CMS_COMPRESS(3)

📦 version 4.4.0, in alpine 3.24, arch rolling

💅 CMS_compress 💅

okay so bestie, this page is CMS_COMPRESS(3) from the Library Functions Manual — section 3, libcrypto — and the whole vibe is create a CMS CompressedData structure. no cap, it's giving crypto paperwork but make it compressed ✨

✨ 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. Literally its whole job. 💅

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. That's the stuff going in, bestie.

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. Cute little headers, okay? 🎀
  • 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. So no, you can't just crank the zlib level, bestie — not today. 😭

🔥 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). So if you get NULL, go ask ERR_get_error(3) what went wrong, fr fr. 👑

🫶 SEE ALSO 🫶

🌈 STANDARDS 🌈

RFC 3274: Compressed Data Content Type for Cryptographic Message Syntax (CMS) — that's the standard, periodt. ⭐

💜 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. History really said "been here a minute," and honestly? iconic. 👑

And the cute little footer tea, since we're not copy-pasting the line: this is from Debian, dated June 8, 2025, and the page is CMS_COMPRESS(3) — mwah, bye bestie 💋✨

📦 where this page comes from