3 ms·

hobbes@namagiri:~$ time openssl dhparam -5 2048 Generating DH parameters, 2048 bit long safe prime, generator 5 This is going to take a long time ..

hobbes@namagiri:~$ time openssl dhparam -5 2048 Generating DH parameters, 2048 bit long safe prime, generator 5 This is going to take a long time ............[snip]......++*++* -----BEGIN DH PARAMETERS----- MIIBCAKCAQEAzshMWp3IBjMW5Aia2wJOvA2EBY32Mn2fMXzlyFDklnRUg8ff/19A YWbRA4RAXrBMxoXEH1LVVpm5l89PGZ3DzjDafuNzNskgUhcAewUXXMQdkOFnPHYc 5F7+3DS8981Q0Q05qscBb26YGb2XaoJygyVj+B87NTvdAPzNU4fW5DyCuxhf5eov ZeZwcC4KZ31Lr7enFcFBjTjxQxW88pP4YhiNYQ1fsFARGJT0X7ksOlRVWFrODu6b nq0Ye/UWe0WB1zzmxz66ZujAwRwAgfmQZd7rILJqg68sxBeg88FlXJUeKfPL/bIT KOl1LhiHr/HkUBfgZRahK0MGcthwiLdFZwIBBQ== -----END DH PARAMETERS----- real 0m37.073s user 0m33.464s sys 0m2.924s hobbes@namagiri:~$ Edit: second run: real 0m35.362s user 0m31.992s sys 0m2.880s Would somebody post the steps I should use to make my own OpenSSH sshd(s) use non-standard DH parameters? Do I need to do anything to my clients?

daveloyall 11y ago
Apparently `/etc/ssh/moduli` is where I should store newly generated DH parameters. See the manpage moduli(5). Hm, the format of the existing file does not match the format of what `openssl dhparam` generated. I'll research...
- schoen 11y ago
  The dhparam command has mostly been used with Apache, as far as I know; ssh provides its own commands as part of ssh-keygen, which are described in the man page you just mentioned. However, the paper authors say "If you use SSH, you should upgrade both your server and client installations to the most recent version of OpenSSH, which prefers Elliptic-Curve Diffie-Hellman Key Exchange." That might ultimately be a simpler and safer course unless you have to deal with old clients that you don't have the ability to upgrade.
2ton_jeff 11y ago
Bit late to the party here, but this is why I wrote https://2ton.com.au/dhtool/ https://2ton.com.au/dhtool/ ... I have 48 CPU cores still dedicated to constantly generating safe primes and generators (have amassed a huge number of 2k, 3k, 4k and 8k DH parameters if anyone's interested) Any of the sizes' most recent 128 parameters can be downloaded in SSH moduli format, e.g. https://2ton.com.au/dhparam/4096/ssh https://2ton.com.au/dhparam/4096/ssh
- pentestercrab 11y ago
  Very interesting. Any chance you'll be attending the Ruxcon security conference in Melbourne?
- AnimalMuppet 11y ago
  If you wanted, you could start a small business selling CDs with those DH parameters. Be of public service and make some money at the same time.
daveloyall 11y ago
The EFF addressed this today. For SSH, they suggested this guide. https://stribika.github.io/2015/01/04/secure-secure-shell.html https://stribika.github.io/2015/01/04/secure-secure-shell.ht... Indeed, the guide recommends generating new contents for the /etc/ssh/moduli file (IFF you keep `diffie-hellman-group-exchange-sha256` enabled).

daveloyall 11y ago

Apparently `/etc/ssh/moduli` is where I should store newly generated DH parameters. See the manpage moduli(5). Hm, the format of the existing file does not match the format of what `openssl dhparam` generated. I'll research...

schoen 11y ago
The dhparam command has mostly been used with Apache, as far as I know; ssh provides its own commands as part of ssh-keygen, which are described in the man page you just mentioned. However, the paper authors say "If you use SSH, you should upgrade both your server and client installations to the most recent version of OpenSSH, which prefers Elliptic-Curve Diffie-Hellman Key Exchange." That might ultimately be a simpler and safer course unless you have to deal with old clients that you don't have the ability to upgrade.

2ton_jeff 11y ago

Bit late to the party here, but this is why I wrote https://2ton.com.au/dhtool/ https://2ton.com.au/dhtool/ ... I have 48 CPU cores still dedicated to constantly generating safe primes and generators (have amassed a huge number of 2k, 3k, 4k and 8k DH parameters if anyone's interested) Any of the sizes' most recent 128 parameters can be downloaded in SSH moduli format, e.g. https://2ton.com.au/dhparam/4096/ssh https://2ton.com.au/dhparam/4096/ssh

pentestercrab 11y ago
Very interesting. Any chance you'll be attending the Ruxcon security conference in Melbourne?
AnimalMuppet 11y ago
If you wanted, you could start a small business selling CDs with those DH parameters. Be of public service and make some money at the same time.

daveloyall 11y ago

The EFF addressed this today. For SSH, they suggested this guide. https://stribika.github.io/2015/01/04/secure-secure-shell.html https://stribika.github.io/2015/01/04/secure-secure-shell.ht... Indeed, the guide recommends generating new contents for the /etc/ssh/moduli file (IFF you keep `diffie-hellman-group-exchange-sha256` enabled).