Re: [Qemu-devel] [PATCH v2 2/7] crypto: make PBKDF iterations configurable for LUKS format

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

On Mon, Sep 12, 2016 at 10:38:25AM -0500, Eric Blake wrote:
> On 09/12/2016 09:13 AM, Daniel P. Berrange wrote:
> > As protection against bruteforcing passphrases, the PBKDF
> > algorithm is tuned by counting the number of iterations
> > needed to produce 1 second of running time. If the machine
> > that the image will be used on is much faster than the
> > machine where the image is created, it can be desirable
> > to raise the number of iterations. This change adds a new
> > 'iter-time' property that allows the user to choose the
> > iteration wallclock time.
> > 
> > Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
> > ---
> 
> > @@ -183,6 +184,11 @@ static QemuOptsList block_crypto_create_opts_luks = {
> >              .type = QEMU_OPT_STRING,
> >              .help = "Name of encryption hash algorithm",
> >          },
> > +        {
> > +            .name = BLOCK_CRYPTO_OPT_LUKS_ITER_TIME,
> > +            .type = QEMU_OPT_NUMBER,
> > +            .help = "Time to spend in PBKDF in milliseconds",
> 
> Worth mentioning the default of 1000?

I tried to keep all the defaults listed just in the QAPI schema,
to avoid remembering to update multiple docs.

> > @@ -1075,6 +1078,16 @@ qcrypto_block_luks_create(QCryptoBlock *block,
> >          goto error;
> >      }
> >  
> > +    if (iters > (ULLONG_MAX / luks_opts.iter_time)) {
> > +        error_setg_errno(errp, ERANGE,
> > +                         "PBKDF iterations %llu too large to scale",
> > +                         (unsigned long long)iters);
> 
> As in 1/7, you seem to prefer casts over PRIu64 :)
> 
> > +++ b/qapi/crypto.json
> > @@ -185,6 +185,9 @@
> >  #                  Currently defaults to 'sha256'
> >  # @hash-alg: #optional the master key hash algorithm
> >  #            Currently defaults to 'sha256'
> > +# @iter-time: #optional number of milliseconds to spend in
> > +#             PBKDF passphrase processing. Currently defaults
> > +#             to 1000. (since 2.8)
> 
> Ah, you mentioned the default here.
> 
> Reviewed-by: Eric Blake <eblake@redhat.com>
> 
> -- 
> Eric Blake   eblake redhat com    +1-919-301-3266
> Libvirt virtualization library http://libvirt.org
> 




Regards,
Daniel
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