Ulf:
But bin2bn is generating BNs from the static P and G which don't have
trailing 0s. It is BN_num_bytes that reports 55 when it should be
saying 56. Is there a better way to determine the length of the key?
Do I just need to know the length of the key apriori? I had allowed the
client to pick the key length, and the server just figured it out from
the output from BN_num_bytes(). Should BN_num_bytes() report 55 instead
of 56 just because the last byte is 0? Maybe it should be renamed
BN_num_bytes_not_counting_trailing_zeroes() :-) At the very least that
limitation should be documented. DH_size() always seems to return the
correct answer, so I'll switch to that.
On the MGF1 thing (new to me, I'll admit), I assume I create the
blowfish key using
MGF1(*bfkey, length_of_blowfish_key, *dh_shared_secret,
length_of_dh_shared_secret);
on both nodes once they have generated the DH_shared_secret. This
appears to be a hash function using SHA1?
Just curious, why is the DH shared key insecure before being run through
MGF1?
thanks for your help!
Ulf Moeller wrote:
>
> On Wed, Nov 01, 2000, Lawrence MacIntyre wrote:
>
> > I use bin2bn to generate the BNs for P and G, and the char lengths are
> > correct AND statically defined in the .h file, so they are the same
> > every time. Then I generate the key using DH_generate_key() and
> > BN_num_bytes() to determine the length of the key. Is that incorrect?
>
> No, it's just that the top byte can be zero, and bin2bn doesn't output
> trailing 0s.
>
> > The reason I found the error is that I use the DH struct to provide a
> > key to the blowfish encryption function and it would fail sometimes.
>
> That is potentially insecure anyway. You should use a key derivation
> function (for example MGF1) to generate a symmetric key from the DH
> shared secret.
--
Lawrence
~
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