Only certificates carrying RSA keys are supported so the recipient certificates
supplied to this function must all contain RSA public keys, though they do not
have to be signed using the RSA algorithm.
EVP_des_ede3_cbc() (triple DES) is the algorithm of choice for S/MIME use
because most clients will support it.
The algorithm passed in the cipher parameter must support ASN1 encoding of
Many browsers implement a sign and encrypt option which is simply an S/MIME
envelopedData containing an S/MIME signed message. This can be readily produced
by storing the S/MIME signed message in a memory BIO and passing it to
The following flags can be passed in the flags parameter.
If the CMS_TEXT flag is set MIME headers for type text/plain are
prepended to the data.
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. If CMS_BINARY is set then
CMS_TEXT is ignored.
OpenSSL will by default identify recipient 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 all recipient certificates do not
have a subject key identifier extension.
If the CMS_STREAM flag is set a partial CMS_ContentInfo structure is
returned suitable for streaming I/O: no data is read from the BIO in.
If the CMS_PARTIAL flag is set a partial CMS_ContentInfo structure is
returned to which additional recipients and attributes can be added before
The data being encrypted is included in the CMS_ContentInfo structure, unless
CMS_DETACHED is set in which case it is omitted. This is rarely used in
practice and is not supported by SMIME_write_CMS().
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
The recipients specified in certs use a CMS KeyTransRecipientInfo info
structure. KEKRecipientInfo is also supported using the flag CMS_PARTIAL
The parameter certs may be NULL if CMS_PARTIAL is set and recipients
added later using CMS_add1_recipient_cert() or CMS_add0_recipient_key().