libcrypto/man/CMS_sign.3

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.Dd $Mdocdate: April 18 2024 $
.Dt CMS_SIGN 3
.Os
.Sh NAME
.Nm CMS_sign
.Nd create a CMS SignedData structure
.Sh SYNOPSIS
.In openssl/cms.h
.Ft CMS_ContentInfo *
.Fo CMS_sign
.Fa "X509 *signcert"
.Fa "EVP_PKEY *pkey"
.Fa "STACK_OF(X509) *certs"
.Fa "BIO *data"
.Fa "unsigned int flags"
.Fc
.Sh DESCRIPTION
.Fn CMS_sign
creates and returns a CMS
.Vt SignedData
structure.
.Fa signcert
is the certificate to sign with,
.Fa pkey
is the corresponding private key.
.Fa 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
.Dv NULL .
.Pp
The data to be signed is read from
.Fa data .
.Pp
Any of the following flags (OR'ed together) can be passed in the
.Fa flags
argument:
.Bl -tag -width Ds
.It Dv CMS_TEXT
Prepend MIME headers for the type text/plain to the data.
Many S/MIME clients expect the signed content to include valid MIME
headers.
.It Dv CMS_NOCERTS
Do not include the signer's certificate in the
.Vt CMS_ContentInfo
structure.
The signer's certificate must still be supplied in the
.Fa signcert
parameter though.
This can reduce the size of the signature if the signer's certificate can
be obtained by other means, for example from a previously signed message.
.It Dv CMS_DETACHED
Omit the data being signed from the
.Vt CMS_ContentInfo
structure.
This is used for
.Vt CMS_ContentInfo
detached signatures which are used in S/MIME plaintext signed messages
for example.
.It Dv CMS_BINARY
Do not translate the supplied content into MIME canonical format
even though that is required by the S/MIME specifications.
This option should be used if the supplied data is in binary format.
Otherwise the translation will corrupt it.
.It Dv CMS_NOATTR
Do not add any
.Vt SignedAttributes .
By default, the
.Fa signerInfos
field includes several CMS
.Vt SignedAttributes
including the signing time, the CMS content type,
and the supported list of ciphers in an
.Vt SMIMECapabilities
attribute.
.It Dv CMS_NOSMIMECAP
Omit just the
.Vt SMIMECapabilities .
If present, the SMIMECapabilities attribute indicates support for the
following algorithms in preference order: 256-bit AES,
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.
.It Dv CMS_USE_KEYID
Use the subject key identifier value to identify signing certificates.
An error occurs if the signing certificate does not have a subject key
identifier extension.
By default, issuer name and serial number are used instead.
.It Dv CMS_STREAM
Only initialize the returned
.Vt CMS_ContentInfo
structure to prepare it for performing the signing operation.
The signing is however
.Em not
performed and the data to be signed is not read from the
.Fa data
parameter.
Signing is deferred until after the data has been written.
In this way, data can be signed in a single pass.
The returned
.Vt CMS_ContentInfo
structure is
.Em not
complete and outputting its contents via a function that does not
properly finalize the
.Vt CMS_ContentInfo
structure will give unpredictable results.
Several functions including
.Xr SMIME_write_CMS 3 ,
.Xr i2d_CMS_bio_stream 3 ,
or
.Xr PEM_write_bio_CMS_stream 3
finalize the structure.
Alternatively, finalization can be performed by obtaining the streaming
ASN1
.Vt BIO
directly using
.Xr BIO_new_CMS 3 .
.It Dv CMS_PARTIAL
Output a partial
.Vt CMS_ContentInfo
structure to which additional signers and capabilities can be
added before finalization.
.El
.Pp
If a signer is specified, it will use the default digest for the signing
algorithm.
This is SHA1 for both RSA and DSA keys.
.Pp
If
.Fa signcert
and
.Fa pkey
are
.Dv NULL ,
then a certificates only CMS structure is output.
.Pp
The function
.Fn CMS_sign
is a basic CMS signing function whose output will be suitable for many
purposes.
For finer control of the output format the
.Fa certs ,
.Fa signcert
and
.Fa pkey
parameters can all be
.Dv NULL
and the
.Dv CMS_PARTIAL
flag set.
Then one or more signers can be added using the function
.Xr CMS_add1_signer 3 ,
non default digests can be used and custom attributes added.
.Xr CMS_final 3
must then be called to finalize the structure if streaming is not
enabled.
.Sh RETURN VALUES
.Fn CMS_sign
returns either a valid
.Vt CMS_ContentInfo
structure or
.Dv NULL
if an error occurred.
The error can be obtained from
.Xr ERR_get_error 3 .
.Sh SEE ALSO
.Xr CMS_add0_cert 3 ,
.Xr CMS_add1_signer 3 ,
.Xr CMS_ContentInfo_new 3 ,
.Xr CMS_final 3 ,
.Xr CMS_sign_receipt 3 ,
.Xr CMS_verify 3
.Sh STANDARDS
RFC 5652: Cryptographic Message Syntax (CMS)
.Bl -dash -compact -offset indent
.It
section 5.1: SignedData Type
.It
section 5.3: SignerInfo Type
.El
.Pp
RFC 8419: Use of Edwards-Curve Digital Signature Algorithm (EdDSA) Signatures
in the Cryptographic Message Syntax (CMS)
.Pp
RFC 8551: Secure/Multipurpose Internet Mail Extensions (S/MIME)
Version\ 4.0 Message Specification,
section 2.5.2: SMIMECapabilities Attribute
.Sh HISTORY
.Fn CMS_sign
first appeared in OpenSSL 0.9.8h
and has been available since
.Ox 6.7 .
.Sh BUGS
Some attributes such as counter signatures are not supported.