CMS_COMPRESS(3) | Library Functions Manual | CMS_COMPRESS(3)

# NAME

`CMS_compress` — create a CMS CompressedData structure

# SYNOPSIS

library “libcrypto”
`#include <openssl/cms.h>`

CMS_ContentInfo *
`CMS_compress`(BIO *in, int comp_nid, unsigned int flags);

# DESCRIPTION

`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. Currently, the default algorithm `NID_zlib_compression` is the only supported algorithm. If zlib support is not compiled in, `CMS_compress`() always returns an error.

in provides the content to be compressed.

Any of the following flags (OR'ed together) can be passed in the flags parameter:

`CMS_TEXT`
  Prepend MIME headers for type text/plain to the data.
`CMS_BINARY`
  Do not translate the supplied content into MIME canonical format, even though that is required by the S/MIME specifications. This option should be used if the supplied data is in binary format. Otherwise, the translation will corrupt it. If `CMS_BINARY` is set, `CMS_TEXT` is ignored.
`CMS_STREAM`
  Return a partial CMS_ContentInfo structure suitable for streaming I/O: no data is read from in. Several functions including SMIME_write_CMS(3), i2d_CMS_bio_stream(3), or PEM_write_bio_CMS_stream(3) can be used to finalize the structure. Alternatively, finalization can be performed by obtaining the streaming ASN1 BIO directly using BIO_new_CMS(3). Outputting the contents of the CMS_ContentInfo structure via a function that does not properly finalize it will give unpredictable results.
`CMS_DETACHED`
  Do not include the compressed data in the CMS_ContentInfo structure. This is rarely used in practice and is not supported by SMIME_write_CMS(3).

Additional compression parameters such as the zlib compression level cannot currently be set.

# RETURN VALUES

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

# SEE ALSO

CMS_ContentInfo_new(3), CMS_uncompress(3)

# STANDARDS

RFC 3274: Compressed Data Content Type for Cryptographic Message Syntax (CMS)

# HISTORY

`CMS_compress`() first appeared in OpenSSL 0.9.8h and has been available since OpenBSD 6.7.

The `CMS_STREAM` flag first appeared in OpenSSL 1.0.0.

June 8, 2025 | Debian