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Re: [kitten] SPAKE and weak checksum types

"Henry B (Hank) Hotz, CISSP" <hbhotz@oxy.edu> Fri, 15 September 2017 04:38 UTC

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From: "Henry B (Hank) Hotz, CISSP" <hbhotz@oxy.edu>
In-Reply-To: <898b0135-7c9d-078d-c213-faf90c5c0417@mit.edu>
Date: Thu, 14 Sep 2017 21:38:28 -0700
Cc: Benjamin Kaduk <kaduk@mit.edu>, kitten@ietf.org
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To: Greg Hudson <ghudson@MIT.EDU>
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Subject: Re: [kitten] SPAKE and weak checksum types
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> On Sep 13, 2017, at 9:20 PM, Greg Hudson <ghudson@MIT.EDU> wrote:
> 
> On 09/13/2017 09:36 PM, Benjamin Kaduk wrote:
>>>>> IIUC you are concerned with the case that someone will stand up a kdc
>>>>> which will opportunistically use SPAKE, but supports older/weaker
>>>>> stuff. By its nature such a beast will be vulnerable to downgrade
>>>>> attacks and you can't solve that in SPAKE.
>>>> 
>>>> If the KDC downgrades itself to encrypted timestamp for DES/RC4 keys,
>>>> only a passive attack is needed, versus an active attack to downgrade
>>>> to encrypted timestamp.
>> 
>> I must still be foggy from recovering from being sick; could you walk
>> me through the passive attack a bit more slowly (or which scenarios are
>> being compared)?
> 
> The particular scenario I was concerned about here (which should not be
> an issue since we appear to have agreement on the text change) was: the
> KDC and the client both permit SPAKE and encrypted timestamp.  The KDC
> decides not to offer SPAKE because the initial reply key is an RC4 key
> and therefore the transcript checksum would use HMAC-MD5.  The passive
> attacker can simply dictionary attack the ciphertext from the client (or
> the KDC).

Wouldn’t the KDC have already sent a preuth-required message which prohibited that enctype? I thought the list of enctypes was separate from the list of PA types.

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