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

Benjamin Kaduk <bkaduk@akamai.com> Fri, 22 January 2016 18:48 UTC

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To: =JeffH <Jeff.Hodges@KingsMountain.com>, IETF TLS WG <tls@ietf.org>
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From: Benjamin Kaduk <bkaduk@akamai.com>
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Date: Fri, 22 Jan 2016 12:48:20 -0600
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Subject: Re: [TLS] Length of a variable-length vector: Could it be an odd multiple?
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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" <bkaduk@akamai.com>; wrote:
> >On 01/20/2016 12:47 PM, Hodges, Jeff wrote:
> >> On 1/13/16, 12:53 PM, "Benjamin Kaduk" <bkaduk@akamai.com>; 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".

Sorry for the confusion.

-Ben