Hey! I'm David, a security consultant at Cryptography Services, the crypto team of NCC Group . This is my blog about cryptography and security and other related topics that I find interesting.

What Diffie-Hellman parameters to use?

posted October 2016

I see some discussions on some mailing lists about what parameters to use for Diffie-Hellman (DH).

It seems like the recent line of papers about weak Diffie-Hellman parameters (Logjam) and Diffie-Hellman backdoors (socat, the RFC 5114, the special primes, ...) has troubled more than one.

Less than two weeks ago, a study from Dorey et al. based on these previous results was released, uncovering many problems in how Diffie-Hellman is implemented (or even backdoored!) in the wild.

This is a non-problem. We don't need a RFC to choose Diffie-Hellman groups. A simple openssl gendh -out keyfile -2 2048 will generate a 2048-bit safe prime along with correct DH parameters for you to use. If you're worried about "special primes" issues, either make it yourself with this command, or pick a larger (let's say 4096-bit safe prime) from a list and verify that it's a safe prime. You can use this tool for that.

But since some people really don't want to do the work, here are some safe parameters you can use.

2048-bit parameters for Diffie-Hellman

Here's is the .pem file containing the parameters:

-----BEGIN DH PARAMETERS-----
MIIBCAKCAQEA7WJTTl5HMXOi8+kEeze7ftMRbIiX+P7tLkmwci30S+P6xc6wG1p4
SwbpPyewFlyasdL2Dd8PkhYFtE1xD3Ssj1De+P8T0UcJn5rCHn+g2+0k/CalysKT
XrobEzihlSLeQO1NsgBt1F1XCMO+6inLVvSGVbb3Cei4q+5Djnc7Yjjq0kxGY6Hd
ds/YQnyc1xdJU8NBi3zO1XY2Uk6BSd+NN5KnLh9zRq8t/b0RiIb/fY9mJ9BCtgPo
2m4AfJE8+5dE1ttpQAJFSlA8Ku3/9Vp8sMMWATVk2Q1z9PdkikKQYRfMPYDBSIa/
8Y2l9Hh7vNYOwXd4WF5Q55RHP46RB+F+swIBAg==
-----END DH PARAMETERS-----

You can parse it yourself by piping it to openssl dh -noout -text. It uses 2 as a generator and this big hexstring as a safe prime:

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

4096-bit parameters for Diffie-Hellman

Here's the .pem file:

-----BEGIN DH PARAMETERS-----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-----END DH PARAMETERS-----

And here is the hexstring value of the safe prime (note that it still uses 2 as a generator):

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
Well done! You've reached the end of my post. Now you can leave me a comment :)

John d'Oh

Why in the world would anyone sane use dh parameters from a random blog on the internets?

@John +1

david

Because some people just search for "what diffie hellman parameters to use" and will use the first answer they find :D

The idea here was to mix my answer with other crappy answers and broken RFCs to raise the overall security.