1=pod
2
3=head1 NAME
4
5CMS_decrypt, CMS_decrypt_set1_pkey_and_peer, CMS_decrypt_set1_pkey - decrypt
6content from a CMS envelopedData structure
7
8=head1 SYNOPSIS
9
10 #include <openssl/cms.h>
11
12 int CMS_decrypt(CMS_ContentInfo *cms, EVP_PKEY *pkey, X509 *cert,
13                 BIO *dcont, BIO *out, unsigned int flags);
14 int CMS_decrypt_set1_pkey_and_peer(CMS_ContentInfo *cms,
15                 EVP_PKEY *pk, X509 *cert, X509 *peer);
16 int CMS_decrypt_set1_pkey(CMS_ContentInfo *cms, EVP_PKEY *pk, X509 *cert);
17
18=head1 DESCRIPTION
19
20CMS_decrypt() extracts and decrypts the content from a CMS EnvelopedData
21or AuthEnvelopedData structure. B<pkey> is the private key of the recipient,
22B<cert> is the recipient's certificate, B<out> is a BIO to write the content to
23and B<flags> is an optional set of flags.
24
25The B<dcont> parameter is used in the rare case where the encrypted content
26is detached. It will normally be set to NULL.
27
28CMS_decrypt_set1_pkey_and_peer() associates the private key B<pkey>, the
29corresponding certificate B<cert> and the originator certificate B<peer> with
30the CMS_ContentInfo structure B<cms>.
31
32CMS_decrypt_set1_pkey() associates the private key B<pkey>, corresponding
33certificate B<cert> with the CMS_ContentInfo structure B<cms>.
34
35=head1 NOTES
36
37Although the recipients certificate is not needed to decrypt the data it is
38needed to locate the appropriate (of possible several) recipients in the CMS
39structure.
40
41If B<cert> is set to NULL all possible recipients are tried. This case however
42is problematic. To thwart the MMA attack (Bleichenbacher's attack on
43PKCS #1 v1.5 RSA padding) all recipients are tried whether they succeed or
44not. If no recipient succeeds then a random symmetric key is used to decrypt
45the content: this will typically output garbage and may (but is not guaranteed
46to) ultimately return a padding error only. If CMS_decrypt() just returned an
47error when all recipient encrypted keys failed to decrypt an attacker could
48use this in a timing attack. If the special flag B<CMS_DEBUG_DECRYPT> is set
49then the above behaviour is modified and an error B<is> returned if no
50recipient encrypted key can be decrypted B<without> generating a random
51content encryption key. Applications should use this flag with
52B<extreme caution> especially in automated gateways as it can leave them
53open to attack.
54
55It is possible to determine the correct recipient key by other means (for
56example looking them up in a database) and setting them in the CMS structure
57in advance using the CMS utility functions such as CMS_set1_pkey(). In this
58case both B<cert> and B<pkey> should be set to NULL.
59
60To process KEKRecipientInfo types CMS_set1_key() or CMS_RecipientInfo_set0_key()
61and CMS_RecipientInfo_decrypt() should be called before CMS_decrypt() and
62B<cert> and B<pkey> set to NULL.
63
64The following flags can be passed in the B<flags> parameter.
65
66If the B<CMS_TEXT> flag is set MIME headers for type B<text/plain> are deleted
67from the content. If the content is not of type B<text/plain> then an error is
68returned.
69
70=head1 RETURN VALUES
71
72CMS_decrypt() returns either 1 for success or 0 for failure.
73The error can be obtained from ERR_get_error(3)
74
75=head1 BUGS
76
77The lack of single pass processing and the need to hold all data in memory as
78mentioned in CMS_verify() also applies to CMS_decrypt().
79
80=head1 SEE ALSO
81
82L<ERR_get_error(3)>, L<CMS_encrypt(3)>
83
84=head1 HISTORY
85
86B<CMS_decrypt_set1_pkey_and_peer> was added in OpenSSL 3.0.
87
88=head1 COPYRIGHT
89
90Copyright 2008-2020 The OpenSSL Project Authors. All Rights Reserved.
91
92Licensed under the Apache License 2.0 (the "License").  You may not use
93this file except in compliance with the License.  You can obtain a copy
94in the file LICENSE in the source distribution or at
95L<https://www.openssl.org/source/license.html>.
96
97=cut
98