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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
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