Feature req: DH prime bitsize query

Janne Snabb snabb at epipe.com
Sun May 27 10:47:26 CEST 2012

On Sun, 27 May 2012, Phil Pennock wrote:

> When gnutls_dh_params_generate2() is used to generate DH parameters of a
> particular size, it has a tendency to overshoot.
> Asking for 2236 bits, a 2237 bit prime seems to be fairly common.


> Could GnuTLS 3 *please* get an API call to find out the size in bits of
> the DH prime in a gnutls_dh_params_t ?  Perhaps even add a query mode to
> certtool?

New version of certtool prints out the number of bits. Are you looking
for this:

$ certtool --dh-info --infile=/var/spool/exim4/gnutls-params-2236
Generator (8 bits): 02

Prime (2240 bits):

The --dh-info option does not seem to be available in Ubuntu packaged
gnutls-bin version 3.0.11+really2.12.14-5ubuntu3 (wtf?) but gnutls-bin
version 3.0.19-2 on debian "sid" has the --dh-info option. I did not
have time now to look how it gets the information.

Appears that I have 2240 bits when Exim had asked for 2236. Which means
that Thunderbird/NSS would not be able to negotiate DHE-RSA with this
server :(.

Janne Snabb / EPIPE Communications
snabb at epipe.com - http://epipe.com/

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