SSL_CTX_SET1_SIGALGS(3SSL)

📦
version 4.0.3 · 13 versions
, in ubuntu stonking

✨ SSL_CTX_SET1_SIGALGS ✨ (the 3SSL manual, bestie edition)

okay so. bestie. let me tell you about the slayest little family of functions in OpenSSL: 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. they set supported signature algorithms. that's the whole vibe. 🌸

💫 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 💅

🦋 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, bestie. 🍓

SSL_CTX_set1_sigalgs() and SSL_set1_sigalgs() set the supported signature algorithms for ctx or ssl. the array slist, of length slistlen, has to be made of pairs of NIDs that correspond to digest and public key algorithms. pairs, okay? like a buddy system. no third-wheeling.

SSL_CTX_set1_sigalgs_list() and SSL_set1_sigalgs_list() also set the supported signature algorithms for ctx or ssl, but this time via the str parameter. str must be a null terminated string that's a colon separated list of elements, and each element is either:

  • a combo of a public key algorithm and a digest, separated by +, or
  • a TLS 1.3-style named SignatureScheme like rsa_pss_pss_sha256.

in the algorithm+hash form, the signature scheme names and the public key algorithm names are case-insensitive — but the digest names are NOT. 🙈

and ooh, fun detail: if a list entry is preceded with the "?" character, it'll just be ignored if an implementation is missing. no drama, no crash, it just ghosts you. 👑

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. other than that, they're literally identical to SSL_CTX_set1_sigalgs(), SSL_set1_sigalgs(), SSL_CTX_set1_sigalgs_list() and SSL_set1_sigalgs_list(). same energy, different target. twin vibes fr fr.

heads up bestie: 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. that's on you, not on OpenSSL. 😭

🔥 NOTES 🔥

if an application wants to let signature algorithms be set as one of many user configurable options, it should consider using the more flexible SSL_CONF API instead. more options, more power, more slay. ✨

the signature algorithms set by a client are used directly in the supported signature algorithm in the client hello message. sending it straight to the group chat, no filter.

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_SERVER_PREFERENCE, their order too. order matters, always does. 💅

the client authentication signature algorithms set by a server are sent in a certificate request message if client authentication is enabled. otherwise? unused. sitting there, unemployed. 💀

similarly, client authentication signature algorithms set by a client are used to determine the set of client authentication shared signature algorithms.

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 no.

currently the supported digest NIDs are: NID_md5, NID_sha1, NID_sha224, NID_sha256, NID_sha384 and NID_sha512. and the supported public key algorithm NIDs are: 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 are "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. iconic. 🌈

⚠️ the use of MD5 as a digest is strongly discouraged due to security weaknesses. like, do not. i'm begging. it's giving 2004. 🚫

⭐ RETURN VALUES ⭐

all these functions return 1 for success and 0 for failure. one for yay, zero for nay. simple as. 💖

🌸 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 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. glow up era. ✨

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>.

and that's the whole page bestie, no cap. this was OpenSSL 4.0.3, dated 2026-09-29, filed under SSL_CTX_SET1_SIGALGS(3SSL). go set some sigalgs, you absolute main character. 🫶💖✨

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