1=pod
2
3=head1 NAME
4
5PKCS7_sign - create a PKCS#7 signedData structure
6
7=head1 SYNOPSIS
8
9PKCS7 *PKCS7_sign(X509 *signcert, EVP_PKEY *pkey, STACK_OF(X509) *certs, BIO *data, int flags);
10
11=head1 DESCRIPTION
12
13PKCS7_sign() creates and returns a PKCS#7 signedData structure. B<signcert>
14is the certificate to sign with, B<pkey> is the corresponsding private key.
15B<certs> is an optional additional set of certificates to include in the
16PKCS#7 structure (for example any intermediate CAs in the chain). 
17
18The data to be signed is read from BIO B<data>.
19
20B<flags> is an optional set of flags.
21
22=head1 NOTES
23
24Any of the following flags (ored together) can be passed in the B<flags> parameter.
25
26Many S/MIME clients expect the signed content to include valid MIME headers. If
27the B<PKCS7_TEXT> flag is set MIME headers for type B<text/plain> are prepended
28to the data.
29
30If B<PKCS7_NOCERTS> is set the signer's certificate will not be included in the
31PKCS7 structure, the signer's certificate must still be supplied in the B<signcert>
32parameter though. This can reduce the size of the signature if the signers certificate
33can be obtained by other means: for example a previously signed message.
34
35The data being signed is included in the PKCS7 structure, unless B<PKCS7_DETACHED> 
36is set in which case it is omitted. This is used for PKCS7 detached signatures
37which are used in S/MIME plaintext signed messages for example.
38
39Normally the supplied content is translated into MIME canonical format (as required
40by the S/MIME specifications) if B<PKCS7_BINARY> is set no translation occurs. This
41option should be used if the supplied data is in binary format otherwise the translation
42will corrupt it.
43
44The signedData structure includes several PKCS#7 autenticatedAttributes including
45the signing time, the PKCS#7 content type and the supported list of ciphers in
46an SMIMECapabilities attribute. If B<PKCS7_NOATTR> is set then no authenticatedAttributes
47will be used. If B<PKCS7_NOSMIMECAP> is set then just the SMIMECapabilities are
48omitted.
49
50If present the SMIMECapabilities attribute indicates support for the following
51algorithms: triple DES, 128 bit RC2, 64 bit RC2, DES and 40 bit RC2. If any
52of these algorithms is disabled then it will not be included.
53
54=head1 BUGS
55
56PKCS7_sign() is somewhat limited. It does not support multiple signers, some
57advanced attributes such as counter signatures are not supported.
58
59The SHA1 digest algorithm is currently always used.
60
61When the signed data is not detached it will be stored in memory within the
62B<PKCS7> structure. This effectively limits the size of messages which can be
63signed due to memory restraints. There should be a way to sign data without
64having to hold it all in memory, this would however require fairly major
65revisions of the OpenSSL ASN1 code.
66
67Clear text signing does not store the content in memory but the way PKCS7_sign()
68operates means that two passes of the data must typically be made: one to compute
69the signatures and a second to output the data along with the signature. There
70should be a way to process the data with only a single pass.
71
72=head1 RETURN VALUES
73
74PKCS7_sign() returns either a valid PKCS7 structure or NULL if an error occurred.
75The error can be obtained from ERR_get_error(3).
76
77=head1 SEE ALSO
78
79L<ERR_get_error(3)|ERR_get_error(3)>, L<PKCS7_verify(3)|PKCS7_verify(3)>
80
81=head1 HISTORY
82
83PKCS7_sign() was added to OpenSSL 0.9.5
84
85=cut
86