[Ntp] Antw: Re: [EXT] Danny's Review (was Re: draft‑ietf‑ntp‑roughtime‑05: tag change makes implementation more complex)

Ulrich Windl <Ulrich.Windl@rz.uni-regensburg.de> Tue, 05 October 2021 08:29 UTC

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Date: Tue, 05 Oct 2021 10:29:22 +0200
From: "Ulrich Windl" <Ulrich.Windl@rz.uni-regensburg.de>
To: <watsonbladd@gmail.com>
Cc: "ntp@ietf.org" <ntp@ietf.org>
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>>> Watson Ladd <watsonbladd@gmail.com> schrieb am 05.10.2021 um 04:16 in
Nachricht
<CACsn0c=mDN6-tb=sP60gDgn6XvypxegWaVNd5yFASO74VHwVGA@mail.gmail.com>om>:
> On Mon, Oct 4, 2021 at 12:28 AM Ulrich Windl
> <Ulrich.Windl@rz.uni-regensburg.de> wrote:
>>
>> >>> Watson Ladd <watsonbladd@gmail.com> schrieb am 28.09.2021 um 03:48 in
>> Nachricht
>> <CACsn0cmks2fdwem1rS+QNzCL1WhNR4890Fi1zpjQrL=E3Y=3fQ@mail.gmail.com>om>:
>> ...
>> >> 9. Hashing algorithm in Section 6.3
>> >>
>> >> It's not clear why the algorithm can only be SHA‑512. This really
should
>> >> be separately referenced so that if SHA‑512 is considered too week in
>> >> can be replaced by a different hashing algorithm like say HMAC‑SHA‑512,
>> >> without having to update the proposed RFC.
>> >
>> > Part of what is required for roughtime is interoperability between all
>> > servers, auditors, and clients. Using only a single hash function and
>> > signature, and then migrating entire versions, is a better approach
>> > than permitting everything.
>>
>> Why not specify that the only supported hash algorithm for this protocol
>> version is SHA-512, while later protocol version might require or support
>> different algorithms?
> 
> I'm happy to add that sentence: it means nothing. Future protocols can
> change a lot of things.
> 
>> For completeness the used/expected hash algorithm could indicated by a tag
>> even in this version, maybe providing a mechanism how to allow/require
>> different algorithms in a later version.
>> That design would avoid some bad compatibility designs in a later version
>> (like NTP's "famous" extension fields).
> 
> I'm not sure why the hash function is so worthy of singling out here,
> perhaps you can explain more. The intention is every version has its
> own single signature algorithm.

If that is part of the design, say so. However having to start a new
incompatible protocol version just to replace a hash algorithm is bad design
IMHO.
Imagine SSL/TLS had a new or different version for every hash algorithm that
is supported.

Regards,
Ulrich

> Sincerely,
> Watson Ladd
>>
>> ...
>>
>> Regards,
>> Ulrich Windl
>>
>>
> 
> 
> -- 
> Astra mortemque praestare gradatim