[Dovecot] 2048-bit Diffie-Hellman parameters

[Marios Titas]

Currently, dovecot generates two primes for Diffie-Hellman key
exchanges: a 512-bit one and a 1024-bit one. In light of recent
events, I think it would be wise to add support for 2048-bit primes as
well, or even better, add a configuration option that lets the user
select a file (or files) containing the DH parameters

In recent years, there has been increased interest in DH especially in
its ephemeral version (DHE) because it provides perfect forward
secrecy. In that context, the use of 1024-bit parameters might not
seem such a terrible idea: if someone cracks the ephemeral key then
they will only gain access to the data exchanged during that
particular session. Therefore, it might not be worth the effort to
crack such a key. But this is certainly not the case for IMAPS: it is
quite likely that the session data will include the user's
credentials.