DH_METH_NEW(3ossl)

💅 DH_METH_NEW(3ossl) — or, "how to build your own DH method, bestie"

Okay so real talk: this whole page is DH_METH_NEW(3ossl), and it's the manual for the little family of functions that let you build up custom DH methods — like, you get to say "no fr, I'll handle the DH math, thank you" and OpenSSL will respect it. ✨

Big mood warning before we even start, though:

🚨 Every single function on this page is deprecated. Like, all of them. Since OpenSSL 3.0. Applications should use the provider APIs instead. Not me being dramatic, that's literally the first line of DESCRIPTION. 😭

🌸 SYNOPSIS 🌸

#include <openssl/dh.h>

The following functions have been deprecated since OpenSSL 3.0, and can be hidden entirely by defining OPENSSL_API_COMPAT with a suitable version value, see openssl_user_macros(7):

DH_METHOD *DH_meth_new(const char *name, int flags);

void DH_meth_free(DH_METHOD *dhm);

DH_METHOD *DH_meth_dup(const DH_METHOD *dhm);

const char *DH_meth_get0_name(const DH_METHOD *dhm);
int DH_meth_set1_name(DH_METHOD *dhm, const char *name);

int DH_meth_get_flags(const DH_METHOD *dhm);
int DH_meth_set_flags(DH_METHOD *dhm, int flags);

void *DH_meth_get0_app_data(const DH_METHOD *dhm);
int DH_meth_set0_app_data(DH_METHOD *dhm, void *app_data);

int (*DH_meth_get_generate_key(const DH_METHOD *dhm))(DH *);
int DH_meth_set_generate_key(DH_METHOD *dhm, int (*generate_key)(DH *));

int (*DH_meth_get_compute_key(const DH_METHOD *dhm))
    (unsigned char *key, const BIGNUM *pub_key, DH *dh);
int DH_meth_set_compute_key(DH_METHOD *dhm,
    int (*compute_key)(unsigned char *key, const BIGNUM *pub_key, DH *dh));

int (*DH_meth_get_bn_mod_exp(const DH_METHOD *dhm))
    (const DH *dh, BIGNUM *r, const BIGNUM *a, const BIGNUM *p,
     const BIGNUM *m, BN_CTX *ctx, BN_MONT_CTX *m_ctx);
int DH_meth_set_bn_mod_exp(DH_METHOD *dhm,
    int (*bn_mod_exp)(const DH *dh, BIGNUM *r, const BIGNUM *a,
                      const BIGNUM *p, const BIGNUM *m, BN_CTX *ctx,
                      BN_MONT_CTX *m_ctx));

int (*DH_meth_get_init(const DH_METHOD *dhm))(DH *);
int DH_meth_set_init(DH_METHOD *dhm, int (*init)(DH *));

int (*DH_meth_get_finish(const DH_METHOD *dhm))(DH *);
int DH_meth_set_finish(DH_METHOD *dhm, int (*finish)(DH *));

int (*DH_meth_get_generate_params(const DH_METHOD *dhm))
    (DH *, int, int, BN_GENCB *);
int DH_meth_set_generate_params(DH_METHOD *dhm,
    int (*generate_params)(DH *, int, int, BN_GENCB *));

Yeah those return types are unhinged — functions returning function pointers returning… listen, it's giving "C from 1972," we deal. 💀

🦋 DESCRIPTION 🦋

So, first thing first, because it matters:

All of the functions described on this page are deprecated. Applications should instead use the provider APIs. Periodt. 🫶

Now the fun part. The DH_METHOD type is a structure used for the provision of custom DH implementations. It provides a set of functions used by OpenSSL for the implementation of the various DH capabilities — think of it as a little vibe check plugin pack: you hand OpenSSL a bundle of callbacks, and it calls them when it needs DH stuff done.

🎀 The constructor-ish ones

DH_meth_new() creates a new DH_METHOD structure. It should be given a unique name and a set of flags. The name should be a NULL terminated string, which will be duplicated and stored in the DH_METHOD object — so it's the caller's responsibility to free the original string. The flags will be used during the construction of a new DH object based on this DH_METHOD. Any new DH object will have those flags set by default.
DH_meth_dup() creates a duplicate copy of the DH_METHOD object passed as a parameter. This might be useful for creating a new DH_METHOD based on an existing one, but with some differences. Slay, copy-paste-with-tweaks energy. ✨
DH_meth_free() destroys a DH_METHOD structure and frees up any memory associated with it. If the argument is NULL, nothing is done. (Iconic behaviour honestly — no crash, no drama.)

💜 Name + flags

DH_meth_get0_name() will return a pointer to the name of this DH_METHOD. This is a pointer to the internal name string and so should not be freed by the caller. Do not free it, bestie, I'm begging. 🙈
DH_meth_set1_name() sets the name of the DH_METHOD to name. The string is duplicated and the copy is stored in the DH_METHOD structure, so the caller remains responsible for freeing the memory associated with the name.
DH_meth_get_flags() returns the current value of the flags associated with this DH_METHOD.
DH_meth_set_flags() provides the ability to set these flags.

🍓 App data (aka the "my stuff" pocket)

The functions DH_meth_get0_app_data() and DH_meth_set0_app_data() provide the ability to associate implementation specific data with the DH_METHOD. It is the application's responsibility to free this data before the DH_METHOD is freed via a call to DH_meth_free(). ⚠️ Like, free your own stuff first, fr fr.

🌈 The callback getters/setters, one by one

DH_meth_get_generate_key() and DH_meth_set_generate_key() get and set the function used for generating a new DH key pair respectively. This function will be called in response to the application calling DH_generate_key(). The parameter for the function has the same meaning as for DH_generate_key().
DH_meth_get_compute_key() and DH_meth_set_compute_key() get and set the function used for computing a new DH shared secret respectively. This function will be called in response to the application calling DH_compute_key(). The parameters for the function have the same meaning as for DH_compute_key().
DH_meth_get_bn_mod_exp() and DH_meth_set_bn_mod_exp() get and set the function used for computing the following value:

r = a ^ p mod m

This function will be called by the default OpenSSL function for DH_generate_key(). The result is stored in the r parameter. This function may be NULL unless using the default generate key function, in which case it must be present. 👀 Read that twice.

DH_meth_get_init() and DH_meth_set_init() get and set the function used for creating a new DH instance respectively. This function will be called in response to the application calling DH_new() (if the current default DH_METHOD is this one) or DH_new_method(). The DH_new() and DH_new_method() functions will allocate the memory for the new DH object, and a pointer to this newly allocated structure will be passed as a parameter to the function. This function may be NULL.
DH_meth_get_finish() and DH_meth_set_finish() get and set the function used for destroying an instance of a DH object respectively. This function will be called in response to the application calling DH_free(). A pointer to the DH to be destroyed is passed as a parameter. The destroy function should be used for DH implementation specific clean up. The memory for the DH itself should not be freed by this function. This function may be NULL. 🚨 (Yes I'm yelling, that's the whole point of a finish callback — clean your stuff, not OpenSSL's stuff.)
DH_meth_get_generate_params() and DH_meth_set_generate_params() get and set the function used for generating DH parameters respectively. This function will be called in response to the application calling DH_generate_parameters_ex() (or DH_generate_parameters()). The parameters for the function have the same meaning as for DH_generate_parameters_ex(). This function may be NULL.

⭐ RETURN VALUES ⭐

DH_meth_new() and DH_meth_dup() return the newly allocated DH_METHOD object or NULL on failure.
DH_meth_get0_name() and DH_meth_get_flags() return the name and flags associated with the DH_METHOD respectively.
All other DH_meth_get_*() functions return the appropriate function pointer that has been set in the DH_METHOD, or NULL if no such pointer has yet been set.
DH_meth_set1_name() and all DH_meth_set_*() functions return 1 on success or 0 on failure. (Yes, 1/0, not 0/-1. We stay unbothered. 💅)

(And yes, DH_new(3) is listed twice in the original. I'm not fixing it. It's canon. 😌)

🔥 HISTORY 🔥

All of these functions were deprecated in OpenSSL 3.0.
The functions described here were added in OpenSSL 1.1.0.

So they lived a good long life from 1.1.0 to 3.0. RIP to a real one. 🕯️

🧚 COPYRIGHT 🧚

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>.

Mwah 💋 — this has been version 3.5.0, dated 2026-09-25, coming at you straight from the DH_METH_NEW(3ossl) manual. Go forth, define your OPENSSL_API_COMPAT, and stay deprecated-but-fabulous. 🦋💖

📦 where this page comes from

opensuse-leap 16.0: openssl-3-doc 3.5.0-160000.11.1 · Apache-2.0
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on 2026-11-04: a language model's rewrite of the original, under its terms