SSL_CTX_SET1_SIGALGS(3ossl)

📦 version 3.5.0, in opensuse-leap 16.0

🌸 NAME 🌸

okay bestie, gather round 💖 this is the manpage for the whole sigalg squad, and yes they all basically do the same cute thing: set supported signature algorithms. Introducing, in order of appearance:

  • 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

(this is the 3ossl manual, iykyk 🫶)

✨ SYNOPSIS ✨

like, the actual signatures, verbatim, no cap:

#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, the array girlies first: SSL_CTX_set1_sigalgs() and SSL_set1_sigalgs() 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. pairs! like matching outfits 👯‍♀️

then the string girlies: SSL_CTX_set1_sigalgs_list() and SSL_set1_sigalgs_list() also set the supported signature algorithms for ctx or ssl, but the str parameter must be a null terminated string consisting of a colon separated list of elements. and 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. slay, flexible 💅

and the cutest little detail: if a list entry is preceded with the "?" character, it will be ignored if an implementation is missing. it's giving "no hard feelings" 🎀

now the client auth remix: 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 context ✨

⚠️ plot twist: 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 babysit, 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. (hot tip if you're building a whole settings menu 👀)

The signature algorithms set by a client are used directly in the supported signature algorithm in the client hello message. like, straight in, no filter.

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. server keeps those receipts to itself 🤫

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 like that in the original, we love a typo moment 😭)

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 no 💀

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. that's the guest list 🎟️

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. yes queen, both ways 🫶

🚨 The use of MD5 as a digest is strongly discouraged due to security weaknesses. like literally do not. 🙈

⭐ RETURN VALUES ⭐

All these functions return 1 for success and 0 for failure. one is iconic, 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");

see how both do the exact same thing? versatility ate 🔥

🌈 SEE ALSO 🌈

go read those next, bestie, i believe in you 🧚

👑 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. the "?" glow-up is recent, periodt ✨

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 🫶 this was SSL_CTX_SET1_SIGALGS(3ossl), brought to you by 3.5.0, dated 2026-09-25 — mwah, go forth and sign your algorithms 💖🎀🌸

📦 where this page comes from