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122 lines
4.9 KiB
Plaintext
122 lines
4.9 KiB
Plaintext
=pod
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=head1 NAME
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CMS_sign - create a CMS SignedData structure
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=head1 SYNOPSIS
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#include <openssl/cms.h>
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CMS_ContentInfo *CMS_sign(X509 *signcert, EVP_PKEY *pkey, STACK_OF(X509) *certs, BIO *data, unsigned int flags);
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=head1 DESCRIPTION
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CMS_sign() creates and returns a CMS SignedData structure. B<signcert> is
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the certificate to sign with, B<pkey> is the corresponding private key.
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B<certs> is an optional additional set of certificates to include in the CMS
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structure (for example any intermediate CAs in the chain). Any or all of
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these parameters can be B<NULL>, see B<NOTES> below.
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The data to be signed is read from BIO B<data>.
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B<flags> is an optional set of flags.
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=head1 NOTES
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Any of the following flags (ored together) can be passed in the B<flags>
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parameter.
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Many S/MIME clients expect the signed content to include valid MIME headers. If
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the B<CMS_TEXT> flag is set MIME headers for type B<text/plain> are prepended
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to the data.
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If B<CMS_NOCERTS> is set the signer's certificate will not be included in the
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CMS_ContentInfo structure, the signer's certificate must still be supplied in
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the B<signcert> parameter though. This can reduce the size of the signature if
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the signers certificate can be obtained by other means: for example a
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previously signed message.
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The data being signed is included in the CMS_ContentInfo structure, unless
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B<CMS_DETACHED> is set in which case it is omitted. This is used for
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CMS_ContentInfo detached signatures which are used in S/MIME plaintext signed
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messages for example.
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Normally the supplied content is translated into MIME canonical format (as
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required by the S/MIME specifications) if B<CMS_BINARY> is set no translation
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occurs. This option should be used if the supplied data is in binary format
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otherwise the translation will corrupt it.
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The SignedData structure includes several CMS signedAttributes including the
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signing time, the CMS content type and the supported list of ciphers in an
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SMIMECapabilities attribute. If B<CMS_NOATTR> is set then no signedAttributes
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will be used. If B<CMS_NOSMIMECAP> is set then just the SMIMECapabilities are
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omitted.
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If present the SMIMECapabilities attribute indicates support for the following
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algorithms in preference order: 256 bit AES, Gost R3411-94, Gost 28147-89, 192
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bit AES, 128 bit AES, triple DES, 128 bit RC2, 64 bit RC2, DES and 40 bit RC2.
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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
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not loaded.
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OpenSSL will by default identify signing certificates using issuer name
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and serial number. If B<CMS_USE_KEYID> is set it will use the subject key
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identifier value instead. An error occurs if the signing certificate does not
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have a subject key identifier extension.
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If the flags B<CMS_STREAM> is set then the returned B<CMS_ContentInfo>
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structure is just initialized ready to perform the signing operation. The
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signing is however B<not> performed and the data to be signed is not read from
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the B<data> parameter. Signing is deferred until after the data has been
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written. In this way data can be signed in a single pass.
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If the B<CMS_PARTIAL> flag is set a partial B<CMS_ContentInfo> structure is
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output to which additional signers and capabilities can be added before
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finalization.
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If the flag B<CMS_STREAM> is set the returned B<CMS_ContentInfo> structure is
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B<not> complete and outputting its contents via a function that does not
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properly finalize the B<CMS_ContentInfo> structure will give unpredictable
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results.
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Several functions including SMIME_write_CMS(), i2d_CMS_bio_stream(),
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PEM_write_bio_CMS_stream() finalize the structure. Alternatively finalization
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can be performed by obtaining the streaming ASN1 B<BIO> directly using
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BIO_new_CMS().
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If a signer is specified it will use the default digest for the signing
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algorithm. This is B<SHA1> for both RSA and DSA keys.
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If B<signcert> and B<pkey> are NULL then a certificates only CMS structure is
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output.
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The function CMS_sign() is a basic CMS signing function whose output will be
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suitable for many purposes. For finer control of the output format the
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B<certs>, B<signcert> and B<pkey> parameters can all be B<NULL> and the
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B<CMS_PARTIAL> flag set. Then one or more signers can be added using the
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function CMS_sign_add1_signer(), non default digests can be used and custom
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attributes added. B<CMS_final()> must then be called to finalize the
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structure if streaming is not enabled.
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=head1 BUGS
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Some attributes such as counter signatures are not supported.
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=head1 RETURN VALUES
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CMS_sign() returns either a valid CMS_ContentInfo structure or NULL if an error
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occurred. The error can be obtained from ERR_get_error(3).
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=head1 SEE ALSO
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L<ERR_get_error(3)|ERR_get_error(3)>, L<CMS_verify(3)|CMS_verify(3)>
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=head1 HISTORY
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CMS_sign() was added to OpenSSL 0.9.8
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The B<CMS_STREAM> flag is only supported for detached data in OpenSSL 0.9.8,
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it is supported for embedded data in OpenSSL 1.0.0 and later.
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=cut
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