The pattern currently is to call get_cache(), generate the encoding
if that failed and then store it with cache(). The latter adopts the
passed encoding and frees any stored encoding. However, the latter means
that if two threads concurrently fail to get a cached encoding and then
both generate and store one, one of the threads might use an encoding
that was freed by the other thread.
Since encodings are not expected to change, we can avoid this issue by
not replacing an existing cache entry and instead return that (while
freeing the passed value instead of the cached one).
Closesstrongswan/strongswan#1231
As per RFC 8032, section 5.1.7 (and section 8.4) we have to make sure s, which
is the scalar in the second half of the signature value, is smaller than L.
Without that check, L can be added to most signatures at least once to create
another valid signature for the same public key and message.
This could be problematic if, for instance, a blacklist is based on hashes
of certificates. A new certificate could be created with a different
signature (without knowing the signature key) by simply adding L to s.
Currently, both OpenSSL 1.1.1 and Botan 2.8.0 are vulnerable to this, which is
why the unit test currently only warns about it.
Tobias Brunner