Import OpenSSL-1.0.1h.
[dragonfly.git] / crypto / openssl / doc / crypto / CMS_decrypt.pod
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1=pod
2
3=head1 NAME
4
5 CMS_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, BIO *dcont, BIO *out, unsigned int flags);
12
13=head1 DESCRIPTION
14
15CMS_decrypt() extracts and decrypts the content from a CMS EnvelopedData
16structure. B<pkey> is the private key of the recipient, B<cert> is the
17recipient's certificate, B<out> is a BIO to write the content to and
18B<flags> is an optional set of flags.
19
20The B<dcont> parameter is used in the rare case where the encrypted content
21is detached. It will normally be set to NULL.
22
23=head1 NOTES
24
25OpenSSL_add_all_algorithms() (or equivalent) should be called before using this
26function or errors about unknown algorithms will occur.
27
28Although the recipients certificate is not needed to decrypt the data it is
29needed to locate the appropriate (of possible several) recipients in the CMS
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30structure.
31
32If B<cert> is set to NULL all possible recipients are tried. This case however
33is problematic. To thwart the MMA attack (Bleichenbacher's attack on
34PKCS #1 v1.5 RSA padding) all recipients are tried whether they succeed or
35not. If no recipient succeeds then a random symmetric key is used to decrypt
36the content: this will typically output garbage and may (but is not guaranteed
37to) ultimately return a padding error only. If CMS_decrypt() just returned an
38error when all recipient encrypted keys failed to decrypt an attacker could
39use this in a timing attack. If the special flag B<CMS_DEBUG_DECRYPT> is set
40then the above behaviour is modified and an error B<is> returned if no
41recipient encrypted key can be decrypted B<without> generating a random
42content encryption key. Applications should use this flag with
43B<extreme caution> especially in automated gateways as it can leave them
44open to attack.
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45
46It is possible to determine the correct recipient key by other means (for
47example looking them up in a database) and setting them in the CMS structure
48in advance using the CMS utility functions such as CMS_set1_pkey(). In this
49case both B<cert> and B<pkey> should be set to NULL.
50
51To process KEKRecipientInfo types CMS_set1_key() or CMS_RecipientInfo_set0_key()
52and CMS_ReceipientInfo_decrypt() should be called before CMS_decrypt() and
53B<cert> and B<pkey> set to NULL.
54
55The following flags can be passed in the B<flags> parameter.
56
57If the B<CMS_TEXT> flag is set MIME headers for type B<text/plain> are deleted
58from the content. If the content is not of type B<text/plain> then an error is
59returned.
60
61=head1 RETURN VALUES
62
63CMS_decrypt() returns either 1 for success or 0 for failure.
64The error can be obtained from ERR_get_error(3)
65
66=head1 BUGS
67
68The lack of single pass processing and the need to hold all data in memory as
69mentioned in CMS_verify() also applies to CMS_decrypt().
70
71=head1 SEE ALSO
72
73L<ERR_get_error(3)|ERR_get_error(3)>, L<CMS_encrypt(3)|CMS_encrypt(3)>
74
75=head1 HISTORY
76
77CMS_decrypt() was added to OpenSSL 0.9.8
78
79=cut