SSL_CTX_SET1_SIGALGS(3ossl)

💅 SSL_CTX_SET1_SIGALGS — the OpenSSL glow-up page ✨

okay bestie, so this is the SSL_CTX_SET1_SIGALGS page from OpenSSL (yes the manual is literally named 3ossl, we love that for it) and it's all about telling your TLS stack which signature algorithms you're willing to vibe with. signature algorithms = the digest + public key algorithm combos used to sign stuff. let's get into it 🫶

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✨ NAME ✨

SSL_CTX_set1_sigalgs, SSL_set1_sigalgs, SSL_CTX_set1_sigalgs_list, SSL_set1_sigalgs_list, SSL_CTX_set1_client_sigalgs, SSL_set1_client_sigalgs, SSL_CTX_set1_client_sigalgs_list, SSL_set1_client_sigalgs_list — set supported signature algorithms 💖

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🌸 SYNOPSIS 🌸

#include <openssl/ssl.h>

long SSL_CTX_set1_sigalgs(SSL_CTX *ctx, const int *slist, long slistlen);
long SSL_set1_sigalgs(SSL *ssl, const int *slist, long slistlen);
long SSL_CTX_set1_sigalgs_list(SSL_CTX *ctx, const char *str);
long SSL_set1_sigalgs_list(SSL *ssl, const char *str);

long SSL_CTX_set1_client_sigalgs(SSL_CTX *ctx, const int *slist, long slistlen);
long SSL_set1_client_sigalgs(SSL *ssl, const int *slist, long slistlen);
long SSL_CTX_set1_client_sigalgs_list(SSL_CTX *ctx, const char *str);
long SSL_set1_client_sigalgs_list(SSL *ssl, const char *str);

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🦋 DESCRIPTION 🦋

okay so first up, the OGs:

SSL_CTX_set1_sigalgs() and SSL_set1_sigalgs() set the supported signature algorithms for ctx or ssl. and the way you do that is with an array — the array slist, of length slistlen, has to be made of pairs of NIDs that map to digest and public key algorithms. pair 'em up, no leftovers, periodt. 👑

then the _list girlies:

SSL_CTX_set1_sigalgs_list() and SSL_set1_sigalgs_list() also set the supported signature algorithms for ctx or ssl, but they take a string instead. the str parameter has to be a null terminated string that's a colon separated list of elements, where each element is either:

• a combination of a public key algorithm and a digest separated by +, or
• a TLS 1.3-style named SignatureScheme such as rsa_pss_pss_sha256 ✨

Signature scheme names and public key algorithm names (but not the digest names) in the algorithm+hash form are case-insensitive. iykyk. and if a list entry is preceded with the "?" character, it will be ignored if an implementation is missing — so polite, so considerate 🎀

and now the client-auth twins:

SSL_CTX_set1_client_sigalgs(), SSL_set1_client_sigalgs(), SSL_CTX_set1_client_sigalgs_list() and SSL_set1_client_sigalgs_list() set signature algorithms related to client authentication. otherwise? they are literally identical to SSL_CTX_set1_sigalgs(), SSL_set1_sigalgs(), SSL_CTX_set1_sigalgs_list() and SSL_set1_sigalgs_list(). twins fr fr 👯

🚨 one thing you NEED to know: all these functions are implemented as macros. and the signature algorithm parameter (integer array or string) is not freed — the application should free it, if necessary. not me forgetting to free and leaking all over the place 😭 don't be like that.

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💜 NOTES 💜

• If an application wishes to allow the setting of signature algorithms as one of many user configurable options, it should consider using the more flexible SSL_CONF API instead. (iconic API, very main character ✨)
• The signature algorithms set by a client are used directly in the supported signature algorithm in the client hello message. 📨
• The supported signature algorithms set by a server are not sent to the client, but they're used to determine the set of shared signature algorithms and — if server preferences are set with SSL_OP_CIPHER_SERVER_PREFERENCE — their order.
• The client authentication signature algorithms set by a server are sent in a certificate request message if client authentication is enabled, otherwise they are unused. 💀
• Similarly, client authentication signature algorithms set by a client are used to determined the set of client authentication shared signature algorithms. (yes, "determined" is spelled "determined"... okay the original says "determined", we move 🫶)
• Signature algorithms will neither be advertised nor used if the security level prohibits them (for example SHA1 if the security level is 4 or more). security level said "not on my watch" 🛑
• Currently the NID_md5, NID_sha1, NID_sha224, NID_sha256, NID_sha384 and NID_sha512 digest NIDs are supported, plus the public key algorithm NIDs EVP_PKEY_RSA, EVP_PKEY_RSA_PSS, EVP_PKEY_DSA and EVP_PKEY_EC. that's the whole guest list, bestie 📋
• The short or long name values for digests can be used in a string (for example "MD5", "SHA1", "SHA224", "SHA256", "SHA384", "SHA512") and the public key algorithm strings "RSA", "RSA-PSS", "DSA" or "ECDSA". ✨
• The TLS 1.3 signature scheme names (such as "rsa_pss_pss_sha256") can also be used with the _list forms of the API. 🔥

⚠️ The use of MD5 as a digest is strongly discouraged due to security weaknesses. MD5 is giving expired milk energy, do not. 🙈

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🌈 RETURN VALUES 🌈

All these functions return 1 for success and 0 for failure. that's it, that's the whole vibe check. 1 = slay, 0 = 😭

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⭐ EXAMPLES ⭐

Set supported signature algorithms to SHA256 with ECDSA and SHA256 with RSA using an array:

const int slist[] = {NID_sha256, EVP_PKEY_EC, NID_sha256, EVP_PKEY_RSA};

SSL_CTX_set1_sigalgs(ctx, slist, 4);

Set supported signature algorithms to SHA256 with ECDSA and SHA256 with RSA using a string:

SSL_CTX_set1_sigalgs_list(ctx, "ECDSA+SHA256:RSA+SHA256");

see how easy that was?? ate and left no crumbs 🍓

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👑 SEE ALSO 👑

ssl(7), SSL_get_shared_sigalgs(3), SSL_CONF_CTX_new(3)

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🧚 HISTORY 🧚

Support for ignoring unknown signature algorithms in SSL_CTX_set1_sigalgs_list(), SSL_set1_sigalgs_list(), SSL_CTX_set1_client_sigalgs_list() and SSL_set1_client_sigalgs_list() was added in OpenSSL 3.3. so the "?" behavior is a newer-era blessing ✨

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💖 COPYRIGHT 💖

Copyright 2015-2025 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 2026-09-29, from the SSL_CTX_SET1_SIGALGS(3ossl) manual. go forth, set those sigalgs, and remember: free your arrays, respect the security level, and never ever use MD5 💅🫶✨