Subject: [IPsec] One more comment to the draft-ietf-ipsecme-aes-ctr-ikev2-05.txt

From: Tero Kivinen <[email-masked]>
Date: Mon, 22 Mar 2010 00:41:26 +0200

In the security considerations section there is a text saying:

>  Additionally, since AES has a 128-bit block size, regardless of the
>  mode employed, the ciphertext generated by AES encryption becomes
>  distinguishable from random values after 2^64 blocks are encrypted
>  with a single key.  Since IKEv2 is not likely to carry traffic in
>  such a high quantity compared with ESP, this won't be a big concern
>  here.  However, when a large amount of traffic appears in the future
>  or under abnormal circumstances, implementations SHOULD generate a
>  fresh key before 2^64 blocks are encrypted with the same key.

The last SHOULD is not really needed as IKEv2 message ID is 32-bits,
and the IKE SA MUST be closed (or rekeyed) before it wraps, thus at
most one IKE SA can have 2^32 messages, each consisting of at max 2^16
bytes, thus maximum number of bytes that may be transmitted over IKEv2
SA is 2^48 bytes. As this 2^48 bytes is much smaller than 2^64 blocks,
this paragraph is not an issue in IKEv2.

I would change the paragraph to be:

   Additionally, since AES has a 128-bit block size, regardless of the
   mode employed, the ciphertext generated by AES encryption becomes
   distinguishable from random values after 2^64 blocks are encrypted
   with a single key.  Since IKEv2 SA cannot carry that much of data,
   this issue is not a concern here.
-- 
[email-masked]