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

I think I agree with this. Just to confirm I understand correctly, you are proposing:


-          Define a subclass of Algorithm that defines key length for each algorithm that cares about it. This length parameter must be supplied in the algorithm identifier passed to importKey when import format is raw. If the input number of bytes exceeds the specified length, the algorithm's importKey function should truncate the input.

-          HKDF-CTR should also have an importKey operation defined. This one does not need the length defined. We could probably use the hash instead (since it will be used in the extract stage).

-          (EC)DH deriveKey then works like this:

o   If the programmer wants to use the raw output of the DH phase 2 as key material, they would use the target algorithm (e.g. AES-CFB) as the derivedKeyType. The implementation would do the DH phase 2 and pass the secret to the algorithm's importKey function as raw input.

o   If the programmer wants to perform an additional KDF, they would use the KDF algorithm (e.g. HKDF-CTR) as the derivedKeyType. The implementation would do the same thing - perform DH phase 2 and pass the secret to the algorithm's importKey method as raw input. The programmer would then call deriveKey again on the resulting key handle to get the final key.

Does that capture your proposal accurately?

From: Mark Watson [mailto:watsonm@netflix.com]
Sent: Tuesday, February 25, 2014 4:35 PM
To: Ryan Sleevi
Cc: public-webcrypto@w3.org
Subject: Re: Define how keys are derived from secret values for deriveKey



On Tue, Feb 25, 2014 at 3:46 PM, Ryan Sleevi <sleevi@google.com<mailto:sleevi@google.com>> wrote:


On Tue, Feb 25, 2014 at 3:33 PM, Mark Watson <watsonm@netflix.com<mailto:watsonm@netflix.com>> wrote:


On Tue, Feb 25, 2014 at 12:47 PM, Ryan Sleevi <sleevi@google.com<mailto:sleevi@google.com>> wrote:


On Tue, Feb 25, 2014 at 12:01 PM, Mark Watson <watsonm@netflix.com<mailto: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