     Foo[32] ;
>
>   Foo        fooSequence<0..2^16-1>; /* will accommodate up to 2048
>                                         Foo instances */
>
> ..because it has a zero-based byte count, as in this example..
>
>
>   opaque Array<0..2^2-1> ;  /* should accommodate
>                                2^2 = 4  1-byte elements */
>
>   /*
>   Array with 4
>   elements in memory:  [ xx xx xx xx ]
>   byte count (hex):       0  1  2  3        3 = 2^2-1
>
>   byte count (binary):   00 01 10 11
>
>   */
>
>
> ..yes?  or am I missing something?

You are missing something.

The encoded length represents the actual number of bytes that will
follow, so that a zero-length array with maximum length 2^16-1 is
encoded as just 00 00.  So, even though the index into the array starts
at zero, the actual length "starts at" 1, just like in C.

-Ben