1=pod
2
3=head1 NAME
4
5CMS_decrypt - decrypt content from a CMS envelopedData structure
6
7=head1 SYNOPSIS
8
9 #include <openssl/cms.h>
10
11 int CMS_decrypt(CMS_ContentInfo *cms, EVP_PKEY *pkey, X509 *cert,
12                 BIO *dcont, BIO *out, unsigned int flags);
13
14=head1 DESCRIPTION
15
16CMS_decrypt() extracts and decrypts the content from a CMS EnvelopedData
17structure. B<pkey> is the private key of the recipient, B<cert> is the
18recipient's certificate, B<out> is a BIO to write the content to and
19B<flags> is an optional set of flags.
20
21The B<dcont> parameter is used in the rare case where the encrypted content
22is detached. It will normally be set to NULL.
23
24=head1 NOTES
25
26Although the recipients certificate is not needed to decrypt the data it is
27needed to locate the appropriate (of possible several) recipients in the CMS
28structure.
29
30If B<cert> is set to NULL all possible recipients are tried. This case however
31is problematic. To thwart the MMA attack (Bleichenbacher's attack on
32PKCS #1 v1.5 RSA padding) all recipients are tried whether they succeed or
33not. If no recipient succeeds then a random symmetric key is used to decrypt
34the content: this will typically output garbage and may (but is not guaranteed
35to) ultimately return a padding error only. If CMS_decrypt() just returned an
36error when all recipient encrypted keys failed to decrypt an attacker could
37use this in a timing attack. If the special flag B<CMS_DEBUG_DECRYPT> is set
38then the above behaviour is modified and an error B<is> returned if no
39recipient encrypted key can be decrypted B<without> generating a random
40content encryption key. Applications should use this flag with
41B<extreme caution> especially in automated gateways as it can leave them
42open to attack.
43
44It is possible to determine the correct recipient key by other means (for
45example looking them up in a database) and setting them in the CMS structure
46in advance using the CMS utility functions such as CMS_set1_pkey(). In this
47case both B<cert> and B<pkey> should be set to NULL.
48
49To process KEKRecipientInfo types CMS_set1_key() or CMS_RecipientInfo_set0_key()
50and CMS_RecipientInfo_decrypt() should be called before CMS_decrypt() and
51B<cert> and B<pkey> set to NULL.
52
53The following flags can be passed in the B<flags> parameter.
54
55If the B<CMS_TEXT> flag is set MIME headers for type B<text/plain> are deleted
56from the content. If the content is not of type B<text/plain> then an error is
57returned.
58
59=head1 RETURN VALUES
60
61CMS_decrypt() returns either 1 for success or 0 for failure.
62The error can be obtained from ERR_get_error(3)
63
64=head1 BUGS
65
66The lack of single pass processing and the need to hold all data in memory as
67mentioned in CMS_verify() also applies to CMS_decrypt().
68
69=head1 SEE ALSO
70
71L<ERR_get_error(3)>, L<CMS_encrypt(3)>
72
73=head1 COPYRIGHT
74
75Copyright 2008-2016 The OpenSSL Project Authors. All Rights Reserved.
76
77Licensed under the OpenSSL license (the "License").  You may not use
78this file except in compliance with the License.  You can obtain a copy
79in the file LICENSE in the source distribution or at
80L<https://www.openssl.org/source/license.html>.
81
82=cut
83