SSL_CTX_SET1_SIGALGS(3ossl)

📦
version 15.1 · 2 versions
, in freebsd 15.1

💖 SSL_CTX_SET1_SIGALGS(3ossl) 💖

okay so bestie, this page lives in the OpenSSL manuals and it's literally all about setting supported signature algorithms — yes the whole squad of 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 and SSL_set1_client_sigalgs_list. eight whole icons 🎀

✨ 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);

🌸 DESCRIPTION 🌸

so here's the tea ☕ — these functions are how you tell OpenSSL which signature algorithms you're willing to vibe with.

🦋 the array girls: SSL_CTX_set1_sigalgs() and SSL_set1_sigalgs()

these two set the supported signature algorithms for ctx or ssl. the array slist of length slistlen must consist of pairs of NIDs corresponding to digest and public key algorithms. like, pairs. two at a time. that's the whole format, no cap 🫶

🩷 the string girls: SSL_CTX_set1_sigalgs_list() and SSL_set1_sigalgs_list()

these ones set the supported signature algorithms for ctx or ssl too, but the str parameter is a null terminated string — 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 — read that again bestie, not the digest names 👀) in the algorithm+hash form are case-insensitive.

Also! If a list entry is preceded with the "?" character, it will be ignored if an implementation is missing. cute little fallback behavior, we love to see it ✨

💅 the client-auth girls

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 identical to SSL_CTX_set1_sigalgs(), SSL_set1_sigalgs(), SSL_CTX_set1_sigalgs_list() and SSL_set1_sigalgs_list(). same energy, different department 💁‍♀️

🎀 the macro moment

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. so like, that memory is yours to manage, bestie 🧚

🔥 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. iykyk ✨

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 are used to determine the set of shared signature algorithms and (if server preferences are set with SSL_OP_CIPHER_SERVER_PREFERENCE) their order. order matters, periodt 🌈

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, the original says "to determined" — we're keeping it, character for character, don't @ me 😭)

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). the security level said no and it meant it 💀

Currently the NID_md5, NID_sha1, NID_sha224, NID_sha256, NID_sha384 and NID_sha512 digest NIDs are supported, and the public key algorithm NIDs EVP_PKEY_RSA, EVP_PKEY_RSA_PSS, EVP_PKEY_DSA and EVP_PKEY_EC.

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. like genuinely bestie, don't. that's not a vibe, that's a vulnerability 🙈

⭐ RETURN VALUES ⭐

All these functions return 1 for success and 0 for failure. one is slay, zero is not 💅

🍓 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");

look at that. two ways to say the exact same thing and both ate 🔥

👑 SEE ALSO 👑

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

🌈 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. a glow-up for the ? prefix, honestly 🦋

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>.

okay that's the whole page bestie, go forth and set your sigalgs responsibly 🫶

mwah 💋 — this was 3.5.6, dated 2026-04-07, and it's the SSL_CTX_SET1_SIGALGS(3ossl) girlies signing off ✨🎀

📦 where this page comes from