un-trusting MD5 in gpg

[David Shaw]

On May 7, 2009, at 4:57 AM, Werner Koch wrote:

> On Thu,  7 May 2009 00:04, dshaw at jabberwocky.com said:
>
>> Blacklist is pretty good.  We could call it "blacklist- 
>> digests" (which
>> gives us a name for "blacklist-ciphers" later).  It's reasonably  
>> clear
>> just from the name.
>
> I agree.  That term is different from disabled and may be used with
> different semantics in other tools.
>
> I would prefer
>
>  --blacklist-digest-algo NAME_or_NUMBER
>
> The reason is that we do not need a specific order as with
> personal-digest-preferences.  It makes implementation (cf. gpgconf)
> easier and better matches other options related to algorithms.

Sounds good to me.  So, thus far, the plan sounds like:

   --blacklist-digest-algo (name or number)
   --no-blacklist-digest-algo (name or number)

Repeating the blacklist-digest-algo option can be done to add more  
than one algorithm to the blacklist.  no-blacklist-digest-algo can be  
used to remove something from the list.  Whoever gets in last (add to  
the list or remove from the list) wins.

A blacklisted digest will cause signature verification to fail with an  
appropriate error message along the lines of "digest algorithm is  
blacklisted" (internally, GPG_ERR_BLACKLISTED_DIGEST or the like).

A key certification created with a blacklisted digest will not be part  
of the web of trust.

A blacklisted digest will also not be usable when creating a signature/ 
certification, with the same sort of error returned.

This does not affect the use of the digest in things like --print-md.

gpg --version will flag blacklisted algorithms by putting them in  
[brackets].

David