Re: [TLS] Wrapping up cached info

Stefan Santesson <stefan@aaa-sec.com> Mon, 17 May 2010 22:25 UTC

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Date: Tue, 18 May 2010 00:24:06 +0200
From: Stefan Santesson <stefan@aaa-sec.com>
To: Stefan Santesson <stefan@aaa-sec.com>, <mrex@sap.com>, Joseph Salowey <jsalowey@cisco.com>
Message-ID: <C8178EA6.AE48%stefan@aaa-sec.com>
Thread-Topic: [TLS] Wrapping up cached info
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Subject: Re: [TLS] Wrapping up cached info
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As Martin pointed out to me privately, the hash used in the finished
calculation becomes known to the client only after receiving the serer
hello.

It would therefore be natural for the client to use the hash function used
to calculate the finished message at the time when the data was cached.

The client would then indicate which hash algorithm it used and upon
acceptance, the server will honor the request.

/Stefan


On 10-05-17 10:34 PM, "Stefan Santesson" <stefan@aaa-sec.com> wrote:

> Guys,
> 
> Where I come from we have a say "don't cross the river to get to the water".
> And to me this proposal to change the finished calculation is just that.
> 
> Look at it.
> 
> The proposal is to bind the cached data by adding a hash of the cached data
> to the finished calculation.
> 
> The proposal is further to avoid hash agility by picking the hash algorithm
> used by TLS's Finished message computation.
> 
> 
> Now there are two ways to achieve this goal.
> 
> 1) The crossing the river to get water approach:  Exchange FNV hashes of the
> cached data in the handshake protocol exchange and then inject hashes of the
> same data into the finished calculation through an alteration of the TLS
> protocol.
> 
> 
> 2) The simple approach: Use the hash algorithm of the finished calculation
> to hash the cached data (according to the current draft).
> 
> 
> Alternative 2 securely bind the hashed data into the finished message
> calculation without altering the algorithm.
> 
> Alternative 2 requires at most a hash algorithm identifier added to the
> protocol, if at all. We don't need to add negotiation since we always use
> the hash of the finished message calculation. Adding this identifier would
> be the only change made to the current draft.
> 
> Alternative 2 don't require additional security analysis. If the hash used
> to calculate the finished message is broken, then we are screwed anyway.
> 
> /Stefan
> 
> 
> 
> 
> 
> 
> 
> 
> 
> On 10-05-17 9:16 PM, "Martin Rex" <mrex@sap.com> wrote:
> 
>> Joseph Salowey wrote:
>>> 
>>> I agree with Uri, that if you determine you need SHA-256 then you should
>>> plan for hash agility.  TLS 1.2 plans for hash agility.
>>> 
>>> What about Nico's proposal where a checksum is used to identify the
>>> cached data and the actual handshake contains the actual data hashed
>>> with the algorithm used in the PRF negotiated with the cipher suite?
>>> 
>>> This way we don't have to introduce hash agility into the extension, but
>>> we have cryptographic hash agility where it matters in the Finished
>>> computation.  Does it solve the problem?
>> 
>> Yes, I think so.
>> This approach should solve the issue at the technical level.
>> 
>> Going more into detail, one would hash/mac only the data that got
>> actually replaced in the handshake, each prefixed by a (locally computed)
>> length field.
>> 
>> -Martin
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> 
> 
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