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 signers certificate will not be included in the
CMS_ContentInfo structure, the signers 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 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
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
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 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 finalization
can be performed by obtaining the streaming ASN1 BIO directly using
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. CMS_final() must then be called to finalize the
structure if streaming is not enabled.