CMS_decrypt - decrypt content from a CMS envelopedData structure
int CMS_decrypt(CMS_ContentInfo *cms, EVP_PKEY *pkey, X509 *cert, BIO *dcont, BIO *out, unsigned int flags);
extracts and decrypts the content from a CMS EnvelopedData
is the private key of the recipient, cert
recipient's certificate, out
is a BIO to write the content to and
is an optional set of flags.
parameter is used in the rare case where the encrypted content
is detached. It will normally be set to NULL.
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
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()
returned an error when all recipient encrypted keys failed to decrypt an
attacker could use this in a timing attack. If the special flag
is set then the above behaviour is modified and an
returned if no recipient encrypted key can be decrypted
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()
this case both cert
should be set to NULL.
To process KEKRecipientInfo types CMS_set1_key()
should be called before CMS_decrypt()
set to NULL.
The following flags can be passed in the flags
If the CMS_TEXT
flag is set MIME headers for type text/plain
deleted from the content. If the content is not of type text/plain
an error is returned.
returns either 1 for success or 0 for failure. The error
can be obtained from ERR_get_error
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()
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