int CMS_decrypt(CMS_ContentInfo *cms, EVP_PKEY *pkey, X509 *cert, BIO *dcont, BIO *out, unsigned int flags);
CMS_decrypt() extracts and decrypts the content from a CMS EnvelopedData structure. pkey is the private key of the recipient, cert is the recipient's certificate, out is a BIO to write the content to and flags is an optional set of flags.
The dcont parameter is used in the rare case where the encrypted content is detached. It will normally be set to NULL.
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 structure.
If cert is set to NULL all possible recipients are tried. This case however is problematic. To thwart the MMA attack (Bleichenbacher's 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 returned.
CMS_decrypt() returns either 1 for success or 0 for failure. The error can be obtained from ERR_get_error(3)
The lack of single pass processing and the need to hold all data in memory as mentioned in CMS_verify() also applies to CMS_decrypt().