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gpg: Do not use weak digest algos if selected by recipient prefs.

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* 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.

Reported-by: Phil Pennock
Signed-off-by: Werner Koch <wk@gnupg.org>
This commit is contained in:
Werner Koch 2020-11-02 13:39:58 +01:00
parent b004701adc
commit 15746d60d4
No known key found for this signature in database
GPG key ID: E3FDFF218E45B72B
4 changed files with 31 additions and 19 deletions
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12
g10/sig-check.c
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@ -488,16 +488,14 @@ check_signature_end_simple (PKT_public_key *pk, PKT_signature *sig,
{
gcry_mpi_t result = NULL;
int rc = 0;
const struct weakhash *weak;
if (!opt.flags.allow_weak_digest_algos)
{
for (weak = opt.weak_digests; weak; weak = weak->next)
if (sig->digest_algo == weak->algo)
{
print_digest_rejected_note(sig->digest_algo);
return GPG_ERR_DIGEST_ALGO;
}
if (is_weak_digest (sig->digest_algo))
{
print_digest_rejected_note (sig->digest_algo);
return GPG_ERR_DIGEST_ALGO;
}
}
/* For key signatures check that the key has a cert usage. We may