TLS: custom DH parameters

[Al2Klimov]

Is your feature request related to a problem? Please describe.

As of #9811 Icinga uses publicly well-known pre-computed DH parameters.

Describe the solution you'd like

As OpenBSD says: if something can be random, make it random.

Describe alternatives you've considered

Let it as-is. Not a security problem, but we ca do "better" than status quo.

Additional context

https://github.com/Icinga/icinga2/pull/9811#issuecomment-1841566043

[Al2Klimov]

Things to consider

bash-3.2$ set -x; for b in 4096 3072 2048 1024 512; do time openssl dhparam -out `mktemp -d`/dhp $b; done
+ for b in 4096 3072 2048 1024 512
++ mktemp -d
+ openssl dhparam -out /var/folders/c2/k37yy5v51qzd5_rfrwd_sd7r0000gn/T/tmp.b5M8ivlD/dhp 4096
Generating DH parameters, 4096 bit long safe prime, generator 2
This is going to take a long time
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........................................................................................................................................................................................................................................................................................................................++*++*++*

real    27m30.612s
user    26m29.741s
sys 0m16.221s
+ for b in 4096 3072 2048 1024 512
++ mktemp -d
+ openssl dhparam -out /var/folders/c2/k37yy5v51qzd5_rfrwd_sd7r0000gn/T/tmp.VZ7cptiF/dhp 3072
Generating DH parameters, 3072 bit long safe prime, generator 2
This is going to take a long time
..............................................+........................................................................................................................................................................................................................................................................................................................+...................................................................................................................................+....................................................................................................................+.+...........................................................................................................................................................................+............+.................................................................................................................................+.............................................+..........................................................................................................................................................................+........................................................+...................................+........................................................................................................................................................................................................................................+............................................................................................................+..................+........................................................................................................................................................................................+.......................................................................................+.......................................................+.........................................................................................................................................+.......................................................................................................................................................................................................+.........+..........+...............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................+........................................................................................................................+..............................................................+..............................................................................................................................................................................................................................................................................................................+...................+....................................................................................................................................................................................+................................................+.................................................................+............................................................................................+...................+.....................................................................................................................................+................................................................+........................................................+......................................................+.....................................................................................+......................................................................+..............................................................................................................................................................................................................................................................................................................................................................................................+....................................................................................................................................................................................................................................................................................................................................................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real    4m57.308s
user    4m51.402s
sys 0m2.457s
+ for b in 4096 3072 2048 1024 512
++ mktemp -d
+ openssl dhparam -out /var/folders/c2/k37yy5v51qzd5_rfrwd_sd7r0000gn/T/tmp.oMzosBgi/dhp 2048
Generating DH parameters, 2048 bit long safe prime, generator 2
This is going to take a long time
......................................................................................+...............................................................+.........................................................................................................+.............................................................+.....................................................................................................................................................................................................................................................................+....................................+..................................................................................................................................................................................+................+.......+.................................................................................................................................................+.....................+................................................................................+................................................+.......................................................................................................................................................................................................................................................................................+.........................................................................................................................+....+..................................................................................................+.................................................................+.......................................................................................................................................................................+.....................+.......................................+.....................................................................+......+........................................................................................................................................................................................................................................................+..............................................................................................+...................................................................................................+.......+............................+.............................................................................................................................................................+..........+..............................................................................................................................+........................................................................+....................+.....+..............+...........................................................+................................+......................................................................................................................................................................................................................................................+........+.....................................................................................................+..................................................................+......................................+.....................+..................................................................................................+.........................................................................................+.............................................+..........+...............................................................................................+........................................................+.............................+.............++*++*++*++*

real    0m24.250s
user    0m23.922s
sys 0m0.159s
+ for b in 4096 3072 2048 1024 512
++ mktemp -d
+ openssl dhparam -out /var/folders/c2/k37yy5v51qzd5_rfrwd_sd7r0000gn/T/tmp.ZCFRf7tE/dhp 1024
Generating DH parameters, 1024 bit long safe prime, generator 2
This is going to take a long time
..........................................................................................................+................................+....................................................+.+..................................................................+........................+.......................................................................................+.......................+.............................................................................+....................................................................................+.......................................................................+.................................................................+........+.....+......+..+................................................................+.............................................+...+.........................+..........+..................................+.....................+.........................................+.............................................................+......................................+..........+................+........................................+............................................................................................................................................................+...............................................................+..............................................+.......................................................................+...+..............+..................................+.................................................................+................................................................................................................................................................................................................................................+....................................+................+...............................................................+....+.......................................................................+.........+............................................................+................+.................................................................+.......................+.............+......+.....................................+............+..............................................................................................................+...............................+.....+............+..............................................+............................................+..................+..........+..........+..................................................................................................................................................................................................+................+......+...................................................+...........................+........+........................+............+...........................+.....................+.............................................................................+........................................................+....................................................+.........+.+...........................+.....................+...........+..........................................+...........................+.............................................................+.........................++*++*++*++*++*

real    0m4.871s
user    0m4.715s
sys 0m0.078s
+ for b in 4096 3072 2048 1024 512
++ mktemp -d
+ openssl dhparam -out /var/folders/c2/k37yy5v51qzd5_rfrwd_sd7r0000gn/T/tmp.fQNTVlIw/dhp 512
Generating DH parameters, 512 bit long safe prime, generator 2
This is going to take a long time
........+....+...+....+................+..............................................................+........+...........+..........+..........................+......+..............+.......+..................+..........+..........................+.......++*++*++*++*++*

real    0m0.132s
user    0m0.115s
sys 0m0.010s
bash-3.2$

[Al2Klimov]

Things to also consider

As generating DH parameters is extremely time consuming, an application should not generate the parameters on the fly. DH parameters can be reused, as the actual key is newly generated during the negotiation.

Typically applications should use well known DH parameters that have built-in support in OpenSSL. The macros SSL_CTX_set_dh_auto() and SSL_set_dh_auto() configure OpenSSL to use the default built-in DH parameters for the SSL_CTX and SSL objects respectively. Passing a value of 1 in the onoff parameter switches the feature on, and passing a value of 0 switches it off. The default setting is off.

– https://www.openssl.org/docs/man3.1/man3/SSL_CTX_set_dh_auto.html

[Al2Klimov]

Oh! They even have pre-computed 8k params: https://github.com/openssl/openssl/blob/986c48c4eb26861f25bc68ea252d8f2aad592735/ssl/t1_lib.c#L3370-L3402

[julianbrost]

Typically applications should use well known DH parameters that have built-in support in OpenSSL. The macros SSL_CTX_set_dh_auto() and SSL_set_dh_auto() configure OpenSSL to use the default built-in DH parameters for the SSL_CTX and SSL objects respectively.

Which is exactly what we do right now (at least if the OpenSSL version supports it):

https://github.com/Icinga/icinga2/blob/420db1565bc44b72c7d15e113d7fa5b2f8fa78f0/lib/base/tlsutility.cpp#L112

Also, keep in mind that, our default cipher string prefers ECDHE over DHE, so whatever we would do wouldn't even affect the majority of connections (if the client supports ECDHE, it will be used):

https://github.com/Icinga/icinga2/blob/420db1565bc44b72c7d15e113d7fa5b2f8fa78f0/lib/base/tlsutility.hpp#L31