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+=pod
+
+=head1 NAME
+
+CMS_add1_signer, CMS_SignerInfo_sign - add a signer to a CMS_ContentInfo signed data structure
+
+=head1 SYNOPSIS
+
+ #include <openssl/cms.h>
+
+ CMS_SignerInfo *CMS_add1_signer(CMS_ContentInfo *cms, X509 *signcert,
+ EVP_PKEY *pkey, const EVP_MD *md,
+ unsigned int flags);
+
+ int CMS_SignerInfo_sign(CMS_SignerInfo *si);
+
+=head1 DESCRIPTION
+
+CMS_add1_signer() adds a signer with certificate B<signcert> and private
+key B<pkey> using message digest B<md> to CMS_ContentInfo SignedData
+structure B<cms>.
+
+The CMS_ContentInfo structure should be obtained from an initial call to
+CMS_sign() with the flag B<CMS_PARTIAL> set or in the case or re-signing a
+valid CMS_ContentInfo SignedData structure.
+
+If the B<md> parameter is B<NULL> then the default digest for the public
+key algorithm will be used.
+
+Unless the B<CMS_REUSE_DIGEST> flag is set the returned CMS_ContentInfo
+structure is not complete and must be finalized either by streaming (if
+applicable) or a call to CMS_final().
+
+The CMS_SignerInfo_sign() function will explicitly sign a CMS_SignerInfo
+structure, its main use is when B<CMS_REUSE_DIGEST> and B<CMS_PARTIAL> flags
+are both set.
+
+=head1 NOTES
+
+The main purpose of CMS_add1_signer() is to provide finer control
+over a CMS signed data structure where the simpler CMS_sign() function defaults
+are not appropriate. For example if multiple signers or non default digest
+algorithms are needed. New attributes can also be added using the returned
+CMS_SignerInfo structure and the CMS attribute utility functions or the
+CMS signed receipt request functions.
+
+Any of the following flags (ored together) can be passed in the B<flags>
+parameter.
+
+If B<CMS_REUSE_DIGEST> is set then an attempt is made to copy the content
+digest value from the CMS_ContentInfo structure: to add a signer to an existing
+structure. An error occurs if a matching digest value cannot be found to copy.
+The returned CMS_ContentInfo structure will be valid and finalized when this
+flag is set.
+
+If B<CMS_PARTIAL> is set in addition to B<CMS_REUSE_DIGEST> then the
+CMS_SignerInfo structure will not be finalized so additional attributes
+can be added. In this case an explicit call to CMS_SignerInfo_sign() is
+needed to finalize it.
+
+If B<CMS_NOCERTS> is set the signer's certificate will not be included in the
+CMS_ContentInfo 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 SignedData structure includes several CMS signedAttributes including the
+signing time, the CMS content type and the supported list of ciphers in an
+SMIMECapabilities attribute. If B<CMS_NOATTR> is set then no signedAttributes
+will be used. If B<CMS_NOSMIMECAP> is set then just the SMIMECapabilities are
+omitted.
+
+OpenSSL will by default identify signing certificates using issuer name
+and serial number. If B<CMS_USE_KEYID> is set it will use the subject key
+identifier value instead. An error occurs if the signing certificate does not
+have a subject key identifier extension.
+
+If present the SMIMECapabilities attribute indicates support for the following
+algorithms in preference order: 256 bit AES, Gost R3411-94, Gost 28147-89, 192
+bit AES, 128 bit AES, triple DES, 128 bit RC2, 64 bit RC2, DES and 40 bit RC2.
+If any of these algorithms is not available then it will not be included: for example the GOST algorithms will not be included if the GOST ENGINE is
+not loaded.
+
+CMS_add1_signer() returns an internal pointer to the CMS_SignerInfo
+structure just added, this can be used to set additional attributes
+before it is finalized.
+
+=head1 RETURN VALUES
+
+CMS_add1_signer() returns an internal pointer to the CMS_SignerInfo
+structure just added or NULL if an error occurs.
+
+=head1 SEE ALSO
+
+L<ERR_get_error(3)>, L<CMS_sign(3)>,
+L<CMS_final(3)>,
+
+=head1 COPYRIGHT
+
+Copyright 2014-2016 The OpenSSL Project Authors. All Rights Reserved.
+
+Licensed under the OpenSSL license (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