Generating Diffie-Hellman parameters efficiently

Question

I am working on an Android project for school and I am supposed to do a DHKE (Diffie Hellman Key Exchange). Everything works well. The problem is that it takes a lot of time (really a lot) to generate DHParameters. Basically, in my code, this is the part that is the most time (and battery) consuming:

KeyPairGenerator kpgDH = KeyPairGenerator.getInstance("DH");
kpgDH.initialize(512);
KeyPair kpDH = kpgDH.generateKeyPair();

As you can see, the key length is only 512 bits, so it's not long, and it still takes at least 30-40 seconds (best case scenario), but it can go up to 400 seconds. I've tested it on several phones: Samsung Galaxy s2 (quad core), Samsung Galaxy s4 (quad core), Samsung Galaxy note 10.1 (quad core).

Does anybody know an alternative to generate more quickly the $p$ and $g$ for the Diffie-Hellman in order to speed up the process?

Answer 1

The standard solution is to generate $g$ and $p$ once during application development, then hardcode $g$ and $p$ in your code. There are standard choices for $p$ and $g$, e.g., documented by NIST in their FIPS series. I suggest using one of those. There is no need to re-generate $g$ or $p$ each time. You can use the same $g$ and $p$ for everyone. See also Is it safer to generate your own Diffie-Hellman primes or to use those defined in RFC 3526? for more details in this vein.

And make sure you use larger parameters. For security, I recommend that the prime $p$ should be at least 2048 bits long, and $g$ should generate a subgroup of size at least $2^{128}$ (or generate the whole group).

ECDHE will provide even better performance than Diffie-Hellman, but you can try the above first. Hardcoding the parameters should already make a huge difference.