RAND_add()/RAND_seed(): fix failure on short input or low entropy
Commit 5b4cb385 (#7382) introduced a bug which had the effect that RAND_add()/RAND_seed() failed for buffer sizes less than 32 bytes. The reason was that now the added random data was used exlusively as entropy source for reseeding. When the random input was too short or contained not enough entropy, the DRBG failed without querying the available entropy sources. This commit makes drbg_add() act smarter: it checks the entropy requirements explicitely. If the random input fails this check, it won't be added as entropy input, but only as additional data. More precisely, the behaviour depends on whether an os entropy source was configured (which is the default on most os): - If an os entropy source is avaible then we declare the buffer content as additional data by setting randomness to zero and trigger a regular reseeding. - If no os entropy source is available, a reseeding will fail inevitably. So drbg_add() uses a trick to mix the buffer contents into the DRBG state without forcing a reseeding: it generates a dummy random byte, using the buffer content as additional data. Related-to: #7449 Reviewed-by: Paul Dale <paul.dale@oracle.com> (Merged from https://github.com/openssl/openssl/pull/7456) (cherry picked from commit 8817215d)
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