Re: Define how keys are derived from secret values for deriveKey

On Tue, Feb 25, 2014 at 4:35 PM, Mark Watson <watsonm@netflix.com> wrote:

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> On Tue, Feb 25, 2014 at 3:46 PM, Ryan Sleevi <sleevi@google.com> wrote:
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>> On Tue, Feb 25, 2014 at 3:33 PM, Mark Watson <watsonm@netflix.com> wrote:
>>
>>>
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>>> On Tue, Feb 25, 2014 at 12:47 PM, Ryan Sleevi <sleevi@google.com> wrote:
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>>>>
>>>>
>>>>
>>>> On Tue, Feb 25, 2014 at 12:01 PM, Mark Watson <watsonm@netflix.com>wrote:
>>>>
>>>>> https://www.w3.org/Bugs/Public/show_bug.cgi?id=24811
>>>>>
>>>>> The deriveKey operation derives a key targeted at a specified algorithm. Both ECDH and DH algorithms output a Secret Value.
>>>>>
>>>>> It is not yet specified how to map from the Secret Value to a key for the specified target algorithm.
>>>>>
>>>>> It seems intuitive to use the "raw" import format for the target algorithm with the Secret Value as the raw input. If we do this we must define how to provide the length of the key and how to convert the secret value to that length. Presently, raw import for symmetric keys e.g. AES-GCM derives the key length from the provided data and fails if the provided data is not one of the supported lengths.
>>>>>
>>>>> It seems valuable to be able to specify the length of the required key independently from the length of the Secret Value.
>>>>>
>>>>>
>>>> I'm not sure I follow what you're proposing here regarding separate
>>>> lengths.
>>>>
>>>
>>> The length of the secret value is whatever the (EC)DH outputs. The
>>> length of the AES key you want could be 128, 192 or 256.
>>>
>>> We can either say the AES key length is some function of the secret
>>> value length, or we could allow the AES key length to be specified directly.
>>>
>>>
>>>>
>>>>>  So, one possibility is to allow the length of the symmetric key to be specified as an input to the import operation and have that operation define the mapping from arbitrary length raw value to a key of the requested length. The deriveKey operations can then refer directly to the "raw" import operations for the derived key algorithm.
>>>>>
>>>>>  ...Mark
>>>>>
>>>>
>>>> Yes, I agree that for the case of deriving symmetric keys - whether
>>>> "directly" (eg: by treating the DH Phase 2 output as a direct input to a
>>>> key) or "indirectly" (eg: by treating the DH Phase 2 output as an input
>>>> into another KDF, like HDKF/Concat) - it's necessary to specify additional
>>>> parameters.
>>>>
>>>> You suggest that it's "one possibility", but I'm curious if you see
>>>> there being any other possibilities.
>>>>
>>>
>>> Not really - I just said "one possibility" to emphasize the tentative
>>> nature of the proposal. But what you suggest below is different from what I
>>> suggested ...
>>>
>>
>> I don't know if it's a matter of not having coffee handy, but I'm at a
>> loss still for parsing some of this email, so I'm again going to try to get
>> clarifications.
>>
>>
>>>
>>>
>>>> Import:
>>>>   - If length is present
>>>>     - If length is 'consistent' with the import data - success
>>>>     - If length is 'inconsistent' with the import data - failure
>>>>  - If length is absent
>>>>   - ?? Fail? Or derived from import data? How is this similar to or
>>>> different than the JWK import cases where alg is optional?
>>>>
>>>
>>> For "pure" import, the length of the data itself is an explicit
>>> indication of the required key length. It can't be absent. We do not have a
>>> separate AesImportParams dictionary and we don't need one (yet - see
>>> below).
>>>
>>
>>> It's different from JWK "alg", because alg is optional and because we
>>> *do* have a place in the method parameters to specify the information
>>> that might be in the alg field. Thus we have to specify the requirement for
>>> consistency.
>>>
>>>
>>>> Derive:
>>>>   - If length is present
>>>>     - If length is less than the maximum output of the key derivation
>>>> step (if any) - create a key from the first (length) bits and feed to
>>>> import("raw")
>>>>    - If length is greater than the maximum output of the key derivation
>>>> step - fail
>>>>  - If length is absent
>>>>    - Fail
>>>>
>>>> Have I missed any other edge conditions?
>>>>
>>>
>>> No. I think what you've written is the right approach, but it's a little
>>> tricky to see how we implement this in the specification.
>>>
>>> The derive logic above could be part of the (EC)DH derive operation. It
>>> has the truncation step which happens before the "raw" AES import
>>> operation. The truncation step needs access to a length field. The
>>> intuitive place to specify this length is in the deriveKeyType method
>>> parameter. e.g.:
>>>
>>> p = deriveKey( { name: "DH", public: PV }, dhPrivateKey, { name:
>>> "AES-GCM", *length: 256* }, false, [ "encrypt", "decrypt" ] )
>>>
>>> However, what is the subclass of the derivedKeyType field ? Presumably
>>> it is something like an AesImportParams.
>>>
>>> So, then, we have an operation which is part of the (EC)DH "derive"
>>> procedures (the truncation) which needs information from an AES-specific
>>> structure. Awkward.
>>>
>>
>> To be clear, my understanding of your proposal is that we add an
>> AesImportParams, and that length is specified. That's why I sought
>> clarification, above, as to what to do. Is "length" optional (implied from
>> context) or is it required (even when it can be inferred?) This is EXACTLY
>> the same conversation we had regarding JWK - a parameter which can be
>> inferred for some operations (AES 'raw' import, JWK with an 'alg'
>> parameter) but not for other operations (AES 'derived' import, JWK without
>> an 'alg' parameter).
>>
>> Please explain why you find this awkward, however, as you didn't really
>> qualify it. derivedKeyType is specifying the "shape" of the key to be
>> derived. The length is part of that shape. Arguably, however, so is the
>> algorithm. For example, it's entirely possible (as discussed on past calls)
>> that things like parity bits might enter the equation if talking 3DES keys
>> - they're both fundamentally shapes of what the derived key is going to
>> look like.
>>
>
> That's not what I find awkward. I'll try and explain that better below. I
> think (correct me if I'm wrong), that we're both moving towards having an
> AesImportParams which explicitly specifies the length and could be used
> both with importKey and deriveKey and then the behaviour is as you
> suggested.
>
>
>>
>>
>>>
>>> We could resolve this by putting the truncation operation into the "raw"
>>> AES import procedures and adding the length as an input to these. If we
>>> wanted true import to work as you have specified, then we'd need those
>>> procedures to have two modes "strict length" mode, in which a length
>>> mis-match is an error and "non-strict length" mode in which truncation can
>>> be performed. This could be a boolean input parameter which is "strict"
>>> when the "raw" input procedures are called from importKey and "non-strict"
>>> when they are called from the derive operation of another algorithm.
>>>
>>> Any better ideas ?
>>>
>>> ...Mark
>>>
>>>
>> I do not parse this at all, but it sounds overly complex and very
>> different than what I was proposing - and how we handle other parameters of
>> a similar type (eg: JWK)
>>
>> I don't see where the issue is with keeping truncation as part of key
>> derivation. That is, it has always been my understanding that a deriveKey()
>> operation is comprised of
>>
>> [1. run the KDF] + [2. do some special steps _within the 'kdf' alg] + [3.
>> import a key with the result]
>>
>> The truncation step is [2]. Parity bits would be [2]. Any other 'shaping'
>> would be [2].
>>
>> If the question is where do you specify the parameters for [2], I still
>> believe that derivedKeyType is the right place.
>>
>
> I mostly agree with this. My problem is just that step [2] is specific to
> the key type. Do we need to have a switch in the derive procedures for the
> different derived key types ? e.g. in the derive operation of DH, do we
> have:
>
> [1]: do the DH phase II, resulting in secret value
> [2]:
> If the name property of normalizedDerivedKeyType is a case-sensitive
> string match for "AES-CTR", "AES-CBC", "AES-GCM", "AES-CMAC", "AES-CFB-8":
>      <truncation steps for AES using length parameter of
> normalizedDerivedKeyAlgorithm which is of type AesImportParams>
> If the name property of normalizedDerivedKeyType is a case-sensitive
> string match for "HMAC":
>     <truncation steps for HMAC using length and hash parameters
> of normalizedDerivedKeyAlgorithm which is of type HmacImportParams>
> etc.
> [3]: call the import operation of normalizedDerivedKeyAlgorithm with
> format "raw", ...
>
> The problem being that step [2] will be duplicated exactly for everything
> that supports deriveKey (ECDH, DH, HKDF, CONCAT KDF, PBKDF2).
>
> My suggestion was just to roll [2] into the import procedure for each
> algorithm. Alternatively, we could define a separate "Import derived key"
> operation for each algorithm that is called from the derive operations and
> that does steps [2] and [3] combined.
>
> ...Mark
>
>
I'm inclined to see something like a separate "import derived key". I don't
think we need to duplicate [3] into it. I think the "import derived key" is
really a statement of "Perform the algorithm-specific part of [2]"

So the description for deriveKey (in general/ alg agnostic)

[1] Perform the derivation defined by deriveKey-KDF
[2] Perform the formatting defined by formatDerivedKey-derivedKeyAlg
[3] Perform the importing defined by importKey-derivedKeyAlg

I think this is fairly similar to what Vijay described.

The question is what is the relation with "derivedKeyAlg"'s algorithm
parameters for "formatDerivedKey" and "importKey". This does imply that
we'd need some sort of derive-algorithm for every algorithm name - even if
it's just a class that is multi-implementing AlgImportParams and
DeriveLengthParams.

Another alternative is that AesImportParams (et-al) do exist and _do_
define a length. the "ImportKey-Alg" template is changed to also take an
additional parameter "truncatable". When importKey-Alg is called by
importKey-generic, truncatable=false. When importKey-Alg is called by
deriveKey-generic, truncatable=true.