Subject: [IPsec] IV in ESP packets for DES and 3DES methods

From: ss murthy nittala <[email-masked]>
Date: Tue, 12 May 2009 12:40:39 +0530

Hi

Thanks for the clarifications regarding IV usage for AES methods.

RFC 2405 (DES) in its implementation note says

"Common practice is to use random data for the first IV and the last 
8 octets of encrypted data from an encryption process as the IV for 
the next encryption process; this logically extends the CBC across 
the packets.It also has the advantage of limiting the leakage of 
information from the random number genrator. No matter which mechnism 
is used, the receiver MUST NOT assume any meaning for this value, 
other than that it is an IV."

But towards the end of the RFC, it says

"For the first block of plaintext, though, the IV takes the place
  of the previous block of ciphertext.  If the IV doesn't differ
  much from the previous IV, and the actual plaintext block doesn't
  differ much from the previous packet's, then the effective
  plaintext won't differ much, either.  This means that you have
  pairs of ciphertext blocks combined with plaintext blocks that
  differ in just a few bit positions.  This can be a wedge for
  assorted cryptanalytic attacks."

What is RFC suggesting here?Anyway we can not avoid the possibility 
of successive plain packets being identical atleast partially.
Is Random number for IV a must or it is ok to get it from the 
previous encrypted packet for DES?

What are the latest observations?

I also want know the same regarding 3DES.

Thanks in advance
-ns murthy

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From: Stephen Kent <[email-masked]>
Date: Tue, 12 May 2009 11:09:41 -0400

2405 is out of date. Its recommendation re using the last 8 octets of 
ciphertext from the previous packet has been replaced with one of 
using a randomly-generated IV for each packet.

Steve

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From: "Murthy N Srinivas-B22237" <[email-masked]>
Date: Tue, 12 May 2009 21:34:33 +0530

Is there a new draft/rfc posted with the change incorporated?
 
-ns murthy

________________________________

From: [email-masked] on behalf of Stephen Kent
Sent: Tue 5/12/2009 8:39 PM
To: Murthy N Srinivas-B22237
Cc: [email-masked]
Subject: Re: [IPsec] IV in ESP packets for DES and 3DES methods

Steve
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From: Stephen Kent <[email-masked]>
Date: Wed, 13 May 2009 13:49:14 -0400

>Is there a new draft/rfc posted with the change incorporated?

DES is deprecated, so I would not expect a revised RFC on that.  AES 
is strongly preferred over 3DES, so there is little incentive to rev 
that doc, although it makes more sense than for DES.

Steve