NAME¶
CMS_decrypt - decrypt content from a CMS envelopedData structure
SYNOPSIS¶
#include <openssl/cms.h>
int CMS_decrypt(CMS_ContentInfo *cms, EVP_PKEY *pkey, X509 *cert, BIO *dcont, BIO *out, unsigned int flags);
DESCRIPTION¶
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.
NOTES¶
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.
RETURN VALUES¶
CMS_decrypt() returns either 1 for success or 0 for failure. The error
can be obtained from
ERR_get_error(3)
BUGS¶
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().
SEE ALSO¶
ERR_get_error(3),
CMS_encrypt(3)
HISTORY¶
CMS_decrypt() was added to OpenSSL 0.9.8