Re: [IPsec] AES key lengths: draft-ietf-ipsecme-esp-ah-reqts

<Paul_Koning@Dell.com> Mon, 10 March 2014 16:13 UTC

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From: <Paul_Koning@Dell.com>
To: <kent@bbn.com>
Thread-Topic: [IPsec] AES key lengths: draft-ietf-ipsecme-esp-ah-reqts
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Date: Mon, 10 Mar 2014 16:12:36 +0000
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Subject: Re: [IPsec] AES key lengths: draft-ietf-ipsecme-esp-ah-reqts
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On Mar 10, 2014, at 12:05 PM, Stephen Kent <kent@bbn.com> wrote:

> Paul
>> On Mar 8, 2014, at 8:08 AM, Black, David <david.black@emc.com> wrote:
>> 
>>>> The next draft changes AES-128-CBC to AES-CBC, and says:
>>>> 
>>>> In the following sections, all AES modes are for 128-bit AES. 192-bit AES
>>>> MAY be supported for those modes, but the requirements here are for 128-bit
>>>> AES.
>>> What about 256-bit AES keys?  They should also be a "MAY".
>> Why not “SHOULD” for 192 and 256 bit keys?
>> 
>> 	paul
> It's good to remember the reason that 256-bits keys for AES were specified,
> i.e., as a hedge against someone building a quantum computer. So, unless the
> data being encrypted is expected to have a lifetime far enough into the future
> as to merit protection against that concern, the extra time needed to perform
> AES-256 vs. AES-128 does not seem justified.
> 
> Steve

That’s a good argument for a user choosing to use AES-128 rather than AES-256.  But it doesn’t really address why “SHOULD implement” isn’t justified — the implementation cost is trivial and if it isn’t used it has no performance impact.

	paul