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.\" ========================================================================
.\"
.IX Title "CMS_SIGN 3"
.TH CMS_SIGN 3 "2023-09-11" "1.1.1w" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.nh
.SH "NAME"
CMS_sign \- create a CMS SignedData structure
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/cms.h>
\&
\& CMS_ContentInfo *CMS_sign(X509 *signcert, EVP_PKEY *pkey, STACK_OF(X509) *certs,
\&                           BIO *data, unsigned int flags);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\fBCMS_sign()\fR creates and returns a \s-1CMS\s0 SignedData structure. \fBsigncert\fR is
the certificate to sign with, \fBpkey\fR is the corresponding private key.
\&\fBcerts\fR is an optional additional set of certificates to include in the \s-1CMS\s0
structure (for example any intermediate CAs in the chain). Any or all of
these parameters can be \fB\s-1NULL\s0\fR, see \fB\s-1NOTES\s0\fR below.
.PP
The data to be signed is read from \s-1BIO\s0 \fBdata\fR.
.PP
\&\fBflags\fR is an optional set of flags.
.SH "NOTES"
.IX Header "NOTES"
Any of the following flags (ored together) can be passed in the \fBflags\fR
parameter.
.PP
Many S/MIME clients expect the signed content to include valid \s-1MIME\s0 headers. If
the \fB\s-1CMS_TEXT\s0\fR flag is set \s-1MIME\s0 headers for type \fBtext/plain\fR are prepended
to the data.
.PP
If \fB\s-1CMS_NOCERTS\s0\fR 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 \fBsigncert\fR 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.
.PP
The data being signed is included in the CMS_ContentInfo structure, unless
\&\fB\s-1CMS_DETACHED\s0\fR 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.
.PP
Normally the supplied content is translated into \s-1MIME\s0 canonical format (as
required by the S/MIME specifications) if \fB\s-1CMS_BINARY\s0\fR is set no translation
occurs. This option should be used if the supplied data is in binary format
otherwise the translation will corrupt it.
.PP
The SignedData structure includes several \s-1CMS\s0 signedAttributes including the
signing time, the \s-1CMS\s0 content type and the supported list of ciphers in an
SMIMECapabilities attribute. If \fB\s-1CMS_NOATTR\s0\fR is set then no signedAttributes
will be used. If \fB\s-1CMS_NOSMIMECAP\s0\fR is set then just the SMIMECapabilities are
omitted.
.PP
If present the SMIMECapabilities attribute indicates support for the following
algorithms in preference order: 256 bit \s-1AES,\s0 Gost R3411\-94, Gost 28147\-89, 192
bit \s-1AES, 128\s0 bit \s-1AES,\s0 triple \s-1DES, 128\s0 bit \s-1RC2, 64\s0 bit \s-1RC2, DES\s0 and 40 bit \s-1RC2.\s0
If any of these algorithms is not available then it will not be included: for example the \s-1GOST\s0 algorithms will not be included if the \s-1GOST ENGINE\s0 is
not loaded.
.PP
OpenSSL will by default identify signing certificates using issuer name
and serial number. If \fB\s-1CMS_USE_KEYID\s0\fR 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.
.PP
If the flags \fB\s-1CMS_STREAM\s0\fR is set then the returned \fBCMS_ContentInfo\fR
structure is just initialized ready to perform the signing operation. The
signing is however \fBnot\fR performed and the data to be signed is not read from
the \fBdata\fR parameter. Signing is deferred until after the data has been
written. In this way data can be signed in a single pass.
.PP
If the \fB\s-1CMS_PARTIAL\s0\fR flag is set a partial \fBCMS_ContentInfo\fR structure is
output to which additional signers and capabilities can be added before
finalization.
.PP
If the flag \fB\s-1CMS_STREAM\s0\fR is set the returned \fBCMS_ContentInfo\fR structure is
\&\fBnot\fR complete and outputting its contents via a function that does not
properly finalize the \fBCMS_ContentInfo\fR structure will give unpredictable
results.
.PP
Several functions including \fBSMIME_write_CMS()\fR, \fBi2d_CMS_bio_stream()\fR,
\&\fBPEM_write_bio_CMS_stream()\fR finalize the structure. Alternatively finalization
can be performed by obtaining the streaming \s-1ASN1\s0 \fB\s-1BIO\s0\fR directly using
\&\fBBIO_new_CMS()\fR.
.PP
If a signer is specified it will use the default digest for the signing
algorithm. This is \fB\s-1SHA1\s0\fR for both \s-1RSA\s0 and \s-1DSA\s0 keys.
.PP
If \fBsigncert\fR and \fBpkey\fR are \s-1NULL\s0 then a certificates only \s-1CMS\s0 structure is
output.
.PP
The function \fBCMS_sign()\fR is a basic \s-1CMS\s0 signing function whose output will be
suitable for many purposes. For finer control of the output format the
\&\fBcerts\fR, \fBsigncert\fR and \fBpkey\fR parameters can all be \fB\s-1NULL\s0\fR and the
\&\fB\s-1CMS_PARTIAL\s0\fR flag set. Then one or more signers can be added using the
function \fBCMS_add1_signer()\fR, non default digests can be used and custom
attributes added. \fBCMS_final()\fR must then be called to finalize the
structure if streaming is not enabled.
.SH "BUGS"
.IX Header "BUGS"
Some attributes such as counter signatures are not supported.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
\&\fBCMS_sign()\fR returns either a valid CMS_ContentInfo structure or \s-1NULL\s0 if an error
occurred. The error can be obtained from \fBERR_get_error\fR\|(3).
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBERR_get_error\fR\|(3), \fBCMS_verify\fR\|(3)
.SH "HISTORY"
.IX Header "HISTORY"
The \fB\s-1CMS_STREAM\s0\fR 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.
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright 2008\-2023 The OpenSSL Project Authors. All Rights Reserved.
.PP
Licensed under the OpenSSL license (the \*(L"License\*(R").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file \s-1LICENSE\s0 in the source distribution or at
<https://www.openssl.org/source/license.html>.