OpenSSL enforces a minimum of 14 bytes (112 bits) on the key size when
used in FIPS-mode (as required by SP 800-131A). So by using an empty
string, instantiation always failed. 32 bytes (256 bits) should be safe
for now.
Closesstrongswan/strongswan#557
If no valid key is configured (e.g. because it's inadvertently uninitialized),
we should not just reuse the previous key.
The `key_set` flag is not necessary anymore because a non-NULL key is set
during initialization since 6b347d5232 ("openssl: Ensure underlying hash
algorithm is available during HMAC init").
Without this we only would learn that the algorithm isn't actually
available (e.g. due to FIPS mode) when set_key() is called later, so there
isn't any automatic fallback to other implementations.
Fixes#3284.
With OpenSSL commit 929b0d70c19f60227f89fac63f22a21f21950823 setting an empty
key fails if no previous key has been set on that HMAC.
In 9138f49e we explicitly added the check we remove now, as HMAC_Update()
might crash if HMAC_Init_ex() has not been called yet. To avoid that, we
set and check a flag locally to let any get_mac() call fail if set_key() has
not yet been called.
Tobias Brunner