https://bugs.exim.org/show_bug.cgi?id=1656
Phil Pennock <pdp@???> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |RESOLVED
Resolution|--- |WONTFIX
--- Comment #2 from Phil Pennock <pdp@???> ---
This patch lowers security, by default.
This option does not control the size of DH parameters accepted; it controls
the maximum size of DH parameters which will be generated. The default value
is 2236 bits. This is a clamp on the size recommended by GnuTLS, because when
asked for a normal size, it recommends a value which was high enough that it
caused interoperability issues with NSS, the crypto library used by Mozilla
products, including Thunderbird.
So the clamp exists to not let the size be too large, for interop, and we
constrain it to allow at least 1024 bits. But in practice, the clamp will be
left alone, at 2236 bits and GnuTLS will be asked to provide DH params of about
that size. (Actually, 10 less, because it's not precise in the values
returned, so would sometimes _still_ break NSS).
The effect of this change is to make it so that an administrator can not
explicitly lower the maximum size down to 1024 bits, should they choose to do
so. There are issues here around "how much can you shoot yourself in the
foot". Changing defaults values has different concerns to changing default
bounds which are imposed upon an administrator without recompilation.
Here, with GnuTLS, we're defaulting to larger than 2048 bits and have been for
a few releases now. I don't see a reason to override administrators who, for
some local reason, need to configure Exim to work well with clients with worse
limits.
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