linux-kernel - Re: [v2 PATCH] crypto: api - Disallow sha1 in FIPS-mode while allowing hmac(sha1)

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Date:   Fri, 14 Jan 2022 07:54:23 -0500
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Stephan Mueller <smueller@...onox.de>,
        Nicolai Stange <nstange@...e.de>,
        Herbert Xu <herbert@...dor.apana.org.au>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Hannes Reinecke <hare@...e.de>, Torsten Duwe <duwe@...e.de>,
        Zaibo Xu <xuzaibo@...wei.com>,
        Giovanni Cabiddu <giovanni.cabiddu@...el.com>,
        David Howells <dhowells@...hat.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
        qat-linux@...el.com, keyrings@...r.kernel.org, simo@...hat.com,
        Eric Biggers <ebiggers@...nel.org>, Petr Vorel <pvorel@...e.cz>
Subject: Re: [v2 PATCH] crypto: api - Disallow sha1 in FIPS-mode while
 allowing hmac(sha1)

On Fri, 2022-01-14 at 13:35 +0100, Stephan Mueller wrote:
> Am Freitag, 14. Januar 2022, 11:55:26 CET schrieb Herbert Xu:
> 
> Hi Herbert,
> 
> > > On an unrelated note, this will break trusted_key_tpm_ops->init() 
> > > in FIPS mode, because trusted_shash_alloc() would fail to get a
> > > hold of sha1. AFAICT, this could potentially make the
> > > init_trusted() module_init to fail, and, as encrypted-keys.ko
> > > imports key_type_trusted, prevent the loading of that one as
> > > well. Not sure that's desired...
> > 
> > Well if sha1 is supposed to be forbidden in FIPS mode why should
> 
> SHA-1 is approved in all use cases except signatures.

Actually, even that's not quite true: you can't use it in a FIPS
compliant system to *generate* signatures, but you can still use it in
a FIPS compliant system to verify legacy signatures (signatures created
before sha-1 was deprecated).  It's still also completely acceptable as
a hash for HMAC.

The supporting document is this one:

https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-131Ar1.pdf

The bottom line is removing SHA-1 to achieve "FIPS compliance" is the
wrong approach.  You just have to make sure you can never use it to
generate signatures.

James