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=pod
=head1 NAME
CMS_decrypt - decrypt content from a CMS envelopedData structure
=head1 SYNOPSIS
#include <openssl/cms.h>
int CMS_decrypt(CMS_ContentInfo *cms, EVP_PKEY *pkey, X509 *cert, BIO *dcont, BIO *out, unsigned int flags);
=head1 DESCRIPTION
CMS_decrypt() extracts and decrypts the content from a CMS EnvelopedData
structure. B<pkey> is the private key of the recipient, B<cert> is the
recipient's certificate, B<out> is a BIO to write the content to and
B<flags> is an optional set of flags.
The B<dcont> parameter is used in the rare case where the encrypted content
is detached. It will normally be set to NULL.
=head1 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 B<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 B<CMS_DEBUG_DECRYPT> is set
then the above behaviour is modified and an error B<is> returned if no
recipient encrypted key can be decrypted B<without> generating a random
content encryption key. Applications should use this flag with
B<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 B<cert> and B<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
B<cert> and B<pkey> set to NULL.
The following flags can be passed in the B<flags> parameter.
If the B<CMS_TEXT> flag is set MIME headers for type B<text/plain> are deleted
from the content. If the content is not of type B<text/plain> then an error is
returned.
=head1 RETURN VALUES
CMS_decrypt() returns either 1 for success or 0 for failure.
The error can be obtained from ERR_get_error(3)
=head1 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().
=head1 SEE ALSO
L<ERR_get_error(3)|ERR_get_error(3)>, L<CMS_encrypt(3)|CMS_encrypt(3)>
=head1 HISTORY
CMS_decrypt() was added to OpenSSL 0.9.8
=cut