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

Benjamin Kaduk <bkaduk@akamai.com> Fri, 22 January 2016 21:53 UTC

Return-Path: <bkaduk@akamai.com>
X-Original-To: tls@ietfa.amsl.com
Delivered-To: tls@ietfa.amsl.com
Received: from localhost (ietfa.amsl.com [127.0.0.1]) by ietfa.amsl.com (Postfix) with ESMTP id B9F991A8A4F for <tls@ietfa.amsl.com>; Fri, 22 Jan 2016 13:53:31 -0800 (PST)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: -2.702
X-Spam-Level:
X-Spam-Status: No, score=-2.702 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RP_MATCHES_RCVD=-0.001, SPF_PASS=-0.001] autolearn=ham
Received: from mail.ietf.org ([4.31.198.44]) by localhost (ietfa.amsl.com [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id XnGj_-y8OPg5 for <tls@ietfa.amsl.com>; Fri, 22 Jan 2016 13:53:29 -0800 (PST)
Received: from prod-mail-xrelay05.akamai.com (prod-mail-xrelay05.akamai.com [23.79.238.179]) by ietfa.amsl.com (Postfix) with ESMTP id C4D2E1A8A4E for <tls@ietf.org>; Fri, 22 Jan 2016 13:53:29 -0800 (PST)
Received: from prod-mail-xrelay05.akamai.com (localhost.localdomain [127.0.0.1]) by postfix.imss70 (Postfix) with ESMTP id DF0013F401F; Fri, 22 Jan 2016 21:53:27 +0000 (GMT)
Received: from prod-mail-relay11.akamai.com (prod-mail-relay11.akamai.com [172.27.118.250]) by prod-mail-xrelay05.akamai.com (Postfix) with ESMTP id BF5463F4014; Fri, 22 Jan 2016 21:53:27 +0000 (GMT)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=akamai.com; s=a1; t=1453499607; bh=ryw910xajQKTucVROlurS/NHI5ZtnnbywKbVg0FjYAM=; l=3157; h=To:References:Cc:From:Date:In-Reply-To:From; b=FAnY6lzrZDevM3zeHp1Xtgdcnq70KB3gcxAFANcGWcQdXWrE2tp26rzH8FgPJMXY1 N2g8LW6PKSpUjG7wtMVmllePxBIAXLSrgpL3kTuHnio/UmFcpHYUOuEmyrsxcOu4S9 1YdpiBIumpBssmS2jLLRR2yUPzs2sBWeje2RTIG0=
Received: from [172.19.0.25] (bos-lpczi.kendall.corp.akamai.com [172.19.0.25]) by prod-mail-relay11.akamai.com (Postfix) with ESMTP id 92D0D202F; Fri, 22 Jan 2016 21:53:27 +0000 (GMT)
To: =JeffH <Jeff.Hodges@KingsMountain.com>
References: <56A2912F.6040100@KingsMountain.com> <56A29E9D.9030305@KingsMountain.com>
From: Benjamin Kaduk <bkaduk@akamai.com>
Message-ID: <56A2A4D7.9050308@akamai.com>
Date: Fri, 22 Jan 2016 15:53:27 -0600
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.5.1
MIME-Version: 1.0
In-Reply-To: <56A29E9D.9030305@KingsMountain.com>
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
Archived-At: <http://mailarchive.ietf.org/arch/msg/tls/02fuZnoa0S76_jyUNITVbP7zXVU>
Cc: IETF TLS WG <tls@ietf.org>
Subject: Re: [TLS] Length of a variable-length vector: Could it be an odd multiple?
X-BeenThere: tls@ietf.org
X-Mailman-Version: 2.1.15
Precedence: list
List-Id: "This is the mailing list for the Transport Layer Security working group of the IETF." <tls.ietf.org>
List-Unsubscribe: <https://www.ietf.org/mailman/options/tls>, <mailto:tls-request@ietf.org?subject=unsubscribe>
List-Archive: <https://mailarchive.ietf.org/arch/browse/tls/>
List-Post: <mailto:tls@ietf.org>
List-Help: <mailto:tls-request@ietf.org?subject=help>
List-Subscribe: <https://www.ietf.org/mailman/listinfo/tls>, <mailto:tls-request@ietf.org?subject=subscribe>
X-List-Received-Date: Fri, 22 Jan 2016 21:53:31 -0000

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" <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".
>
> 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