SSL_CTX_SET_SECURITY_LEVEL(3SSL)

📦

version 4.0.3 · 14 versions

, in ubuntu stonking

🌸 SSL_CTX_set_security_level 🌸 (from the OpenSSL 3SSL manual)

✨ NAME ✨

okay bestie, gather round, because this whole pile of functions is the SSL/TLS security framework 💅:

SSL_CTX_set_security_level, SSL_set_security_level
SSL_CTX_get_security_level, SSL_get_security_level
SSL_CTX_set_security_callback, SSL_set_security_callback
SSL_CTX_get_security_callback, SSL_get_security_callback
SSL_CTX_set0_security_ex_data, SSL_set0_security_ex_data
SSL_CTX_get0_security_ex_data, SSL_get0_security_ex_data

🎀 SYNOPSIS 🎀

literally the whole cast list, typed out exactly so you can copy-paste like the icon you are:

#include <openssl/ssl.h>

void SSL_CTX_set_security_level(SSL_CTX *ctx, int level);
void SSL_set_security_level(SSL *s, int level);

int SSL_CTX_get_security_level(const SSL_CTX *ctx);
int SSL_get_security_level(const SSL *s);

void SSL_CTX_set_security_callback(SSL_CTX *ctx,
                                   int (*cb)(const SSL *s, const SSL_CTX *ctx,
                                             int op, int bits, int nid,
                                             void *other, void *ex));

void SSL_set_security_callback(SSL *s,
                               int (*cb)(const SSL *s, const SSL_CTX *ctx,
                                         int op, int bits, int nid,
                                         void *other, void *ex));

int (*SSL_CTX_get_security_callback(const SSL_CTX *ctx))(const SSL *s,
                                                         const SSL_CTX *ctx,
                                                         int op, int bits,
                                                         int nid, void *other,
                                                         void *ex);
int (*SSL_get_security_callback(const SSL *s))(const SSL *s,
                                               const SSL_CTX *ctx, int op,
                                               int bits, int nid, void *other,
                                               void *ex);

void SSL_CTX_set0_security_ex_data(SSL_CTX *ctx, void *ex);
void SSL_set0_security_ex_data(SSL *s, void *ex);

void *SSL_CTX_get0_security_ex_data(const SSL_CTX *ctx);
void *SSL_get0_security_ex_data(const SSL *s);

🦋 DESCRIPTION 🦋

so, the ✨vibe✨ of this page is: you get to tell OpenSSL how much security you want, and OpenSSL says "say less" and enforces it everywhere.

SSL_CTX_set_security_level() and SSL_set_security_level() set the security level to level. And if you don't set it? bestie, the library default security level is what you get. no notes. 💅
SSL_CTX_get_security_level() and SSL_get_security_level() hand you back the security level you're currently running.
SSL_CTX_set_security_callback(), SSL_set_security_callback(), SSL_CTX_get_security_callback() and SSL_get_security_callback() get or set the security callback attached to ctx or s. If you never set one, a default security callback is used. what those parameters mean and how the default callbacks behave is all written out below, keep reading 🫶
SSL_CTX_set0_security_ex_data(), SSL_set0_security_ex_data(), SSL_CTX_get0_security_ex_data() and SSL_get0_security_ex_data() set the extra data pointer that gets passed to the ex parameter of the callback. this value goes to the callback verbatim — OpenSSL does not peek at it, so set it to literally any application-specific value that's convenient for you. iconic. 💖

👑 DEFAULT CALLBACK BEHAVIOUR 👑

if an application doesn't set its own security callback, the default callback steps in. its whole goal is to give you sane defaults, no cap. and every level means something specific, so here's the tea:

Level 0

Everything is permitted. This retains compatibility with previous versions of OpenSSL. (yes, everything. we're being feral but backwards-compatible 💀)

Level 1

The security level corresponds to a minimum of 80 bits of security. Any parameters offering below 80 bits of security are excluded. As a result RSA, DSA and DH keys shorter than 1024 bits and ECC keys shorter than 160 bits are prohibited. Any cipher suite using MD5 for the MAC is also prohibited. Any cipher suites using CCM with a 64 bit authentication tag are prohibited. Note that signatures using SHA1 and MD5 are also forbidden at this level as they have less than 80 security bits. Additionally, SSLv3, TLS 1.0, TLS 1.1 and DTLS 1.0 are all disabled at this level.

Level 2

Security level set to 112 bits of security. As a result RSA, DSA and DH keys shorter than 2048 bits and ECC keys shorter than 224 bits are prohibited. In addition to the level 1 exclusions any cipher suite using RC4 is also prohibited. Compression is disabled.

Level 3

Security level set to 128 bits of security. As a result RSA, DSA and DH keys shorter than 3072 bits and ECC keys shorter than 256 bits are prohibited. In addition to the level 2 exclusions cipher suites not offering forward secrecy are prohibited. Session tickets are disabled.

Level 4

Security level set to 192 bits of security. As a result RSA, DSA and DH keys shorter than 7680 bits and ECC keys shorter than 384 bits are prohibited. Cipher suites using SHA1 for the MAC are prohibited.

Level 5

Security level set to 256 bits of security. As a result RSA, DSA and DH keys shorter than 15360 bits and ECC keys shorter than 512 bits are prohibited.

bestie these key sizes get big. 🔥 we're talking main-character-sized RSA keys, so know what you're signing up for.

🌈 APPLICATION DEFINED SECURITY CALLBACKS 🌈

an application defined security callback gets installed with SSL_CTX_set_security_callback() or SSL_set_security_callback() and it replaces the default callback. it gets consulted every single time the library needs to decide whether an algorithm, key, parameter or protocol version meets the configured security level. it should return 1 to permit the operation and 0 to reject it. and a rejected parameter is not advertised or used, as appropriate. periodt. 💅

The callback arguments are:

s, ctx — The SSL or SSL_CTX the check applies to. One of the two is NULL: checks originating from a connection pass s (with ctx NULL), and checks originating from a context pass ctx (with s NULL).
op — The operation being checked, one of the SSL_SECOP_* values below.
bits — The number of bits of security the parameter provides, or 0 when not meaningful for op.
nid — An identifier whose meaning depends on op, or NID_undef when not applicable.
other — A pointer to the object being checked, whose type depends on op as listed below. It is NULL when no object is associated with the operation.
ex — The extra data pointer previously set with SSL_CTX_set0_security_ex_data() or SSL_set0_security_ex_data(), or NULL.

Several operations have _SUPPORTED, _SHARED and _CHECK variants, and they are NOT the same thing, okay?

The supported variant checks a parameter before advertising support for it.
The shared variant checks a parameter when selecting from the options offered by both peers.
The check variant checks a parameter the peer has selected before accepting it.

The defined operations, and the corresponding type of other, are:

🌸 the operations 🌸

SSL_SECOP_CIPHER_SUPPORTED, SSL_SECOP_CIPHER_SHARED, SSL_SECOP_CIPHER_CHECK — Advertise/select/accept a TLS cipher suite. other is the SSL_CIPHER.
SSL_SECOP_CURVE_SUPPORTED, SSL_SECOP_CURVE_SHARED, SSL_SECOP_CURVE_CHECK — Advertise/select/accept a named group used for key exchange, either an elliptic curve or a finite-field DH group. nid is the group and other points to its two byte TLS codepoint.
SSL_SECOP_SIGALG_SUPPORTED, SSL_SECOP_SIGALG_SHARED, SSL_SECOP_SIGALG_CHECK, SSL_SECOP_SIGALG_MASK — Advertise/select/accept a signature algorithm used in the handshake; SSL_SECOP_SIGALG_MASK is used when building the mask of permitted signature algorithms. nid is its hash and other points to its two byte TLS codepoint.
SSL_SECOP_VERSION — Negotiate a protocol version. nid is the version, for example TLS1_2_VERSION, and other is NULL.
SSL_SECOP_TMP_DH — Use an ephemeral DH key. other is the EVP_PKEY.
SSL_SECOP_EE_KEY, SSL_SECOP_CA_KEY — Accept an end entity or CA certificate public key. other is the X509 certificate.
SSL_SECOP_TICKET, SSL_SECOP_COMPRESSION — Issue or accept a session ticket, or enable record compression. other is NULL.

💜 NOTES 💜

The default security level can be configured when OpenSSL is compiled by setting -DOPENSSL_TLS_SECURITY_LEVEL=level. If not set then 2 is used.
The security framework disables or reject parameters inconsistent with the set security level. In the past this was difficult as applications had to set a number of distinct parameters (supported ciphers, supported curves supported signature algorithms) to achieve this end and some cases (DH parameter size for example) could not be checked at all. 😭 (the old days were rough, bestie)
By setting an appropriate security level much of this complexity can be avoided. slay. ✨
The bits of security limits affect all relevant parameters including cipher suite encryption algorithms, supported ECC curves, supported signature algorithms, DH parameter sizes, certificate key sizes and signature algorithms. This limit applies no matter what other custom settings an application has set: so if the cipher suite is set to ALL then only cipher suites consistent with the security level are permissible.
See SP800-57 for how the security limits are related to individual algorithms.
Some security levels require large key sizes for non-ECC public key algorithms which can severely degrade performance. For example 256 bits of security requires the use of RSA keys of at least 15360 bits in size.

🫠 yeah. that's not a typo. that's the price of level 5.

Some restrictions can be gracefully handled: for example cipher suites offering insufficient security are not sent by the client and will not be selected by the server. Other restrictions such as the peer certificate key size or the DH parameter size will abort the handshake with a fatal alert. 💀
Attempts to set certificates or parameters with insufficient security are also blocked. For example trying to set a certificate using a 512 bit RSA key or a certificate with a signature with SHA1 digest at level 1 using SSL_CTX_use_certificate(). Applications which do not check the return values for errors will misbehave: for example it might appear that a certificate is not set at all because it had been rejected. (check your return values, bestie, i'm begging 🥺)

💖 RETURN VALUES 💖

SSL_CTX_set_security_level() and SSL_set_security_level() do not return values.
SSL_CTX_get_security_level() and SSL_get_security_level() return a integer that represents the security level with SSL_CTX or SSL, respectively.
SSL_CTX_set_security_callback() and SSL_set_security_callback() do not return values.
SSL_CTX_get_security_callback() and SSL_get_security_callback() return the pointer to the security callback or NULL if the callback is not set.
SSL_CTX_get0_security_ex_data() and SSL_get0_security_ex_data() return the extra data pointer or NULL if the ex data is not set.

🎀 SEE ALSO 🎀

ssl(7)

⭐ HISTORY ⭐

These functions were added in OpenSSL 1.1.0.

okay that's half the story, bestie, and there's more where that came from 🌸 don't go anywhere —

💖 COPYRIGHT 💖

Okay bestie, legal era incoming, and honestly? Respect the paperwork. ✨

Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance with the License. Yeah — it's giving "read the terms, queen," no cap. 🫶

You can grab a copy of the License right here:

in the file LICENSE in the source distribution, or
at <https://www.openssl.org/source/license.html> 🌸

Not me being legally obligated to say this, but fr fr: the license is not optional and the vibes are "compliance is the main character." 💅

And that's a wrap on SSL_CTX_SET_SECURITY_LEVEL(3SSL), bestie — you ate the whole thing. 🔥 This one ships as OpenSSL 4.0.3, dated 2026-09-29. Now go set your security levels responsibly, icon. 👑🎀🦋

📦 where this page comes from

ubuntu stonking: libssl-doc 4.0.3-1ubuntu1 · Apache-2.0
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-11-03: a language model's rewrite of the original, under its terms