Re: What happended to SecretAgreee?

On Thu, Feb 27, 2014 at 2:07 PM, Ryan Sleevi <sleevi@google.com> wrote:

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> On Thu, Feb 27, 2014 at 1:14 PM, Jim Schaad <ietf@augustcellars.com>wrote:
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>> I do not believe that one can ever go directly from the ZZ value to  a
>> symmetric key for AES-CBC.  This is just really bad security behavior for
>> any number of reasons.  However this seems to be what is being proposed on
>> some level by the current discussion.
>>
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> Ok, let's unpack what you said here. One certainly can. Your argument is
> whether you SHOULD. That's entirely different discussion.
>

It's a good and timely one, though.

If the only derived key algorithms allowed with (EC)DH were the KDFs, this
would simplify the specification considerably. I've been drafting the text
that allows an AES key, etc., to be derived directly from the DH secret
value (by truncation) and we have to jump through several hoops for that
(mainly to derive the length to which the shared secret should be
truncated, since this derivation is dependent on the target algorithm).

If it is bad practice to go directly from the secret value to an AES key
(say), I would argue that we should not support that as a first class
operation. If you really want to do it you can deriveBits on the DH key and
then import those as "raw" AES bits.

...Mark


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>> This means that no, I do not believe that DH is ever a key derivation
>> algorithm under any circumstances.   It is a method com computing a shared
>> secret.  From that shared secret one can then apply a key derivation
>> algorithm (identity is not one) to get a key for some other purpose.  There
>> is no reason to believe that any sub-portion of ZZ is not biased.  That is
>> the reason for doing the PRF on it.
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>> The entire concept of doing a deriveBits direction from the algorithm DH
>> therefore makes absolutely no sense to me.  There is no KDF function that
>> is being applied to ZZ to get the bits you want as output.
>>
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> It's quite simple (and has been made clear since the very first time we
> talked about DH). How "ZZ" is used can vary depending on what protocol
> you're using - some use it with protocol-specific KDFs.
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> Or you're polyfilling your own KDF that requires ZZ.
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>> I would also expect that in terms of extractability, if the private key
>> is marked as not extractable, then ZZ and any portion of ZZ is also marked
>> as being not extractible.  This would be passed on to the KDF function.  It
>> would also mean that deriveBits could never work on such a function since
>> it would export part of the computed shared secret.
>>
>> The reason that I put the "and maybe a new key derivation algorithm" in
>> parentheticals is because I am not sure that I believe that it needs to be
>> supported t the current time by the WebCrypto specification.  This is not
>> uncommon practice that one generates a Master Secret key from a shared
>> secret, and then uses that key value to generate other shared secret keys
>> that are used for specific purposes.  One example would be to have a
>> different MAC key for I send to you from you send to me.
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> If you don't want to allow that, don't support the deriveBits operation. I
> don't see where the angst is.
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> On a technical level, there are use cases for it. On a technical level,
> the API supports it.
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> It seems your entire argument is that you don't like it? I'm not trying to
> be dismissive, I'm trying to make a clear distinction between "can" and
> "should", so that we can make sure the "can" is correct BEFORE discussing
> the "should".
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>> From: Ryan Sleevi [mailto:sleevi@google.com]
>> Sent: Thursday, February 27, 2014 12:26 PM
>> To: Jim Schaad
>> Cc: public-webcrypto@w3.org
>> Subject: Re: What happended to SecretAgreee?
>>
>> Yes, Key agreement algorithms (which DH Phase 2 is - agreement of the
>> secret Z based on the exchanged parameters) is treated as a key derivation
>> algorithm.
>>
>> Can you provide any examples of algorithms or parameters you do not
>> believe fits into the deriveKey mechanism?
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>> I wasn't sure if your "and maybe a new key derivation algorithm"
>> indicated a degree of uncertainty. If so, yes, that is exactly the workflow
>> - for example, if you wanted to feed Z into HDKF to extract/expand.
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>> On Thu, Feb 27, 2014 at 12:21 PM, Jim Schaad <ietf@augustcellars.com>
>> wrote:
>> No, that is not true.
>>
>> secretAgreement when from a key agree algorithm to a key derivation
>> algorithm
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>> deriveBits and deriveKey go from a key derivation algoritm to  either a
>> byte array or a symmetric keying algorithm (or maybe a new key derivation
>> algorithm)
>>
>> jim
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>>
>> From: Ryan Sleevi [mailto:sleevi@google.com]
>> Sent: Thursday, February 27, 2014 12:05 PM
>> To: Jim Schaad
>> Cc: public-webcrypto@w3.org
>> Subject: Re: What happended to SecretAgreee?
>>
>> The names were changed, but the behaviours the same.
>>
>> deriveBits and deriveKey.
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>> On Thu, Feb 27, 2014 at 12:00 PM, Jim Schaad <ietf@augustcellars.com>
>> wrote:
>> At one point, I thought there was an agreement to add a new function to
>> the SubtleCrypto interface called secretAgreement.  This never happened.
>>
>> Was there a decision that I missed where this either was either not
>> actually decided or was reversed?
>>
>> Jim
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