OpenSSL_add_all_algorithms() (or equivalent) should be called before using this
function or errors about unknown algorithms will occur.
Although the recipients certificate is not needed to decrypt the data it is
needed to locate the appropriate (of possible several) recipients in the CMS
If cert is set to NULL all possible recipients are tried. This case however
is problematic. To thwart the MMA attack (Bleichenbachers attack on
PKCS #1 v1.5 RSA padding) all recipients are tried whether they succeed or
not. If no recipient succeeds then a random symmetric key is used to decrypt
the content: this will typically output garbage and may (but is not guaranteed
to) ultimately return a padding error only. If CMS_decrypt() just returned an
error when all recipient encrypted keys failed to decrypt an attacker could
use this in a timing attack. If the special flag CMS_DEBUG_DECRYPT is set
then the above behaviour is modified and an error is returned if no
recipient encrypted key can be decrypted without generating a random
content encryption key. Applications should use this flag with
extreme caution especially in automated gateways as it can leave them
open to attack.
It is possible to determine the correct recipient key by other means (for
example looking them up in a database) and setting them in the CMS structure
in advance using the CMS utility functions such as CMS_set1_pkey(). In this
case both cert and pkey should be set to NULL.
To process KEKRecipientInfo types CMS_set1_key() or CMS_RecipientInfo_set0_key()
and CMS_ReceipientInfo_decrypt() should be called before CMS_decrypt() and
cert and pkey set to NULL.
The following flags can be passed in the flags parameter.
If the CMS_TEXT flag is set MIME headers for type text/plain are deleted
from the content. If the content is not of type text/plain then an error is