=pod

=head1 NAME

PKCS7_sign - create a PKCS#7 signedData structure

=head1 SYNOPSIS

 #include <openssl/pkcs7.h>

 PKCS7 *PKCS7_sign(X509 *signcert, EVP_PKEY *pkey, STACK_OF(X509) *certs,
                   BIO *data, int flags);

=head1 DESCRIPTION

PKCS7_sign() creates and returns a PKCS#7 signedData structure. B<signcert> is
the certificate to sign with, B<pkey> is the corresponding private key.
B<certs> is an optional additional set of certificates to include in the PKCS#7
structure (for example any intermediate CAs in the chain).

The data to be signed is read from BIO B<data>.

B<flags> is an optional set of flags.

Any of the following flags (ored together) can be passed in the B<flags>
parameter.

Many S/MIME clients expect the signed content to include valid MIME headers. If
the B<PKCS7_TEXT> flag is set MIME headers for type B<text/plain> are prepended
to the data.

If B<PKCS7_NOCERTS> is set the signer's certificate will not be included in the
PKCS7 structure, the signer's certificate must still be supplied in the
B<signcert> parameter though. This can reduce the size of the signature if the
signers certificate can be obtained by other means: for example a previously
signed message.

The data being signed is included in the PKCS7 structure, unless
B<PKCS7_DETACHED> is set in which case it is omitted. This is used for PKCS7
detached signatures which are used in S/MIME plaintext signed messages for
example.

Normally the supplied content is translated into MIME canonical format (as
required by the S/MIME specifications) if B<PKCS7_BINARY> is set no translation
occurs. This option should be used if the supplied data is in binary format
otherwise the translation will corrupt it.

The signedData structure includes several PKCS#7 authenticatedAttributes
including the signing time, the PKCS#7 content type and the supported list of
ciphers in an SMIMECapabilities attribute. If B<PKCS7_NOATTR> is set then no
authenticatedAttributes will be used. If B<PKCS7_NOSMIMECAP> is set then just
the SMIMECapabilities are omitted.

If present the SMIMECapabilities attribute indicates support for the following
algorithms: triple DES, 128 bit RC2, 64 bit RC2, DES and 40 bit RC2. If any of
these algorithms is disabled then it will not be included.

If the flags B<PKCS7_STREAM> is set then the returned B<PKCS7> structure is
just initialized ready to perform the signing operation. The signing is however
B<not> performed and the data to be signed is not read from the B<data>
parameter. Signing is deferred until after the data has been written. In this
way data can be signed in a single pass.

If the B<PKCS7_PARTIAL> flag is set a partial B<PKCS7> structure is output to
which additional signers and capabilities can be added before finalization.

If the flag B<PKCS7_STREAM> is set the returned B<PKCS7> structure is B<not>
complete and outputting its contents via a function that does not properly
finalize the B<PKCS7> structure will give unpredictable results.

Several functions including SMIME_write_PKCS7(), i2d_PKCS7_bio_stream(),
PEM_write_bio_PKCS7_stream() finalize the structure. Alternatively finalization
can be performed by obtaining the streaming ASN1 B<BIO> directly using
BIO_new_PKCS7().

If a signer is specified it will use the default digest for the signing
algorithm. This is B<SHA1> for both RSA and DSA keys.

The B<certs>, B<signcert> and B<pkey> parameters can all be
B<NULL> if the B<PKCS7_PARTIAL> flag is set. One or more signers can be added
using the function PKCS7_sign_add_signer(). PKCS7_final() must also be
called to finalize the structure if streaming is not enabled. Alternative
signing digests can also be specified using this method.

If B<signcert> and B<pkey> are NULL then a certificates only
PKCS#7 structure is output.

In versions of OpenSSL before 1.0.0 the B<signcert> and B<pkey> parameters must
B<NOT> be NULL.

=head1 BUGS

Some advanced attributes such as counter signatures are not supported.

=head1 RETURN VALUES

PKCS7_sign() returns either a valid PKCS7 structure or NULL if an error
occurred.  The error can be obtained from ERR_get_error(3).

=head1 SEE ALSO

L<ERR_get_error(3)>, L<PKCS7_verify(3)>

=head1 HISTORY

The B<PKCS7_PARTIAL> flag, and the ability for B<certs>, B<signcert>,
and B<pkey> parameters to be B<NULL> were added in OpenSSL 1.0.0.

The B<PKCS7_STREAM> flag was added in OpenSSL 1.0.0.

=head1 COPYRIGHT

Copyright 2002-2016 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file LICENSE in the source distribution or at
L<https://www.openssl.org/source/license.html>.

=cut