Help on Diffie Hellman key exchange

[Tomas Mraz]

On Wed, 2019-11-13 at 11:11 -0500, Jason Qian wrote:
> Hi Tomas,
> 
>    Using DH_compute_key_padded() seems fixed the problem.
>    
>   I have one more question regarding a similar issue but this time is
> about AES key generation.
> 
>  I think the problem is related to  RAND_seed or  RAND_bytes (someone
> also mentioned on another thread).
> 
>     RAND_seed(seed, ::strlen(seed));
>     RAND_bytes(buf, keySize / 8);
> 

I do not understand what is the problem you have. But nevertheless -
you should not need to call RAND_seed() unless you are running the code
on some very special platform where no method of automatical seeding of
the OpenSSL RNG is available.

Your RAND_bytes() call should be fine to produce an AES key of bit
length keySize.

> 
>   
> 
> 
> 
> On Tue, Nov 12, 2019 at 10:50 AM Jason Qian <jqian at tibco.com> wrote:
> > Thanks Tomas, 
> > 
> > I will try that.
> > 
> > On Tue, Nov 12, 2019 at 3:14 AM Tomas Mraz <tmraz at redhat.com>
> > wrote:
> > > On Mon, 2019-11-04 at 17:34 -0500, Jason Qian via openssl-users
> > > wrote:
> > > > Hi
> > > >   
> > > >    We have an application that does the Diffie Hellman key
> > > exchange
> > > > (OpenSSL/1.1.0f).
> > > >    It works fine, but under heavy loaded conditions, sometimes
> > > an
> > > > invalide secret been generated and other side couldn't decrypt
> > > the
> > > > data (the secret seems offset by one).      
> > > > 
> > > >    The client side is c++ and the server side is java.
> > > > 
> > > >     DH_compute_key(secretKey, bnY, m_DH); 
> > > > 
> > > >    Someone in the openssl group also talks about a similar
> > > issue, but
> > > > not sure if have a solution. 
> > > 
> > > Could it be a padding issue? I.E. use DH_compute_key_padded()
> > > instead.
> > > 
-- 
Tomáš Mráz
No matter how far down the wrong road you've gone, turn back.
                                              Turkish proverb
[You'll know whether the road is wrong if you carefully listen to your
conscience.]