# Ssl\_dhparam and recommended-private-length **URL:** \ **Category:** NGINX\ **Created:** [January 3, 2011, 9:58am UTC](https://www.ruby-forum.com/t/ssl-dhparam-and-recommended-private-length/201221 "2011-01-03T09:58:48Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![dubstep](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/cdc98d/32.png) [@dubstep](https://www.ruby-forum.com/u/dubstep)\ **Post date:** [January 3, 2011, 9:58am UTC](https://www.ruby-forum.com/t/ssl-dhparam-and-recommended-private-length/201221/1 "2011-01-03T09:58:48Z")
Hi, Nginx offers a possibility to import DH parameters (prime p and generator g) from external file. The parameters (p and g) have to be generated with “openssl dpharam.” In the default setting, openssl calculates strong primes and uses either 2 or 5 as generator. The private exponent in modular exponentiation is then very long, its bit representation is comparable in length to the prime modulus, e.g. 2047 bits with 2048 bits long modulus. However, there is an extra option in openssl dhparam to generate DSA type parameters for Diffie-Hellman key exchange. The main advantage is that the recommended exponent length is much shorter, e.g. 224 or 256 bits with 2048 bits long modulus. The length of the exponent has direct impact on computational complexity because it determines the number of multiplications and squarings in modular exponentiation. For example, one could go down from 2048 multiplications and squarings to 256 or 224. At the moment, nginx passes only p and g parameters to SSL. It would be nice to pass also the recommended exponent length to openssl via dh-\>length provided that the recommended-private-length is present in pem file. For the recommended-private-length parameter to be present in PEM file, the DH parameters have to be generated with “openssl dhparam -dsaparam” Posted at Nginx Forum: > **[ssl\_dhparam and recommended-private-length](https://forum.nginx.org/read.php?2,163037,163037#msg-163037)** > > Hi, Nginx offers a possibility to import DH parameters (prime p and generator g) from external file. The parameters (p and g) have to be generated with "openssl dpharam." In the default setting, openssl calculates strong primes and uses... ---
**Author:** ![timo2](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/ee7513/32.png) [@timo2](https://www.ruby-forum.com/u/timo2)\ **Post date:** [January 3, 2011, 1:06pm UTC](https://www.ruby-forum.com/t/ssl-dhparam-and-recommended-private-length/201221/2 "2011-01-03T13:06:25Z")
Well, it seems that openssl can handle it by itself if the recommended exponent length is in the pem file. Nginx uses openssl routines to decode PEM file in ngx\_ssl\_dhparam routine. So the recommended exponent length should be taken into account. Can anyone more experienced confirm that? However, the default values (hardcoded in nginx source) are 1024 bit long safe prime p and generator g=2. NIST recommends to use at least 2048 bits long primes with at least 224-bit prime order subgroup starting from 1 January 2011 🙂 Does using any 2048 group from RFC 5114 as a default one make more sense? Posted at Nginx Forum: > **[Re: ssl\_dhparam and recommended-private-length](https://forum.nginx.org/read.php?2,163037,163078#msg-163078)** > > Hi, Nginx offers a possibility to import DH parameters (prime p and generator g) from external file. The parameters (p and g) have to be generated with "openssl dpharam." In the default setting, openssl calculates strong primes and uses...