dovecot - Jun 2018 - upgrade 2.2 to 2.3, diffie-hellman, ssl_min_protocol

hi sorry if question was asked already. Was reading
https://wiki2.dovecot.org/Upgrading/2.3

first I'm confused on diffie hellman parameters file. I never set up  
ssl-parameters.dat before (should i have? do I have one that was  
automatically made for me by dovecot?)

Do I need to make a fresh dh.pem? The upgrade doc tells how to convert  
ssl-parameters.dat but how to make a new one?

other question is if I copy ssl_min_protocol from example config into  
my existing config is that enough? do experts on this list recommend  
any tweaks that increase client requirements more than dovecot  
developers are comfortable with but will ensure more secure protocol  
usage?


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> On 22 June 2018 at 10:18 tai74 at vfemail.net wrote:
> 
> 
> 
> hi sorry if question was asked already. Was reading
> https://wiki2.dovecot.org/Upgrading/2.3
> 
> first I'm confused on diffie hellman parameters file. I never set up  
> ssl-parameters.dat before (should i have? do I have one that was  
> automatically made for me by dovecot?)
> 
> Do I need to make a fresh dh.pem? The upgrade doc tells how to convert  
> ssl-parameters.dat but how to make a new one?
> 
2.2 makes the ssl-parameters.dat automatically. You can choose to either use
that with the instructions given, or you can make a fresh one using openssl
gendh 4096 > dh.pem

Note that this will require quite a lot of entropy, so you should probably
ensure that you run it on a laptop or with virtual machine that has some entropy
source/helper.
> other question is if I copy ssl_min_protocol from example config into  
> my existing config is that enough? do experts on this list recommend  
> any tweaks that increase client requirements more than dovecot  
> developers are comfortable with but will ensure more secure protocol  
> usage?
> 
ssl_min_protocols defines the minimum TLS protocol the server supports. We
recommend TLSv1, but if you want, you can experiment with TLSv1.2, which will
decrease client compability a bit.

Aki
> 
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On Fri, 22 Jun 2018, Aki Tuomi wrote:
>> Do I need to make a fresh dh.pem? The upgrade doc tells how to convert 
>> ssl-parameters.dat but how to make a new one?
>
> ... or you can make a fresh one using openssl
> gendh 4096 > dh.pem
This also works

 	openssl dhparam -out dh.pem 4096
> Note that this will require quite a lot of entropy, so you should
> probably ensure that you run it on a laptop or with virtual machine
> that has some entropy source/helper.
It can take an extraordinary amount of time for long keys.  Most of
the time/entropy is taken up to produce a "safe" prime (p) such that
(p-1)/2 is also prime to resist some factoring algorithms.

However, recent advances make this condition obsolete [*] and not
really safer, so a much faster way to generate a DH key is

 	openssl dhparam -dsaparam -out dh.pem 4096

DH generation is a one time operation, so if you're paranoid and you've
got time to burn, go ahead and generate the "safe" DH key.

[*] https://security.stackexchange.com/questions/42415/openvpn-dhparam)

Joseph Tam <jtam.home at gmail.com>