Re: [Cfrg] I-D Action: draft-irtf-cfrg-hash-to-curve-04.txt

"Blumenthal, Uri - 0553 - MITLL" <uri@ll.mit.edu> Tue, 16 July 2019 13:30 UTC

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From: "Blumenthal, Uri - 0553 - MITLL" <uri@ll.mit.edu>
To: =?utf-8?B?QmrDtnJuIEhhYXNl?= <bjoern.haase@endress.com>
CC: Dan Harkins <dharkins@lounge.org>, "cfrg@irtf.org" <cfrg@irtf.org>
Thread-Topic: [Cfrg] I-D Action: draft-irtf-cfrg-hash-to-curve-04.txt
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Date: Tue, 16 Jul 2019 13:30:43 +0000
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Subject: Re: [Cfrg] I-D Action: draft-irtf-cfrg-hash-to-curve-04.txt
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I am with Dan and Björn. Please add plain SWU back.

Regards,
Uri

Sent from my iPhone

> On Jul 16, 2019, at 09:18, Björn Haase <bjoern.haase@endress.com>; wrote:
> 
> Hi to all,
> 
> One of the clear advantages of using the less-efficient "plain" SWU method with the three calculations of the Legendre symbol as defined in section
> 
> "5.3.2.  Shallue-Woestijne-Ulas Method"
> 
> of the older draft -03 is that we keep safely away from the patents. Yes, its less efficient, but frankly speaking, in my opinion people really concerned about efficiency should use Montgomery or (twisted) Edwards curves anyway (and use Elligator2) and likely for most real-world-protocols the mapping will be not the bottleneck but rather the scalar multiplications.
> 
> This might be a reason to stick with plain SWU or at least include plain SWU as an option in the RFC.
> 
> Björn.
> 
> 
> 
> 
> 
> Mit freundlichen Grüßen I Best Regards 
> 
> Dr. Björn Haase 
> 
> Senior Expert Electronics | TGREH Electronics Hardware
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> -----Ursprüngliche Nachricht-----
> Von: Cfrg <cfrg-bounces@irtf.org>; Im Auftrag von Dan Harkins
> Gesendet: Dienstag, 16. Juli 2019 11:26
> An: cfrg@irtf.org
> Betreff: Re: [Cfrg] I-D Action: draft-irtf-cfrg-hash-to-curve-04.txt
> 
> 
>   Hello,
> 
>   This draft seems like a departure from the direction -03 was going,
> which is unfortunate.
> 
>   Version -03 had several methods of hashing whose preconditions made them
> appropriate for certain curves. Importantly, though, it had SWU which will
> work with basically any Weierstrass curve. Now it seems the focus is on 
> highly
> optimized and curve-specific methods and ciphersuites which fix the curve
> and hash algorithm. SWU is now optimized to work only on certain 
> pairing-friendly
> curves.
> 
>   Would it be possible to add back the -03 SWU as a generic template that
> can be instantiated with a curve and a hash function? That was how I was
> planning on using this soon-to-be RFC.
> 
>   And a comment on -04. The Simple SWU method now has a check whether u=0
> to prevent divide-by-zero. In the event it is, the algorithm outputs
> B/(Z * A) as x. Doesn't this leak information? If I, as a passive observer,
> notice x = B/(Z * A) then I know that hash_to_curve(m) returned 0. I know
> the probability of u=0 is astronomically small but if the possibility is
> going to be addressed why not reduce the output of the hash modulo (p-2)
> and then add 2 to always place 1 < u < p?
> 
>   regards,
> 
>   Dan.
> 
>> On 7/8/19 4:32 PM, internet-drafts@ietf.org wrote:
>> A New Internet-Draft is available from the on-line Internet-Drafts directories.
>> This draft is a work item of the Crypto Forum RG of the IRTF.
>> 
>>         Title           : Hashing to Elliptic Curves
>>         Authors         : Armando Faz-Hernandez
>>                           Sam Scott
>>                           Nick Sullivan
>>                           Riad S. Wahby
>>                           Christopher A. Wood
>>    Filename        : draft-irtf-cfrg-hash-to-curve-04.txt
>>    Pages           : 60
>>    Date            : 2019-07-08
>> 
>> Abstract:
>>    This document specifies a number of algorithms that may be used to
>>    encode or hash an arbitrary string to a point on an elliptic curve.
>> 
>> 
>> The IETF datatracker status page for this draft is:
>> https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdatatracker.ietf.org%2Fdoc%2Fdraft-irtf-cfrg-hash-to-curve%2F&amp;data=02%7C01%7Cbjoern.haase%40endress.com%7Cae3fd8bde95d49afaef008d709cfa996%7C52daf2a93b734da4ac6a3f81adc92b7e%7C1%7C0%7C636988659817711977&amp;sdata=DTtAtT0Ec2pyscnrisBC7dJkkgcNqGSe6pl8IrTHfP8%3D&amp;reserved=0
>> 
>> There are also htmlized versions available at:
>> https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Ftools.ietf.org%2Fhtml%2Fdraft-irtf-cfrg-hash-to-curve-04&amp;data=02%7C01%7Cbjoern.haase%40endress.com%7Cae3fd8bde95d49afaef008d709cfa996%7C52daf2a93b734da4ac6a3f81adc92b7e%7C1%7C0%7C636988659817711977&amp;sdata=UdjS%2BluBqR%2F6%2FFSTRaevzruWEU8xqWVcGZBZ5PDX4lI%3D&amp;reserved=0
>> https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdatatracker.ietf.org%2Fdoc%2Fhtml%2Fdraft-irtf-cfrg-hash-to-curve-04&amp;data=02%7C01%7Cbjoern.haase%40endress.com%7Cae3fd8bde95d49afaef008d709cfa996%7C52daf2a93b734da4ac6a3f81adc92b7e%7C1%7C0%7C636988659817711977&amp;sdata=LCWM4Q8iXV3phnMTTMg0E9HuVgR%2BHHZCvI9lg4I8ILU%3D&amp;reserved=0
>> 
>> A diff from the previous version is available at:
>> https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.ietf.org%2Frfcdiff%3Furl2%3Ddraft-irtf-cfrg-hash-to-curve-04&amp;data=02%7C01%7Cbjoern.haase%40endress.com%7Cae3fd8bde95d49afaef008d709cfa996%7C52daf2a93b734da4ac6a3f81adc92b7e%7C1%7C0%7C636988659817721981&amp;sdata=CR%2F43lACEBS0g8uW7GNEL7H4aZHQxqnmeEWD%2Ft8bRmk%3D&amp;reserved=0
>> 
>> 
>> Please note that it may take a couple of minutes from the time of submission
>> until the htmlized version and diff are available at tools.ietf.org.
>> 
>> Internet-Drafts are also available by anonymous FTP at:
>> ftp://ftp.ietf.org/internet-drafts/
>> 
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