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+=pod
+
+=head1 NAME
+
+ CMS_sign - create a CMS SignedData structure
+
+=head1 SYNOPSIS
+
+ #include <openssl/cms.h>
+
+ CMS_ContentInfo *CMS_sign(X509 *signcert, EVP_PKEY *pkey, STACK_OF(X509) *certs, BIO *data, unsigned int flags);
+
+=head1 DESCRIPTION
+
+CMS_sign() creates and returns a CMS 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 CMS
+structure (for example any intermediate CAs in the chain). Any or all of
+these parameters can be B<NULL>, see B<NOTES> below.
+
+The data to be signed is read from BIO B<data>.
+
+B<flags> is an optional set of flags.
+
+=head1 NOTES
+
+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<CMS_TEXT> flag is set MIME headers for type B<text/plain> are prepended
+to the data.
+
+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 data being signed is included in the CMS_ContentInfo structure, unless
+B<CMS_DETACHED> is set in which case it is omitted. This is used for
+CMS_ContentInfo 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<CMS_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 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.
+
+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.
+
+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 the flags B<CMS_STREAM> is set then the returned B<CMS_ContentInfo>
+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<CMS_PARTIAL> flag is set a partial B<CMS_ContentInfo> structure is
+output to which additional signers and capabilities can be added before
+finalization.
+
+If the flag B<CMS_STREAM> is set the returned B<CMS_ContentInfo> structure is
+B<not> complete and outputting its contents via a function that does not
+properly finalize the B<CMS_ContentInfo> structure will give unpredictable
+results.
+
+Several functions including SMIME_write_CMS(), i2d_CMS_bio_stream(),
+PEM_write_bio_CMS_stream() finalize the structure. Alternatively finalization
+can be performed by obtaining the streaming ASN1 B<BIO> directly using
+BIO_new_CMS().
+
+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.
+
+If B<signcert> and B<pkey> are NULL then a certificates only CMS structure is
+output.
+
+The function CMS_sign() is a basic CMS signing function whose output will be
+suitable for many purposes. For finer control of the output format the
+B<certs>, B<signcert> and B<pkey> parameters can all be B<NULL> and the
+B<CMS_PARTIAL> flag set. Then one or more signers can be added using the
+function CMS_sign_add1_signer(), non default digests can be used and custom
+attributes added. B<CMS_final()> must then be called to finalize the
+structure if streaming is not enabled.
+
+=head1 BUGS
+
+Some attributes such as counter signatures are not supported.
+
+=head1 RETURN VALUES
+
+CMS_sign() returns either a valid CMS_ContentInfo 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)|ERR_get_error(3)>, L<CMS_verify(3)|CMS_verify(3)>
+
+=head1 HISTORY
+
+CMS_sign() was added to OpenSSL 0.9.8
+
+The B<CMS_STREAM> flag is only supported for detached data in OpenSSL 0.9.8,
+it is supported for embedded data in OpenSSL 1.0.0 and later.
+
+=cut