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RE: Why KEM?, RE: Charter Update

2002-05-01 10:33:14
Thanks Russ.  I did manage to look at your slides when you first posted them
to the list on April 17th.  I found them very helpful.   However, I think
Robert has raised some good points (based on the issues claimed in your
slides) that question the motivation for even considering a move to KEM.
I've attached his email below for your convenience.  I think we have chosen
a very good direction already with OAEP and am not convinced that we need to
change this.  
---- Robert's email of 04/19/2002 ---

-----Original Message-----
From: Robert Zuccherato [mailto:robert(_dot_)zuccherato(_at_)entrust(_dot_)com]
Sent: Friday, April 19, 2002 10:46 AM
To: 'Housley, Russ'; ietf-smime(_at_)imc(_dot_)org
Subject: RE: Charter Update


Having not attended the Minneapolis meeting I must say that I was very
surprised by your recommendation to drop OAEP as the MUST implement key
transport mechanism with AES in favour of KEM.  It wasn't all that long ago
that you were attempting to get everyone (S/MIME, TLS, X9.44) to agree on
requiring OAEP with AES as a method of transitioning to OAEP from PKCS#1
v1.5.  Partly as a result of that effort, implementations of OAEP have
started to appear (e.g. OpenSSL) and transitions to OAEP can actually now
start to occur.  If we are going to introduce a new MUST algorithm, and thus
additional uncertainty about what to use and how to transition, we really
should have a good reason.

In your presentation you say that KEM has better security proofs.  That may
be, however, OAEP is still secure.  No actual weaknesses in it have been
found.  On RSA's website there is a description of recent results on OAEP
that says "... it makes little sense replacing OAEP with a "more secure"
encoding method, because if a CCA2 adversary is able to break RSAEP-OAEP,
then she will be able to break RSAEP equipped with any encoding method (if
maybe slightly less efficiently)."  (
<> )
Thus, there is no need to introduce KEM for security reasons.

Your presentation also lists some standards that already include KEM.
However, all of the ones that are listed except TLS also specify OAEP (ANSI
X9.44, IEEE P1363, ISO/IEC 18033-2, PKCS#1, S/MIME).  TLS, while it
specifies a variant of KEM, doesn't actually use anything that is compatible
with the KEM that S/MIME (and the other groups listed) would be using.  I
would also like to point out that XML Encryption has OAEP as a required key
transport method.  At this point it does seem like OAEP is starting to get
adopted by other groups and thus introducing KEM now seems to be

It is true that with OAEP the message length is bounded.  However, for our
requirements here, is that really an issue?  

For these reasons I think we should reconsider your proposal to use RSA-KEM
instead of RSA-OAEP in draft-ietf-smime-aes-alg.

Robert Zuccherato 
Entrust, Inc. 


-----Original Message-----
From: Housley, Russ [mailto:rhousley(_at_)rsasecurity(_dot_)com]
Sent: Wednesday, May 01, 2002 1:22 PM
To: Mike Just
Cc: ietf-smime(_at_)imc(_dot_)org
Subject: Re: Why KEM?, RE: Charter Update


I think that I did respond to Robert's question.  At IETF 53, I gave a
presentation on this subject.  You can see the slides at
<> .


At 01:14 PM 5/1/2002 -0400, Mike Just wrote:

It's not clear to me that there is consensus on this path (unfortunately,
there seemed to be no reaction on either side to this email) as reflected in
the latest version of the Charter.

I'd like to re-iterate Robert's concerns (from his e-mail of 04/19/2002) and
ask the fundamental question: Why are we even considering KEM when we
already have a sufficient solution with OAEP?


-----Original Message----- 
From: Housley, Russ [ mailto:rhousley(_at_)rsasecurity(_dot_)com
<mailto:rhousley(_at_)rsasecurity(_dot_)com> ] 
Sent: Wednesday, April 24, 2002 5:14 PM 
To: ietf-smime(_at_)imc(_dot_)org 
Subject: RE: Charter Update 

Is the WG consensus that RSA-OAEP, RSA-KEM, and AES should be in separate, 
independent documents.  This would allow AES to be used with any of the key 
management techniques? 


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