SSL_COMP_ADD...ESSION_METHOD(3ossl)

💖 NAME

SSL_COMP_add_compression_method, SSL_COMP_get_compression_methods, SSL_COMP_get0_name, SSL_COMP_get_id, SSL_COMP_free_compression_methods — okay so these are the besties that handle SSL/TLS integrated compression methods 🫶 (yes, from SSL_COMP_ADD...ESSION_METHOD(3ossl), the OpenSSL crew, hi ✨)

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✨ SYNOPSIS

Like, first you gotta let it know what's up:

#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, and bestie… it's been deprecated since OpenSSL 1.1.0 💀 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 this list? It's globally maintained for all SSL operations within this application. So no, you cannot set it for specific SSL_CTX or SSL objects. 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 😭
• SSL_COMP_get0_name() — returns the name of the compression method comp.
• SSL_COMP_get_id() — returns the id of the compression method comp.
• SSL_COMP_free_compression_methods() — releases any resources acquired to maintain the internal table of compression methods. Very "cleaning up my room before guests arrive" energy 🧹

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🌸 NOTES

Okay so here's the tea ☕ The TLS standard (or SSLv3) allows the integration of compression methods into the communication. But the TLS RFC does not specify compression methods or their corresponding identifiers, so there is currently no compatible way to integrate compression with unknown peers. Which is why it is therefore currently not recommended to integrate compression into applications 🙈

Applications for non-public use may agree on certain compression methods. But heads up — using different compression methods with the same identifier will lead to connection failure 💀 not cute.

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. The way there's just… no per-connection knob? Iconic in a bad way ngl 😭

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. There is no way to restrict the list of compression methods supported on a per connection basis. Same vibe, same lack of control 🎀

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()

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🔥 RETURN VALUES

SSL_COMP_add_compression_method() may return the following values:

• 0 — the operation succeeded. slay 💅
• 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. (Yeah, that's literally what the page says — "name" — we don't ask questions, we just report 🫶)

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🎀 SEE ALSO

• ssl(7)

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⭐ HISTORY

So SSL_COMP_free_compression_methods() was deprecated in OpenSSL 1.1.0. And SSL_COMP_get0_name() and SSL_comp_get_id() were added in OpenSSL 1.1.0d. Little glow-up timeline for you ✨

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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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okay that's the whole page bestie, no cap, nothing skipped 🫶

mwah 💋 — this has been SSL_COMP_ADD...ESSION_METHOD(3ossl), OpenSSL version 3.5.6, dated 2026-04-0. go be iconic with ur compression methods ✨