Hi Bonno,
On 08/08/16 14:20, Bonno Bloksma wrote:
> Hi,
>
> Starting a new easy-rsa setup from scratch. New CA etc.
> I noticed in my old config I had a dh4096.pem file which I had supposedly
> build by temporarily increasing the KEY_SIZE line
> export KEY_SIZE=4096
> ... I think.
correct, the build-dh script does little more than
$OPENSSL dhparam -out ${KEY_DIR}/dh${KEY_SIZE}.pem ${KEY_SIZE}
> However, I can no longer find why I did this, supposedly to get a larger base
> for my TLS / session keys... I think.
>
> 1) Am I right? Does a larger dhxxxx.pem file indeed result in a larger pool
> for OpenVPN to get TLS/session keys from?
> 2) If 1) is correct and there is indeed a use for a bigger dh file, can I
> indeed create a new dh4096.pem file by temporarily increase the KEY_SIZE, run
> build-dh and then set it back to what I have?
> 3) Is there any use in creating an even bigger dh file, lets say a 8192 bit
> version?
>
The short-cut answer is: your RSA key size should match your DH size,
where "RSA key" is that of the openvpn sever. You can find lots of
answers/info at e.g.
http://security.stackexchange.com/questions/48300/why-should-the-dh-keylength-match-rsa-length-in-tls
In the "DHE_RSA" cipher suite, security is relative to both that offered
by Diffie-Hellman, and that offered by RSA. "In general", the overall
security will be that of the weakest of the two; so:
it is harmful if the DH key is weaker than the RSA key, because
then overall security is lowered (compared to what the RSA key alone
offers);
it is harmful if the DH key is stronger than the RSA key, because
you do not get actual extra security (it is limited by that of the RSA
key), but you still have to pay for the extra large key (larger
messages, higher computational cost, and loss of interoperability with
limited clients).
It so happens that DH and RSA seem to offer similar strength when used
with keys of similar sizes, i.e. DH modulo a 1024-bit prime can be said
to be somewhat as strong as RSA with a 1024-bit modulus (but a non-prime
modulus, of course). So this gives the rule that the DH key and the RSA
key should have the same length.
HTH,
JJK
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