SSL_COMP_ADD...ESSION_METHOD(3ossl)

💖 SSL_COMP_ADD...ESSION_METHOD 💖

okay bestie, so this is the 3ossl manual page, and it's literally all about how OpenSSL handles SSL/TLS integrated compression methods. yes, compression. in your TLS. we'll get to why that's a whole thing in a sec 🫶

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✨ NAME ✨

SSL_COMP_add_compression_method, SSL_COMP_get_compression_methods, SSL_COMP_get0_name, SSL_COMP_get_id, SSL_COMP_free_compression_methods — handle SSL/TLS integrated compression methods. that's the whole vibe. 💅

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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 has been deprecated since OpenSSL 1.1.0, and can be hidden 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);

⚠️ heads up queen: that last function is deprecated. it's giving legacy. not me telling you to avoid it or anything, just saying the docs said it first.

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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 bestie, this list is globally maintained for all SSL operations within this application. it literally cannot be set for specific SSL_CTX or SSL objects. so no per-context customization, it's one list for everybody. periodt.
• SSL_COMP_get_compression_methods() — returns a stack of all of the available compression methods, or NULL on error. iconic.
• SSL_COMP_get0_name() — returns the name of the compression method comp. slay.
• SSL_COMP_get_id() — returns the id of the compression method comp. yes.
• SSL_COMP_free_compression_methods() — releases any resources acquired to maintain the internal table of compression methods. cleanup era. 🧚

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👑 NOTES 👑

okay so this is where it gets real, so stay with me. 💜

the TLS standard (or SSLv3) does allow 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. it is therefore currently not recommended to integrate compression into applications. no cap. applications for non-public use may agree on certain compression methods though. and important: using different compression methods with the same identifier will lead to connection failure. 💀

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. none. zero. you get the whole list or nothing.

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 either. it's giving no chill.

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 them go 🦋

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🌈 RETURN VALUES 🌈

SSL_COMP_add_compression_method() may return the following values:

• 0 — the operation succeeded. we love to see it. ✨
• 1 — the operation failed. check the error queue to find out the reason. go investigate bestie, the answer is in there.

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.

🙈 yes, that last one really does say it returns the name — we're just relaying what the page says, don't shoot the messenger.

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🦋 SEE ALSO 🦋

• ssl(7)

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⭐ HISTORY ⭐

the SSL_COMP_free_compression_methods() function was deprecated in OpenSSL 1.1.0. and the SSL_COMP_get0_name() and SSL_comp_get_id() functions were added in OpenSSL 1.1.0d. glow-up timeline fr fr.

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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 version 3.5.9, dated 2026-09-2, brought to you by the 3ossl girlies. go forth and compress responsibly (or, y'know, don't — the notes kinda said don't 💅). mwah! 🫶✨