Avoid recipient-compatibility SHA1

[Werner Koch]

On Fri, 30 Oct 2020 00:10, Phil Pennock said:

> I just sent a message to N recipients, and I think one of them probably
> has some preference algorithm in their key details, because this one
> mail was signed using SHA1, not my defaults.

Fixed:

commit 15746d60d492f5792e4a179ab0a08801b4049695 
Author: Werner Koch <wk at gnupg.org>
Date:   Mon Nov 2 13:39:58 2020 +0100

    gpg: Do not use weak digest algos if selected by recipient prefs.
    
    * g10/misc.c (is_weak_digest): New.
    (print_digest_algo_note): Use it here.
    * g10/sig-check.c (check_signature_end_simple): Use it.
    * g10/sign.c (hash_for): Do not use recipient_digest_algo if it is in
    the least of weak digest algorithm.
    --
    
    If a message is signed and encrypted to several recipients, the to be
    used digest algorithm is deduced from the preferences of the
    recipient.  This is so that all recipients are able to check the the
    signature.  However, if the sender has a declared an algorithm as
    week, that algorithm shall not be used - in this case we fallback to
    the standard way of selecting an algorithm.
    
    Note that a smarter way of selecting the algo is to check this while
    figuring out the algorithm - this needs more testing and thus we do it
    the simple way.

or in short if any of the preferences would lead to a weak algo the
feature of selecting the digest algo from the preferences is disabled.

I intend to put this also in to 2.2.24.

> recipient.  That's fine.  I'd rather create pressure for people to fix
> their systems to use modern cryptography than cater to their brokenness
> with sensitive messages.

People won't update their keys - that just does not work.  Ignoring the
preferences is a better way here.


Salam-Shalom,

   Werner

-- 
Die Gedanken sind frei.  Ausnahmen regelt ein Bundesgesetz.
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