CMS_compress — create a CMS CompressedData structure
CMS_ContentInfo *CMS_compress(BIO *in, int comp_nid, unsigned int flags);
CMS_compress() creates and returns a CMS CompressedData structure. comp_nid is the compression algorithm to use or NID_undef to use the default algorithm (zlib compression). in is the content to be compressed. flags is an optional set of flags.
The only currently supported compression algorithm is zlib using the NID NID_zlib_compression.
If zlib support is not compiled into OpenSSL then CMS_compress() will return an error.
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.
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.
The compressed data 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 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 BIO directly using BIO_new_CMS().
Additional compression parameters such as the zlib compression level cannot currently be set.
CMS_compress() returns either a CMS_ContentInfo structure or NULL if an error occurred. The error can be obtained from ERR_get_error(3).
CMS_compress() was added to OpenSSL 0.9.8 The CMS_STREAM flag was first supported in OpenSSL 1.0.0.