Re: [IPsec] Updating IPsec algorithm requirements

[Paul Koning <[e-mail]>]

Excerpt of message (sent 6 November 2009) by David McGrew:
> Hi Paul, Yaron, and IPsec ME WG participants,
> 
> I would like to propose an update to the algorithm requirements, as  
> outlined below.
> ...
> Currently, AES in Counter Mode (AES-CTR)[RFC3686] is recommended as a
> SHOULD in "Cryptographic Algorithm Implementation Requirements for
> Encapsulating Security Payload (ESP) and Authentication Header (AH)",
> RFC 4835.  AES-CTR is a useful algorithm because it admits efficient
> high speed implementations.  However, it provides no authentication.

... and therefore no confidentiality either, if used by itself, via
the Bellovin attack.

>  From RFC3686: "With AES-CTR, it is trivial to use a valid ciphertext
> to forge other (valid to the decryptor) ciphertexts. Thus, it is
> equally catastrophic to use AES-CTR without a companion authentication
> function. Implementations MUST use AES-CTR in conjunction with an
> authentication function, such as HMAC-SHA-1-96 [HMAC-SHA]."
> Unfortunately, none of the authentication algorithms currently defined
> for IPsec (HMAC, XCBC-MAC) admit efficient high speed
> implementations.  Thus the need for authentication undermines the
> efficiency of AES-CTR.
> 
> AES-GCM was designed specifically to overcome this problem.  ...
> 
> It is especially important that AES-GCM be recommended over the use of
> AES-CTR with HMAC-SHA-256, ...

I agree.  For the reasons you gave, and also to remove the temptation
to run AES-CTR without authentication for performance reasons, even
though the standard says not to do this.

       paul