CMS_sign - create a CMS SignedData structure
CMS_ContentInfo *CMS_sign(X509 *signcert, EVP_PKEY *pkey, STACK_OF(X509) *certs, BIO *data, unsigned int flags);
creates and returns a CMS SignedData structure.
is the certificate to sign with, pkey
corresponding private key. 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 NULL
The data to be signed is read from BIO data
is an optional set of flags.
Any of the following flags (ored together) can be passed in the flags
Many S/MIME clients expect the signed content to include valid MIME headers. If
flag is set MIME headers for type text/plain
prepended to the data.
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 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
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 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 CMS_NOATTR
is set then no
signedAttributes will be used. If 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
OpenSSL will by default identify signing certificates using issuer name and
serial number. If 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 CMS_STREAM
is set then the returned CMS_ContentInfo
structure is just initialized ready to perform the signing operation. The
signing is however not
performed and the data to be signed is not read
from the 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 CMS_PARTIAL
flag is set a partial CMS_ContentInfo
is output to which additional signers and capabilities can be added before
If the flag CMS_STREAM
is set the returned CMS_ContentInfo
structure is not
complete and outputting its contents via a function
that does not properly finalize the CMS_ContentInfo
structure will give
Several functions including SMIME_write_CMS()
structure. Alternatively finalization can be performed by obtaining the
streaming ASN1 BIO
directly using BIO_new_CMS()
If a signer is specified it will use the default digest for the signing
algorithm. This is SHA1
for both RSA and DSA keys.
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
parameters can all be
and the 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.
must then be called to finalize the
structure if streaming is not enabled.
Some attributes such as counter signatures are not supported.
returns either a valid CMS_ContentInfo structure or NULL if an
error occurred. The error can be obtained from ERR_get_error(3)
was added to OpenSSL 0.9.8
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.