=pod

=head1 NAME

 CMS_encrypt - create a CMS envelopedData structure

=head1 SYNOPSIS

 #include <openssl/cms.h>

 CMS_ContentInfo *CMS_encrypt(STACK_OF(X509) *certs, BIO *in, const EVP_CIPHER *cipher, unsigned int flags);

=head1 DESCRIPTION

CMS_encrypt() creates and returns a CMS EnvelopedData structure. B<certs>
is a list of recipient certificates. B<in> is the content to be encrypted.
B<cipher> is the symmetric cipher to use. B<flags> is an optional set of flags.

=head1 NOTES

Only certificates carrying RSA, Diffie-Hellman or EC keys are supported by this
function.

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 B<cipher> parameter must support ASN1 encoding of
its parameters. 

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
CMS_encrypt().

The following flags can be passed in the B<flags> parameter.

If the B<CMS_TEXT> flag is set MIME headers for type B<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 B<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 B<CMS_BINARY> is set then
B<CMS_TEXT> is ignored.

OpenSSL will by default identify recipient certificates using issuer name
and serial number. If B<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 B<CMS_STREAM> flag is set a partial B<CMS_ContentInfo> structure is
returned suitable for streaming I/O: no data is read from the BIO B<in>.

If the B<CMS_PARTIAL> flag is set a partial B<CMS_ContentInfo> structure is
returned to which additional recipients and attributes can be added before
finalization.

The data being encrypted is included in the CMS_ContentInfo structure, unless
B<CMS_DETACHED> is set in which case it is omitted. This is rarely used in
practice and is not supported by SMIME_write_CMS().

=head1 NOTES

If the flag B<CMS_STREAM> is set the returned B<CMS_ContentInfo> structure is
B<not> complete and outputting its contents via a function that does not
properly finalize the B<CMS_ContentInfo> structure will give unpredictable
results.

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 B<BIO> directly using
BIO_new_CMS().

The recipients specified in B<certs> use a CMS KeyTransRecipientInfo info
structure. KEKRecipientInfo is also supported using the flag B<CMS_PARTIAL>
and CMS_add0_recipient_key().

The parameter B<certs> may be NULL if B<CMS_PARTIAL> is set and recipients
added later using CMS_add1_recipient_cert() or CMS_add0_recipient_key().

=head1 RETURN VALUES

CMS_encrypt() returns either a CMS_ContentInfo structure or NULL if an error
occurred. The error can be obtained from ERR_get_error(3).

=head1 SEE ALSO

L<ERR_get_error(3)|ERR_get_error(3)>, L<CMS_decrypt(3)|CMS_decrypt(3)>

=head1 HISTORY

CMS_decrypt() was added to OpenSSL 0.9.8
The B<CMS_STREAM> flag was first supported in OpenSSL 1.0.0.

=cut