cert-digest-algo clarification

[Robert J. Hansen]

On 7/12/2012 11:39 AM, Sam Smith wrote:
> Say I want to tell everyone, "Hey, I prefer you use SHA256 when 
> communicating with me." What command should I use to communicate
> this? "default-preference-list" right?

There's a difference between what you can enforce and what you might be
able to suggest.

The OpenPGP spec requires that no OpenPGP implementation will ever use
any algorithm except those that are listed on your certificate as ones
that your implementation understands.  This list of "I can understand
the following algorithms" can be found by 'gpg --edit-key [keyid] showpref'.

Some OpenPGP implementations, such as GnuPG, will treat that set of
capabilities as a list of preferences.  If your prefs show up as "SHA256
SHA-1", for instance, an OpenPGP implementation would be forbidden from
using RIPEMD160, but would be able to use SHA1.  GnuPG would likewise be
forbidden from using RIPEMD160, but would be more likely to use SHA-1
than SHA256.

GnuPG might still use SHA-1, though!  If the sender is using a DSA-1k
key and does not have --enable-dsa2 active, SHA256 is disallowed for the
signature, so GnuPG will have to fall back to SHA-1.

The takeaway here is that the capabilities shown on your certificate
("gpg --edit-key [keyid] showpref") MAY be used as a preference list,
are not guaranteed to be used as a preference list, and even if using an
OpenPGP implementation that treats it as a preference list you may wind
up getting stuck with SHA-1 anyway.

> So "personal-digest-preferences" overrides this?

No.  personal-digest-preferences declares which digest algorithms you
prefer to use and in which order.  The certificate preferences declare
which algorithms you are *capable* of using (and, for some
implementations, which algorithms you prefer *other people* to use and
in which order).