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Subject: Re: [pkcs11] Concerns with SHA512/t

On 11/10/2016 01:08 PM, Johnson Darren wrote:
I agree, that section is very odd.
I raised the same questions/concerns in the SHA1/SHA2 proposal.  It was an open ended question in my document that I completely forgot to raise on the last two calls.  I apologize for that.

My questions were along the same lines as Ferenc's below and I agree completely.  If we want to support a CKM_SHA512_T_HMAC_GENERAL mechanism, then it does need two inputs.  Ideally using a specific mechanism parameter as defined below by Ferenc.


I think it was added in anticipation of future SHA-2 like hash proposals. I think that's unlikely at this stage, and if there were, we would probably want to add and explicit mechanism for each of those proposals.

The only other use would be someone experimenting with non- standard SHA-2 hashes or some kind of parametrized SHA-512/t type hash. Is someone on the list using something like this?


-----Original Message-----
From: pkcs11@lists.oasis-open.org [mailto:pkcs11@lists.oasis-open.org] On Behalf Of Valerie Fenwick
Sent: Wednesday, November 09, 2016 3:33 PM
To: pkcs11@lists.oasis-open.org
Subject: [pkcs11] Concerns with SHA512/t

Hi FOlks -

While reviewing some other content in this area, a member of my team brought up this concern. Any opinions?

thank you,


-------- Forwarded Message --------
Subject: Re: Fwd: [pkcs11] SHA-1 and SHA2 information in v2.40
Date: Wed, 14 Sep 2016 16:14:31 +0200
From: Ferenc R.
Organization: Oracle Corporation
To: Valerie Fenwick <valerie.fenwick@oracle.com>

Hi, Valerie,

    what I noticed is the following, please forward it to whom it might concern:

there is a digest mechanism, called SHA-512/t where t is a parameter, the length in bits of the produced hash.

there are HMAC mechanisms, each based on a particular hash function, called General-length <hash_function>-HMAC.

Now, at the description of the mechanisms involving SHA-512/224,
SHA-512/256 and SHA-512/t (sections 2.22, 2.23 and 2.24) there are some obvious copy/paste/edit errors, but with the general-length SHA-512/t-HMAC (2.25.3 - even the name is wrong in the standard) there is a fundamental problem: this mechanism has two parameters, one for t and another for the desired output length (in PKCS#11 terms it means a parameter that is a pair of numbers, not just one number as for the other general mechanisms). I think it also adds to the confusion that the t parameter of the SHA-512/t digest mechanism is defined as CK_MAC_GENERAL_PARAMS, although it is not a MACing mechanism.

I think the remedy can be one of two things:

1. do not allow a general-length SHA-512/t-HMAC mechanism
     (based on the argument that if you want a k-bit MAC, you
     can always just use the HMAC based on SHA-512/t where t=k) 2. define  new parameter types CK_SHA_512_T_PARAMS
      and CK_SHA_512_T_MAC_GENERAL_PARAMS and use those for
      the mechanisms described in 2.25
      (based on the argument that a k-bit MAC truncated from the
      output of the HMAC based on SHA-512/t for t>k is not the same
      as the HMAC based on SHA-512/t where t=k, so this allows
      more flexibility)

I would vote for #2 above.

In the meantime, I am implementing #1 as it is now closer to the current edition of the standard.


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