RE: [pkcs11] Concerns with SHA512/t

[Johnson Darren <darren.johnson@gemalto.com>]

Hi,

This is my opinion... but I don't feel too strongly on it.  So if the group feels differently, I will not object :)

I think we should fix it instead of moving it to the legacy list.  Mostly just for completeness of the standard.  I don’t think there is a strong argument for not supporting General-length HMACs for SHA512/t.

If we believe there is an argument for not supporting it... then it would apply to General-length SHA512/224 and SHA512/256 as well as they are also truncated HMACs. (ie why truncate something that is already truncated?).

I would argue that the T value is not the same as the  General-length truncation value.  The T value is also used to define the initial hash values/constants.  I think that difference is significant enough that it should be treated separately from the General-length truncation value.

DJ

-----Original Message-----
From: Valerie Fenwick [mailto:valerie.fenwick@oracle.com]
Sent: Monday, November 14, 2016 6:26 AM
To: Johnson Darren <darren.johnson@gemalto.com>; pkcs11@lists.oasis-open.org
Subject: Re: [pkcs11] Concerns with SHA512/t

Talking about this with my team now, another proposal is:

What if we get rid of the mechanism (ie move it to historical) CK_SHA_512_T_MAC_GENERAL_PARAMS ?

The T is already providing the truncation that general provides.
(ie why would you truncate something is already truncated?)

thoughts?

Valerie

On 11/10/2016 9:08 PM, Johnson Darren wrote:
> Hi,
> 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.
>
> Thanks
> Darren
>
> -----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,
>
> Valerie
>
>
> -------- 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.
>
> Ferenc
>
>
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--
Valerie Fenwick, http://bubbva.blogspot.com/ @bubbva Solaris Cryptographic & Key Management Technologies, Manager Oracle Corporation: 4180 Network Circle, Santa Clara, CA, 95054.
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