LIBRESSL-SSL_CTX_SET_TMP_DH_CALLBACK(3)

📦 version 4.4.0, in alpine 3.24, arch rolling

Okay bestie, welcome to the SSL_CTX_SET_TMP_DH_CALLBACK(3) page from the libssl / libcrypto Library Functions Manual — LibreSSL edition. We're doing DH keys for ephemeral key exchange, and yes, it's giving crypto main character energy. 💖

🌸 NAME 🌸

  • SSL_CTX_set_tmp_dh_callback, SSL_CTX_set_tmp_dh, SSL_set_tmp_dh_callback, SSL_set_tmp_dh — handle DH keys for ephemeral key exchange.

✨ SYNOPSIS ✨

library “libssl” libcrypto #include <openssl/ssl.h>

void
SSL_CTX_set_tmp_dh_callback(SSL_CTX *ctx,
    DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));

long
SSL_CTX_set_tmp_dh(SSL_CTX *ctx, DH *dh);

void
SSL_set_tmp_dh_callback(SSL *ssl,
    DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength);

long
SSL_set_tmp_dh(SSL *ssl, DH *dh);

💅 DESCRIPTION 💅

Okay so here's the group chat summary, but with all the receipts:

  • SSL_CTX_set_tmp_dh_callback() — sets the callback function for ctx to be used when DH parameters are required, and that callback is tmp_dh_callback. The callback is inherited by all ssl objects created from ctx.
  • SSL_CTX_set_tmp_dh() — sets DH parameters to be used by ctx. The key is inherited by all ssl objects created from ctx.
  • SSL_set_tmp_dh_callback() — sets the callback only for ssl.
  • SSL_set_tmp_dh() — sets the parameters only for ssl.
💅 These functions apply to SSL/TLS servers only. Server-side bestie behavior, no client chaos.

When using a cipher with RSA authentication, an ephemeral DH key exchange can take place. In these cases, the session data are negotiated using the ephemeral/temporary DH key, and the key supplied and certified by the certificate chain is only used for signing. Anonymous ciphers (without a permanent server key) also use ephemeral DH keys.

Using ephemeral DH key exchange yields forward secrecy, as the connection can only be decrypted when the DH key is known. By generating a temporary DH key inside the server application that is lost when the application is left, it becomes impossible for attackers to decrypt past sessions, even if they get hold of the normal (certified) key, as this key was only used for signing. Forward secrecy ate and left no crumbs. 🔥

In order to perform a DH key exchange, the server must use a DH group (DH parameters) and generate a DH key. The server will always generate a new DH key during the negotiation.

As generating DH parameters is extremely time consuming, an application should not generate the parameters on the fly but supply the parameters. DH parameters can be reused, as the actual key is newly generated during the negotiation. The risk in reusing DH parameters is that an attacker may specialize on a very often used DH group. Applications should therefore generate their own DH parameters during the installation process using the openssl(1) dhparam application. This application guarantees that "strong" primes are used.

Files dh2048.pem and dh4096.pem in the apps directory of the current version of the OpenSSL distribution contain the ‘SKIP’ DH parameters, which use safe primes and were generated verifiably pseudo-randomly. These files can be converted into C code using the -C option of the openssl(1) dhparam application. Generation of custom DH parameters during installation should still be preferred to stop an attacker from specializing on a commonly used group. The file dh1024.pem contains old parameters that must not be used by applications.

🚨 Not dh1024.pem, bestie. The original says those are old parameters that must not be used by applications. Periodt.

An application may either directly specify the DH parameters or can supply the DH parameters via a callback function.

Previous versions of the callback used the is_export and keylength parameters to control parameter generation for export and non-export cipher suites. Modern servers that do not support export ciphersuites are advised to either use SSL_CTX_set_tmp_dh() or alternatively, use the callback but ignore keylength and is_export and simply supply at least 2048-bit parameters in the callback.

🎀 RETURN VALUES 🎀

  • SSL_CTX_set_tmp_dh() and SSL_set_tmp_dh() do return 1 on success and 0 on failure. Check the error queue to find out the reason of failure.

🦋 EXAMPLES 🦋

Set up DH parameters with a key length of 2048 bits. Error handling is partly left out.

Command-line parameter generation:

openssl dhparam -out dh_param_2048.pem 2048

Code for setting up parameters during server initialization:

SSL_CTX ctx = SSL_CTX_new();
...

/* Set up ephemeral DH parameters. */
DH *dh_2048 = NULL;
FILE *paramfile;
paramfile = fopen("dh_param_2048.pem", "r");
if (paramfile) {
        dh_2048 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
        fclose(paramfile);
} else {
        /* Error. */
}
if (dh_2048 == NULL) {
        /* Error. */
}
if (SSL_CTX_set_tmp_dh(ctx, dh_2048) != 1) {
        /* Error. */
}

🔥 SEE ALSO 🔥

👑 HISTORY 👑

  • SSL_CTX_set_tmp_dh_callback() and SSL_CTX_set_tmp_dh() first appeared in SSLeay 0.8.0 and have been available since OpenBSD 2.4.
  • SSL_set_tmp_dh_callback() and SSL_set_tmp_dh() first appeared in OpenSSL 0.9.2b and have been available since OpenBSD 2.6.

Debian 🌸 June 8, 2025 🌸 SSL_CTX_SET_TMP_DH_CALLBACK(3) — that's the whole page, bestie. Go forth and be iconic with your DH params. 💅✨

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