Subject: [ietf-dkim] RFC4871bis - whether to drop -- SHA1 support

From: Dave CROCKER <[email-masked]>
Date: Sat, 30 May 2009 10:17:09 -0700

Folks,

In:

   <http://mipassoc.org/pipermail/ietf-dkim/2009q2/011959.html>

Steve Atkins posted a list of suggested DKIM features to drop.

This note is intended to anchor a discussion thread for discusses one of those
features, namely:

>     Drop support for SHA1 entirely. It's beginning to look  
> cryptographically very dubious, and is being dropped by pretty much  
> everyone else. Even if the attacks against it don't affect the way  
> it's used in DKIM it seems unwise to suggest it be used at all. At the  
> very least it seems a poor "marketing" move to include an algorithm  
> that's been dropped by most everyone else as insecure before this spec  
> is finalized.
> 
>     "Verifiers MUST support rsa-sha256 and MAY support rsa-sha1.  
> Signers SHOULD sign using rsa-sha256 and SHOULD NOT sign using rsa- 
> sha1." might provide enough wiggle room to allow existing code time to  
> migrate away from SHA1.

Please discuss arguments for and against dropping this.

d/
-- 

   Dave Crocker
   Brandenburg InternetWorking
   bbiw.net

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---

From: "Olivier MJ Crepin-Leblond" <[email-masked]>
Date: Sun, 31 May 2009 00:46:36 +0200

>> sha1." might provide enough wiggle room to allow existing code time to

Seeing that the message I received this suggestion in, is signed by the 
mipassoc.org server with an rsa-sha1 key, I find this suggestion curious. 
Dropping support altogether for SHA1 might indeed alienate many currently 
installed systems.
I'd opt for "Verifiers MUST support rsa-sha256 and MUST support rsa-sha1", 
whilst keeping the SHOULD/SHOULD NOT emphasis as described above, in order 
to eventually have every signer use rsa-sha256.

To optimise is okay, but to start dropping/alienating currently installed 
base is something I'd consider unwise at this point in time.

Kind regards,

-- 
Olivier MJ Crépin-Leblond, PhD
http://www.gih.com/ocl.html