RE: [PATCH 4/7] crypto: testmgr - Added testvectors for the ofb(sm4) & cfb(sm4) skciphers

[Pascal Van Leeuwen <pvanleeuwen@verimatrix.com>]

> -----Original Message-----
> From: Eric Biggers <ebiggers@kernel.org>
> Sent: Sunday, September 15, 2019 10:21 PM
> To: Pascal Van Leeuwen <pvanleeuwen@verimatrix.com>
> Cc: Pascal van Leeuwen <pascalvanl@gmail.com>; linux-crypto@vger.kernel.org;
> antoine.tenart@bootlin.com; herbert@gondor.apana.org.au; davem@davemloft.net
> Subject: Re: [PATCH 4/7] crypto: testmgr - Added testvectors for the ofb(sm4) & cfb(sm4)
> skciphers
> 
> On Wed, Sep 11, 2019 at 07:34:31PM +0000, Pascal Van Leeuwen wrote:
> > > -----Original Message-----
> > > From: Eric Biggers <ebiggers@kernel.org>
> > > Sent: Wednesday, September 11, 2019 6:06 PM
> > > To: Pascal van Leeuwen <pascalvanl@gmail.com>
> > > Cc: linux-crypto@vger.kernel.org; antoine.tenart@bootlin.com; herbert@gondor.apana.org.au;
> > > davem@davemloft.net; Pascal Van Leeuwen <pvanleeuwen@verimatrix.com>
> > > Subject: Re: [PATCH 4/7] crypto: testmgr - Added testvectors for the ofb(sm4) & cfb(sm4)
> > > skciphers
> > >
> > > On Wed, Sep 11, 2019 at 12:38:21PM +0200, Pascal van Leeuwen wrote:
> > > > Added testvectors for the ofb(sm4) and cfb(sm4) skcipher algorithms
> > > >
> > >
> > > What is the use case for these algorithms?  Who/what is going to use them?
> > >
> > > - Eric
> > >
> > SM4 is a Chinese replacement for 128 bit AES, which is mandatory to be used for many
> > Chinese use cases. So they would use these whereever you would normally use ofb(aes)
> > or cfb(aes). Frankly, I'm not aware of any practicle use cases for these feedback
> > modes, but we've been supporting them for decades and apparently the Crypto API
> > supports them for AES as well. So they must be useful for something ...
> >
> > The obvious advantage over CBC mode was that they only require the encrypt part of
> > the cipher, but that holds for the (newer) CTR mode as well. So, my guess would be
> > some legacy uses cases from before the time CTR mode and AEAD's became popular.
> >
> > Maybe someone remembers why these were added for AES in the first place?
> >
> 
> So if you have no idea why they should be added, why are you adding them?
>
Because our hardware supports these modes and I added support for this to
the inside-secure driver, so it made sense to be able to test that as well.
That's the primary reason for adding them, otherwise I would not have bothered.

> - Eric

Regards,
Pascal van Leeuwen
Silicon IP Architect, Multi-Protocol Engines @ Verimatrix
www.insidesecure.com