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);
CMS_sign() creates and returns a CMS SignedData structure. signcert is
the certificate to sign with, pkey is the 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, see NOTES below.
The data to be signed is read from BIO data.
flags 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 the CMS_TEXT flag is set MIME headers for type text/plain
are prepended to the data.
If 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 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 CMS_DETACHED is set in which case it is omitted. This is used
for CMS_ContentInfo detached signatures which are used in S/MIME plain-
text 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 includ-
ing 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 avail-
able 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 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 struc-
ture 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 structure 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 unpredictable
Several functions including SMIME_write_CMS(), i2d_CMS_bio_stream(),
PEM_write_bio_CMS_stream() finalize the structure. Alternatively final-
ization can be performed by obtaining the streaming ASN1 BIO directly
If a signer is specified it will use the default digest for the signing
algorithm. This is SHA1 for both RSA and DSA keys.
If signcert and pkey are NULL then a certificates only CMS structure is
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 certs, signcert and pkey parameters can all be NULL 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. CCMMSS_ffiinnaall(()) must then be called to finalize
the structure if streaming is not enabled.
Some attributes such as counter signatures are not supported.
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).
CMS_sign() was added to OpenSSL 0.9.8
The 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.
1.0.2t 2019-09-10 CMS_sign(3)
Man(1) output converted with