Re: [PATCH 08/18] crypto: add crypto tests for single block DES-ECB and DES-CBC

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Jul 08, 2021 at 01:50:54PM -0500, Eric Blake wrote:
> On Tue, Jul 06, 2021 at 10:59:14AM +0100, Daniel P. Berrangé wrote:
> > The GNUTLS crypto provider doesn't support DES-ECB, only DES-CBC.
> 
> I had to go research these terms; DES-ECB is weaker (each block
> encrypted on its own), DES-CBC is stronger (the encryption of later
> blocks depend on the earlier text).  Makes sense that GNUTLS has
> dropped support for the weaker form.
> 
> > We can use the latter to simulate the former, if we encrypt only
> > 1 block (8 bytes) of data at a time, using a all-zeros IV. This
> 
> using an all-zeros
> 
> > is a very inefficient way to use the QCryptoCipher APIs, but
> > since the VNC authentication challenge is only 16 bytes, this
> > is acceptable. No other part of QEMU should be using DES. This
> > test case demonstrates the equivalence of ECB and CBC for the
> > single-block case.
> 
> Agreed - both on the inefficiency (we're throwing away all the work
> spent on chaining the later blocks - thankfully there is only one such
> block in our 16-byte challenge), and on the fact that DES should be
> avoided where possible (our sole use is due to VNC's less-than-stellar
> "security").

Actually there isn't any work wasted chaining blocks, because we're
only writing one block of data.

The inefficiency is because we have to constantly re-create the
cipher context object after every 8 bytes. This massively dominates
over the cipher speed.

> 
> > 
> > Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> > ---
> >  tests/unit/test-crypto-cipher.c | 23 +++++++++++++++++++++++
> >  1 file changed, 23 insertions(+)
> 
> Reviewed-by: Eric Blake <eblake@redhat.com>

Regards,
Daniel
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