RE: What happended to SecretAgreee?

From: Ryan Sleevi [mailto:sleevi@google.com] 
Sent: Thursday, February 27, 2014 3:22 PM
To: Jim Schaad
Cc: public-webcrypto@w3.org
Subject: Re: What happended to SecretAgreee?



On Thu, Feb 27, 2014 at 3:11 PM, Jim Schaad <ietf@augustcellars.com> wrote:
Ryan,

If you want to look at some of the deeper security requirements.

http://r.duckduckgo.com/l/?kh=-1&uddg=http%3A%2F%2Fwww.iacr.org%2Fconferences%2Feurocrypt2004%2Fslides%2Fsession10talk3.ppt

contains one argument over why a KDF function is needed and you cannot use ZZ directly as keying material.  Given that one cannot assume that ZZ is uniform, since not all values are possible, directly using ZZ as keying material is a security attack point.

This means that my argument is MUST not SHOULD.

Jim

I fear you have misunderstood me, and saw the "RFC" ness of the capitalization rather than the intended emphasis.

Let me phrase it differently:

There are two things under discussion:
1) Whether something is technically possible
2) Whether something is good security practice

There is a reason this is called SubtleCrypto - there are subtleties here.

You originally argued that there is no legitimate reason to want ZZ - as a Key or as Bits. To this point, you're mistaken. We've discussed since the beginning why you would want this.

[JLS] Other than as an input to a KDF function, the above statement is completely correct.  There is no legitimate reason to get ZZ.  I would far rather expose ZZ by the following steps than by the use of deriveBits - Run a secretAgreement on the DH keys to get a KDF key, then run export on the KDF key to get ZZ.   This allows you to get to ZZ for use in a polyFill KDF.

The next question is - even if there is legitimate reason, should we allow it. This is the same discussion about "raw" AES, the same discussion about MD5, the same discussion about 3DES, the same discussion about CFB8, etc.

It seems to me that your argument is "We shouldn't allow X because X is hard to get right", but for every other value of X != "diffie-hellman phase 2", we've said "That's why this is called SubtleCrypto".

I don't see how the reference materially changes that discussion, or that there are legitimate use cases.
 


From: Ryan Sleevi [mailto:sleevi@google.com]
Sent: Thursday, February 27, 2014 2:07 PM
To: Jim Schaad
Cc: public-webcrypto@w3.org
Subject: Re: What happended to SecretAgreee?



On Thu, Feb 27, 2014 at 1:14 PM, Jim Schaad <ietf@augustcellars.com> wrote:
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.

Ok, let's unpack what you said here. One certainly can. Your argument is whether you SHOULD. That's entirely different discussion.


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.

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.

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.

Or you're polyfilling your own KDF that requires ZZ.


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.


If you don't want to allow that, don't support the deriveBits operation. I don't see where the angst is.

On a technical level, there are use cases for it. On a technical level, the API supports it.

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".





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?

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.

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

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


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.

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

Received on Friday, 28 February 2014 01:58:50 UTC