SSL_COMP_ADD...ESSION_METHOD(3ossl)

💅 SSL_COMP_ADD...ESSION_METHOD — the 3ossl of it all

okay bestie, pull up a chair 🎀 this one's a manpage from the 3ossl fam and it's all about SSL/TLS integrated compression methods — how to add them, list them, name them, id them, and (if you're on old code) free them. it's giving "niche but powerful", no cap.

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✨ SYNOPSIS ✨

#include <openssl/ssl.h>

int SSL_COMP_add_compression_method(int id, COMP_METHOD *cm);
STACK_OF(SSL_COMP) *SSL_COMP_get_compression_methods(void);
const char *SSL_COMP_get0_name(const SSL_COMP *comp);
int SSL_COMP_get_id(const SSL_COMP *comp);

and then there's this one, which is a whole deprecated situation, so listen up 🫶:

the function below has been deprecated since OpenSSL 1.1.0, and you can hide it entirely by defining OPENSSL_API_COMPAT with a suitable version value — peep openssl_user_macros(7) for the deets:

void SSL_COMP_free_compression_methods(void);

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🌸 DESCRIPTION 🌸

• SSL_COMP_add_compression_method() — adds the compression method cm with the identifier id to the list of available compression methods. and like, this list is globally maintained for all SSL operations within this application — you literally cannot set it per SSL_CTX or per SSL object. one list to rule them all, periodt 👑
• SSL_COMP_get_compression_methods() — returns a stack of all of the available compression methods, or NULL on error. iconic little stack 🥞
• SSL_COMP_get0_name() — returns the name of the compression method comp. simple, clean, slay.
• SSL_COMP_get_id() — returns the id of the compression method comp. yeah.
• SSL_COMP_free_compression_methods() — releases any resources acquired to maintain the internal table of compression methods. the cleanup queen 🧹 (but remember: deprecated, see above!)

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🦋 NOTES 🦋

so here's the tea ☕ the TLS standard (or SSLv3) does allow compression methods to be integrated into the communication. but the TLS RFC does not specify compression methods or their corresponding identifiers, so rn there's no compatible way to integrate compression with unknown peers. translation: it is currently not recommended to integrate compression into applications 😭

applications for non-public use may agree on certain compression methods — y'all can vibe on your own protocol. but using different compression methods with the same identifier will lead to connection failure. 💀 not the vibe.

An OpenSSL client speaking a protocol that allows compression (SSLv3, TLSv1) will unconditionally send the list of all compression methods enabled with SSL_COMP_add_compression_method() to the server during the handshake. and unlike the mechanisms to set a cipher list, there is no method available to restrict the list of compression method on a per connection basis. soz 🙈

An OpenSSL server will match the identifiers listed by a client against its own compression methods and will unconditionally activate compression when a matching identifier is found. again: no way to restrict the list of compression methods supported on a per connection basis. it's all or nothing, bestie.

if enabled during compilation, the OpenSSL library will have the following compression methods available:

• COMP_zlib()
• COMP_brotli()
• COMP_brotli_oneshot()
• COMP_zstd()
• COMP_zstd_oneshot()

look at that lineup 🔥 five whole methods, we love to see it.

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🎀 RETURN VALUES 🎀

SSL_COMP_add_compression_method() may return the following values:

• 0 — the operation succeeded. we ate. 💅
• 1 — the operation failed. check the error queue to find out the reason. 🔍

SSL_COMP_get_compression_methods() returns the stack of compressions methods or NULL on error.

SSL_COMP_get0_name() returns the name of the compression method or NULL on error.

SSL_COMP_get_id() returns the name of the compression method or -1 on error.

okay yes, the page says "name" for SSL_COMP_get_id() too 😭 i'm not making that up, i'm literally just reading the manpage to you. and yes, "compressions methods" — the plural is a little quirky, we're keeping it fr fr.

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🌈 SEE ALSO 🌈

• ssl(7)

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👑 HISTORY 👑

• SSL_COMP_free_compression_methods() was deprecated in OpenSSL 1.1.0.
• SSL_COMP_get0_name() and SSL_comp_get_id() were added in OpenSSL 1.1.0d.

note the lowercase c in SSL_comp_get_id() — that's exactly how the page spells it, so i'm spelling it that way too 🫡✨

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💜 COPYRIGHT 💜

Copyright 2001-2020 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.

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and that's the whole page bestie 💖 this was OpenSSL 3.5.0, dated 2026-09-2, signing off with the biggest 3ossl energy. go forth and compress responsibly (or don't, honestly, the docs say don't) 🧚⭐